The CNS and normal neuromuscular and musculoskeletal function Week 2 Flashcards
How does pain occur ?
Pain occurs when sensory nerve endings called nociceptors (also referred to as pain receptors) come into contact with a painful or noxious stimulus. The resulting painful impulse travels from the sensory nerve ending, enters the dorsal spinal cord, and travels to diverse parts the brain via nerve tracts in the spinal cord and brainstem. The brain processes the pain sensation and quickly makes a motor response in an attempt to cease the action causing the pain.
What are somatosensory pathways ?
Sensory pathways consist of the chain of neurons, from receptor organ to cerebral cortex, that are responsible for the perception of sensations.
The sensory information processed by the somatosensory systems travels along different anatomical pathways depending on the information carried.
For example, the posterior column-medial lemniscal pathway carries discriminative touch and proprioceptive information from the body, and the main sensory trigeminal pathway carries this information from the face. Whereas, the spinothalamic pathways carry crude touch, pain and temperature information from the body, and the spinal trigeminal pathway carries this information from the face.
What are the common features in a somatosensory pathway ?
Within each somatosensory pathway,
-The 1° afferent is a pseudounipolar neuron that has its cell body located in a peripheral (spinal or cranial) ganglion. It has a peripheral axon that forms or innervates somatosensory receptors and a central process that synapses with 2° afferent neuron(s) in a spinal cord or brain stem nucleus.
-The 2° afferent may synapse with 3° afferent neurons in the spinal cord or may ascend the neuraxis to synapse with 3° afferent neurons in the thalamus.
-There is a decussation (i.e., axons crossing the midline to the opposite side of the spinal cord or brain stem) in each somatosensory pathway below the level of the thalamus.
-All somatosensory pathways include a thalamic nucleus. The thalamic neurons send their axons in the posterior limb of the internal capsule to end in the cerebral cortex.
Most somatosensory pathways terminate in the parietal lobe of the cerebral cortex.
Describe the different somatosensory axons ?
-The Group I and II 1° afferent axons, which form the muscle/tendon receptors and carry body proprioceptive information, have the largest diameter and the thickest myelin of all the somatosensory 1° afferent axons.
-The Type C 1° afferent axons, which form free nerve endings and carry dull pain, deep pain, crude touch or warm/hot information, are the smallest 1° afferent axons and are unmyelinated.
-The Type Aδ1° afferent axons, which form free nerve endings and carry sharp pain or cool/cold information, are thinly myelinated and larger than the Type C axons.
-The Type Aβ 1° afferent axons, which form encapsulated endings in skin and joints or hair follicle endings or Merkel disks in skin, are myelinated and have diameter less than Group I afferents and greater than the Type Aδ 1° afferent axons.
Which peripheral axons have the greatest conduction velocity ?
-the larger and more heavily myelinated the axon, the greater its conduction velocity
-Consequently, the 1° afferent axons carrying information required for fine motor control and rapid reflex responses (i.e., those forming body proprioceptors) conduct action potentials rapidly, whereas those carrying information about body and object temperature conduct action potentials at a much slower rate
What are the two main classes of nerve fibers associated with the transmission of pain ?
There are two major classes of nerve fibers associated with the transmission of pain:
-Unmyelinated C fibers (small and slow)
-Myelinated A-delta fibers (myelinated and fast)
Unmeylinated C fibers:
- The unmyelinated C fibers respond to thermal, mechanical, and chemical stimuli and produce the sensation of dull, diffuse, aching, burning, and delayed pain.
Myelinated A -delta fibers:
-The myelinated A-delta fibers respond to mechanical (pressure) stimulus and produce the sensation of sharp, localized, fast pain.
What is the spinothalamic tract ?
One of the most important central pain pathways is the spinothalamic tract, which originates in the spinal cord and extends to the thalamus. This spinal tract transmits sensory information related to pain, temperature, and crude touch. Another prominent pathway is the spinoreticular tract, which is involved in nociceptive processing. The spinoreticular tract is similar to the spinothalamic tract in that it is excited by similar sensory fibers. Rather than ascending to the thalamus however, spinoreticular neurons terminate within the brainstem reticular formation
What is pain ?
What is nociception ?
Is pain good or bad ?
Characterizing pain based on its origin in the body and duration
- We can categorize pain into visceral pain and soamtic pain.
- Visceral pain normally occurs in organs of the thorax and abdominal cavity. Normally very poorly localized, we don’t always know exactly where the pain is coming from.
- Somatic pain we normally feel on our skin, muscles, joints…..
this type of pain is more localized and we can normally tell where the pain is coming from - We can also categorise pain based on the duration of how long it lasts for. Acute and chronic pain.
Describe the general pain pathway ?
- This information ascends upwards using first, second, and third-order neurons.
- First-order neurons receive impulses from skin and proprioceptors and send them to the spinal cord. They then synapse with second-order neurons. Second-order neurons live in the dorsal horn and send impulses to the thalamus and cerebellum
- Lastly, third-order neurons pick up these impulses in the thalamus and relay it to the somatosensory portion of the cerebrum. Somatosensory sensations are pressure, pain, temperature, and the body’s senses.
First-order neurones :
-These are pseudounipolar neurones which have cells bodies within the dorsal root ganglion.
-They have one axon which splits into two branches, a peripheral branch (which extends towards the peripheries) and a central branch (which extends centrally into the spinal cord/brainstem).
Second-order neurones:
-The cell bodies of these neurones are found in the Rexed laminae of the spinal cord, or in the nuclei of the cranial nerves within the brain stem.
- These neurones then decussate in the anterior white commissure of the spinal cord and ascend cranially in the spinothalamic tract to the ventral posterolateral (VPL) nucleus of the thalamus.
Third-order neurones:
-The cell bodies of third-order neurones lie within the VPL of the thalamus.
-They project via the posterior limb of the internal capsule to terminate in the primary somatosensory cortex.
- The primary somatosensory cortex is somatotopically organized. -
-Therefore, pain signals initiated in the hand will terminate in the area of the cortex dedicated to sensations of the hand.
What is the somatosensory cortex ?
-The somatosensory cortex is a region of the brain which is responsible for receiving and processing sensory information from across the body, such as touch, temperature, and pain.
- Located: in the postcentral gyrus of the parietal lobe, and lies behind the primary motor cortex of the frontal lobe.
- The somatosensory cortex receives tactile information from the body, including sensations such as touch, pressure, temperature, and pain. This sensory information is then carried to the brain via neural pathways to the spinal cord, brainstem, and thalamus.
-This information is then projected to the somatosensory cortex, which in turn has numerous connections with other brain areas in order to process the sensory information.
-The somatosensory cortex uses sensory information to initiate important movements that may be required to deal with particular situations.
Describe the somatosensory pathway ?
Somatosensory pathways are typically comprised of three neurons: primary, secondary, and tertiary.
The primary neurons are the sensory receptors within the periphery of the somatosensory cortex which are able to detect various stimuli such as touch or temperature. The secondary neurons are located within the spinal cord and brainstem and act as a relay station.
Afferent pathways (which carry signals to the central nervous system) in the spinal cord and brainstem working by passing information from the periphery and the rest of the body to the brain. These will then terminate in either the thalamus or the cerebellum.
The tertiary neurons, which are located within the thalamus and cerebellum, will then project to the somatosensory cortex. This will then aid in forming a sensory homunculus, which is a representational map of the body.
Understand the anatomy and physiology of nociceptors and the ‘pain pathways’
-Some first-order neurones have specialist receptors called nociceptors which are activated through various noxious stimuli.
-Nociceptors exist at the free nerve endings of the primary afferent neurone.
- Since nociceptors are free nerve endings this means they are unencapsulated cutaneous receptors
-Similar to other sensory modalities, each nociceptor has its own receptive field. This means one nociceptor will transduce the signal of pain when a particular region is skin is stimulated. The size of receptive fields varies throughout the body and there is often overlap with neighbouring fields.
-Areas such as the fingertips have smaller receptive fields than areas such as the forearm. In addition, they have a larger density of free nerve endings within this receptive field. This difference is important as it allows for greater acuity in detecting a sensory stimulus.
-The size of cortical representation in the somatosensory cortex of a particular body part is also related to the size of the receptive fields in that body part. For example, because the fingertips have small receptive fields, and thus a greater degree of sensory acuity, they have a larger cortical representation.
Types of nociceptors ?
Nociceptors can be found in the skin, muscle, joints, bone, and organs (other than the brain) and can fire in response to a number of different stimuli. Different types of nociceptors exist:
-Mechanical nociceptors – detect the distension of skin (stretch) and pressure which elicit sharp, pricking pain.
-Chemical nociceptors – detect exogenous and endogenous chemical agents, such as prostanoids, histamines etc.
-Thermal and mechano-thermal nociceptors – detect thermal sensations that elicit slow and burning, or cold and sharp in nature, pain.
Polymodal nociceptors – detect mechanical, thermal, and chemical stimuli.
Silent receptors are dormant in normal joints and will be unresponsive to stimuli such as heat or pressure. They become responsive only after tissue damage causes the release of inflammatory molecules in conditions such as arthritis. It is believed that patients with arthritis often complain of joint pain at rest due to these receptors suddenly becoming active and causing spontaneous neuronal firing. Additionally, these silent nociceptors may become mechanosensitive following inflammatory induction and can then contribute to pain with movement in patients with arthritis.
Primary afferent fibers (axons) ?
- Primary afferents are sensory neurons (axons or nerve fibers) in the peripheral nervous system that transduce information about mechanical, thermal, and chemical states of the body and transmit it to sites in the central nervous system.
- Primary afferent sensory neurons are the gateway by which sensory information is transmitted from the peripheral tissues to the spinal cord and brain
- The cell bodies of primary afferent sensory nerve fibers are located in the dorsal root ganglia (DRG) and trigeminal ganglia.
- Anatomically, there are two broad groups of sensory nerve fibers: myelinated A-fibers and smaller diameter, unmyelinated C-fibers
- the large diameter axons which are A (alpha) and A (beta) are very rapidly conducting. They are mainly associated with low threshold mechanoreceptors so touch
- Pain and temperature nociceptors have A (delta) fibers.
- Polymodal nociceptors have C fibers.
What is a pseudounipolar neuron ?
- A pseudounipolar neuron is a type of neuron which has one extension from its cell body. This type of neuron contains an axon that has split into two branches.
- Pseudounipolar neurons are sensory neurons that have no dendrites, the branched axon serving both functions. The peripheral branch extends from the cell body to organs in the periphery including skin, joints and muscles, and the central branch extends from the cell body to the spinal cord
Different types of afferent receptors ?
Alternatively, in the sensory system, afferent fibers can be classified by sizes with category specifications depending on if they innervate the skins or muscles
Describe primary afferent fibers: c type ?
- The majority of primary afferent neurons that transmit noxious stimuli are C-fibers which can be divided into two different classes.
Peptide rich C fibers:
- In the adult, peptide-rich C-fibers express neuropeptides such as calcitonin gene-related peptide (CGRP) and substance P (SP), are regulated by nerve growth factor (NGF) and express tyrosine receptor kinase A (trkA), the cognate receptor for NGF.
-These fibers are largely nociceptive and terminate more superficially within the dorsal horn
Peptide poor C fibers:
- the peptide-poor C-fibers contain the enzyme fluoride-resistant acid phosphatase and selectively bind the lectin Griffonia simplicifolia isolectin B4 (IB4)
-Additionally these nerve fibers express the purinergic receptor known as P2X3 , and respond to glial cell line-derived neurotrophic factor (GDNF) as they express the GDNF receptor complex, which includes glial cell line-derived factor receptor (GFR-alpha) subunits and receptor tyrosine kinase c-Ret (c-RET) .
-These peptide-poor nerve fibers terminate almost exclusively within the deeper parts of lamina II of the spinal dorsal horn
Describe first and second pain ?
- The two different types of fibers that transmit nociception, Alpha (delta) and C fibers do so at different speeds.
- First pain is transmitted by the A (delta fibers)
- Second pain is transmitted by the C fibers.
Transduction of pain:
- different noxious stimuli will open different receptors channels
- Eg. Capsaicin binds to TRPV1 receptor
-Eg. Menthol binds to TRPM8 receptor
Eg. Mustard oil binds to TRPA1 receptor
Nociceptive stimuli activate TRP channels located on nerve endings, which cause first-order neurons to depolarize and fire action potentials. Action potential frequency determines stimulus intensity. A delta fibers release glutamate onto second-order neurons, while C fibers release neuropeptide neurotransmitters. First-order neurons are found in the dorsal root ganglion (stimulus from the body) and trigeminal ganglia (stimulus from the face).
Transduction of pain basic
- The free nerve endings activate different receptor types. This generates a receptor potential which depolarizes the membrane.
- The cell bodies are located in the dorsal root ganglion.
- The action potential enters the spinal chord via the dorsal horn.
- There are sodium channels on the nociceptor. Mutations in these can lead to congenital analgesia.
What is congenital analgesia ?
Congenital insensitivity to pain is a condition that inhibits the ability to perceive physical pain. From birth, affected individuals never feel pain in any part of their body when injured
The sodium channel in a nociceptor can be blocked, which reduces pain
Transmission of the primary afferent in the spinal cord ?
The pathway is known as the spinothalamic ( nociceptive pathway)
- Signals from mechanical, chemical, thermal, and mechano-thermal nociceptors are transmitted to the dorsal horn of the spinal cord predominantly via Aδ fibres. These myelinated fibres have a low threshold for firing and a fast conduction speed. Hence, they are responsible for transmitting the first pain felt.
In addition, Aδ fibres permit the localisation of pain and form the afferent pathway for the reflexes elicited by pain.
Aδ fibres predominantly terminate in Rexed laminae I where they mainly release the neurotransmitter, glutamate.
Polymodal nociceptors transmit their signals into the dorsal horn through C fibres. C fibres are unmyelinated and a slow conduction speed. For this reason, C fibres are responsible for the secondary pain we feel which is often dull, deep, and throbbing in nature. These fibres typically have large receptive fields and therefore lead to poor localisation of pain.
Compared to Aδ fibres, C fibres have a high threshold for firing. However, noxious stimuli can cause sensitisation of C fibres and reduce their threshold for firing.
C fibres predominately terminate in Rexed laminae II (known as substantia gelatinosa) and release the neurotransmitter substance P.
Other neurotransmitters are released by primary afferent neurons terminating within the spinal cord such as aspartate and vasoactive peptide.
What is another name for the spinothalamic tract ?
- Another name for the spinothalamic tract is the antero-lateral system.
- The tract ahs two pathways a anterior and lateral pathway.
1) Anterior spinothalamic tract carries crude touch and pressure sensation
2) Lateral spinothalamic tract carries pain and temperature sensation.
Summary of nociceptive pathway ?
What is the dorsal column medial lemniscus pathway ?
- The dorsal column–medial lemniscus pathway (DCML) (also known as the posterior column-medial lemniscus pathway, PCML) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (position) from the skin and joints.
- It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain.
- The first order neuron travels from the periphery to the spinal cord via the Fasciculus cutaneous or the fasciculus gracilis.
- Anything below T7 ascend the spinal cord in the ipsilateral gracile fasciculus
- Anything which is T6 and above and cervical roots collect in the ipsilateral cuneate fasciculus.
- of the gracile and cuneate fasciculi are collectively called the posterior funiculus or posterior column.
- ascends the spinal cord in the posterior funiculus up to the medulla without synapsing or decussating
In the medulla:
-the 1° afferents in the gracile fasciculus synapse in the gracile nucleus
-the 1° afferents in the cuneate fasciculus synapse in the cuneate nucleus.
-the axons of the gracile and cuneate nuclei (2° afferents) pass anteriorly and decussate to form the medial lemniscus, contralateral to their cells of origin.
-above the level of the gracile and cuneate nuclei, each half of the body is represented contralaterally (e.g., left half of body in right medial lemniscus) within the medial lemniscal pathway.
The medial lemniscus is a second-order neuron of the dorsal column-medial lemniscus pathway (DCML),
The 2° medial lemniscal afferents
ascend the brain stem in the medial lemniscus to the diencephalon.
terminate in the ventral posterolateral (VPL) nucleus of the thalamus.
carrying cutaneous information terminate in the core of the VPL.
carrying proprioceptive information terminate in the surrounding shell of the VPL.
The axons of the VPL 3° afferent neurons
travel in the posterior limb of the internal capsule.
terminate in the postcentral gyrus and posterior paracentral lobule of the parietal lobe.
The postcentral gyrus and posterior paracentral lobule
are called the primary somatosensory cortex.
are the primary cortical receiving areas of the somatosensory system.
Somatosensory vs nociceptive pathway
- Pain on the right side of the body is processed on the left side of the brain.
- Pain on the left side of the body s processed on the right side of the brain.
Test showed Alpha (delta) and Alpha (beta) activated similar maps in the human cortex.
Nociceptive pathway for the face and head.
- The nociceptive pathway for pain in the face and head is called the main sensory trigeminal pathway.
- peripheral processes are located in the trigeminal (predominantly), facial, glossopharyngeal and vagus nerves.
- form mechanoreceptors in the skin, mucous membranes, muscles and joints of the face. The relationship between receptor type formed and the axon Type/Group are similar to those of the medial lemniscal 1° afferents.
- have pseudounipolar cell bodies in the cranial ganglia of the trigeminal, facial, glossopharyngeal and vagus nerves
- send their central axons to the brain stem.
- synapse in the main sensory trigeminal nucleus (2° afferents)
The main sensory trigeminal 2° afferent axons:
-decussate immediately on leaving the main sensory trigeminal nucleus.
-join the contralateral ventral trigeminal lemniscus.
-above the level of the main sensory trigeminal nucleus (i.e., the mid pons), carries information about the contralateral face (i.e., the right ventral trigeminal lemniscus carries information about the left side of the face).
The 2° main sensory trigeminal afferents in the ventral trigeminal lemniscus:
-ascend to the diencephalon.
-terminate in the ventral posteromedial (VPM) nucleus of the thalamus.
The axons of the 3° main sensory trigeminal afferents (VPM neurons) :
-travel in the posterior limb of the internal capsule.
-end in the postcentral gyrus of the parietal lobe.
- Trigeminal pain pathway. Pain sensation from face and mouth is carried by three peripheral nerve branches (V1, V2 and V3) of trigeminal nerve whose cell bodies sit in trigeminal ganglion (TG) and project centrally to synapse with the second order neurons in the trigeminal spinal nucleus caudalis (VC).
- The second order neurons then ascend to terminate in thalamus. From thalamus, nociceptive information is projected to primary somatosensory cortex (SI) where pain processing occurs. Abbreviations: V1: ophthalmic branch; V2: Maxillary branch; V3; mandibular branch of trigeminal nerve.
Describe dissociated sensory loss ( aka Brown Sequard Syndrome )
-Brown-Sequard syndrome (BSS) is a rare neurological condition characterized by a lesion in the spinal cord which results in weakness or paralysis on one side of the body and a loss of sensation on the opposite side.
- The sensory loss is particularly strong on the same side (ipsilateral) as the injury to the spine. These sensations are accompanied by a loss of the sense of pain and of temperature (hypalgesia) on the side of the body opposite (contralateral) to the side at which the injury was sustained.
Explain the phenomenon of referred pain and phantom limb pain.
What is referred pain ?
Referred pain is when the pain you feel in one part of your body is actually caused by pain or injury in another part of your body. For example, an injured pancreas could be causing pain in your back, or a heart attack could be triggering pain in your jaw
What is phantom limb pain ?
Phantom limb pain is the perception of pain or discomfort in a limb that is no longer there
Explain the influence of higher brain centers on pain perception.
The gate control theory of pain describes how non-painful sensations can override and reduce painful sensations. A painful, nociceptive stimulus stimulates primary afferent fibers and travels to the brain via transmission cells. Increasing activity of the transmission cells results in increased perceived pain. Conversely, decreasing activity of transmission cells reduces perceived pain. In the gate control theory, a closed “gate” describes when input to transmission cells is blocked, therefore reducing the sensation of pain. An open “gate” describes when input to transmission cells is permitted, therefore allowing the sensation of pain.
When you experience a negative feeling, such as pain from a bump or an itch from a bug bite, a common reaction is an attempt to eliminate the feeling by rubbing the painful bump or scratching the itchy bite. Gate control theory asserts that activation of nerves that do not transmit pain signals, called nonnociceptive fibers, can interfere with signals from pain fibers, thereby inhibiting pain.
Central modulation of pain: Dissociation
Describe physiological (acute) pain vs Pathological (chronic) pain ?
Pain may be broadly classified into physiological and pathological pain. Nociceptive and inflammatory pains are physiological pain states, as they are protective and adaptive, whereas pathological pain is nonprotective and maladaptive.
Describe sensitization to pain ?
What is fibromyalgia ?
Fibromyalgia is a condition that causes widespread pain and extreme tiredness.
Outline the various approaches used in the management of pain.
Describe pain management with opioids ?
Descending Modulation of Pain:
-Within the central nervous system, there are three types of opioid receptors which regulate the neurotransmission of pain signals. These receptors are called mu, delta, and kappa opioid receptors.
-They are all G protein-coupled receptors and their activation leads to a reduction in neurotransmitter release and cell hyperpolarisation, reducing cell excitability. Exogenous opioids, such as morphine, provide excellent analgesia by acting on these receptors. Likewise, our body contains endogenous opioids which can modulate pain physiologically. There are three types of endogenous opioids:
-Β-endorphins – which predominately binds to mu opioid receptors
-Dynorphins – which predominately bind to kappa opioid receptors
-Enkephalins – which predominately bind to delta opioid receptors
-Opioids can regulate pain on a number of levels, both within the spinal cord, brain stem, and cortex. Within the spinal cord, both dynorphins and enkephalins can act to reduce the transmission of pain signals in the dorsal horn. This is because the post-synaptic ends of second-order neurones have opioid receptors within the membrane. In addition, the pre-synaptic ends of first-order neurones contain opioid receptors.
-When endogenous opioids act on these receptors it reduces neurotransmitter release from the first-order neurone, and causes hyperpolarisation of the second-order neurone. Together, this reduces the firing of action potentials in the second-order neurone, blocking the transmission of pain signals.
Calcium channels close so calcium can’t come inside
Potrasium channels open so potassium can’t come in
This causes hyperpolarisation
Common clinical opioid analgesics ?
What are the common features in a somatosensory pathway ?
Within each somatosensory pathway,
-The 1° afferent is a pseudounipolar neuron that has its cell body located in a peripheral (spinal or cranial) ganglion. It has a peripheral axon that forms or innervates somatosensory receptors and a central process that synapses with 2° afferent neuron(s) in a spinal cord or brain stem nucleus.
-The 2° afferent may synapse with 3° afferent neurons in the spinal cord or may ascend the neuraxis to synapse with 3° afferent neurons in the thalamus.
-There is a decussation (i.e., axons crossing the midline to the opposite side of the spinal cord or brain stem) in each somatosensory pathway below the level of the thalamus.
-All somatosensory pathways include a thalamic nucleus. The thalamic neurons send their axons in the posterior limb of the internal capsule to end in the cerebral cortex.
Most somatosensory pathways terminate in the parietal lobe of the cerebral cortex.
Describe the different somatosensory axons ?
-The Group I and II 1° afferent axons, which form the muscle/tendon receptors and carry body proprioceptive information, have the largest diameter and the thickest myelin of all the somatosensory 1° afferent axons.
-The Type C 1° afferent axons, which form free nerve endings and carry dull pain, deep pain, crude touch or warm/hot information, are the smallest 1° afferent axons and are unmyelinated.
-The Type Aδ1° afferent axons, which form free nerve endings and carry sharp pain or cool/cold information, are thinly myelinated and larger than the Type C axons.
-The Type Aβ 1° afferent axons, which form encapsulated endings in skin and joints or hair follicle endings or Merkel disks in skin, are myelinated and have diameter less than Group I afferents and greater than the Type Aδ 1° afferent axons.
Which peripheral axons have the greatest conduction velocity ?
-the larger and more heavily myelinated the axon, the greater its conduction velocity
-Consequently, the 1° afferent axons carrying information required for fine motor control and rapid reflex responses (i.e., those forming body proprioceptors) conduct action potentials rapidly, whereas those carrying information about body and object temperature conduct action potentials at a much slower rate
What is an action potential ?
Steps for an action potential ?
- An action potential begins at the axon hillock as a result of depolarisation. During depolarisation voltage-gated sodium ion channels open due to an electrical stimulus. As the sodium ions rush back into the cell, their positive charge changes potential inside the cell from negative to more positive.
- If a threshold potential is reached, then an action potential is produced. Action potentials will only occur if a threshold is reached. Additionally, if the threshold is reached, then the response of the same magnitude is always elicited, irrespective of the strength of the stimulus. Hence, action potentials are described as “all-or-nothing“.
-Once the cell has been depolarised the voltage-gated sodium ion channels begin to close. The positive potential inside the cell causes voltage-gated potassium channels to open and K+ ions now move down their electrochemical gradient out of the cell. As the K+ moves out of the cell, the membrane potential becomes more negative and starts to approach the resting potential.
- Typically, repolarisation overshoots the resting membrane potential, making the membrane potential more negative. This is known as hyperpolarisation. It is important to note that the Na+/K+ ATPase is not involved in the repolarisation process following an action potential.
What does conduction non-decremental spread mean?
Conduction without decrement means that action potentials transmitted down an axon will not decrease in amplitude
Action potential time course
-The resting membrane potential of cells varies depending on the cell type. For neurones, it typically sits between -50 and -75mV.
Refractory period ?
Every action potential is followed by a refractory period. This period can be further divided into:
the absolute refractory period which occurs once the sodium channels close after an AP. Sodium channels then enter an inactive state during which they cannot be reopened, regardless of the membrane potential.
and
the relative refractory period which occurs when sodium channels slowly come out of the inactivation. During this period the neurone can be excited with stimuli stronger than the one normally needed to initiate an AP. Early on in the relative refractory period, the strength of the stimulus required is very high. Gradually, it becomes smaller throughout the relative refractory period as more sodium channels recover from the inactivation stage.
-In neurones, K+ and organic anions are typically found at a higher concentration within the cell than outside, whereas Na+ and Cl- are typically found in higher concentrations outside the cell.
-Hence, K+ ions would be moving out of the cells, while Na+ and Cl- ions would be moving into the cell
- These concentration gradients are maintained by the action of the Na+/K+ ATPase via active transport, which in turn allows the membrane potential to be maintained.
Why do action potentials only travel in one direction ?
Propagation of Action Potentials
Action potentials are propagated along the axons of neurones via local currents. Local currents induce depolarisation of the adjacent axonal membrane and where this reaches a threshold, further action potentials are generated. The areas of the membrane that have recently depolarised will not depolarise again due to the refractory period – meaning that the action potential will only travel in one direction.
These local currents would eventually decrease in charge until a threshold is no longer reached. The distance that this would take depends on the membrane capacitance and resistance:
-Membrane capacitance – the ability to store charge. The lower capacitance results in a greater distance before the threshold is no longer reached.
-Membrane resistance – depends on the number of ion channels open. The lower the number of channels open, the greater membrane resistance is. A higher membrane resistance results in a greater distance before the threshold is no longer reached.
Myelinated axons ?
In order to allow rapid conduction of electrical signals through a neurone and make them more energy-efficient certain neuronal axons are covered by a myelin sheath. The myelin sheath surrounds the axon to form an insulating layer. Further information on the myelin sheath can be found here.
Along a myelinated axon, there are periodic gaps where there is no myelin and the axonal membrane is exposed. These gaps are called Nodes of Ranvier. In contrast to myelinated sections of the axon that lack voltage-gated ion channels, nodes of Ranvier harbour a high density of ion channels. For this reason, an action potential can only occur at the nodes.
The myelin sheath speeds up the conduction by increasing the membrane resistance and reducing the membrane capacitance. Hence, the action potential is able to propagate along the neurone at a higher speed than would be possible in unmyelinated neurons. The electrical signals are rapidly conducted from one node to the next, where is causes depolarisation of the membrane. If the depolarisation exceeds the threshold, it initiates another action potential which is conducted to the next node. In this manner, an action potential is rapidly conducted down a neurone. This is known as saltatory conduction.
The cell membrane ?
- The cell membrane acts as a selective filter, allowing the free movement of some molecules across it while tightly controlling the movement of others. Passage of a specific substance across the membrane depends on multiple factors including its electric charge, molar mass and the polarity of the molecule.
- Movement of uncharged substances, like O2, CO2, urea, alcohol and glucose, depends only on their concentration gradient. The cell membrane is permeable to these molecules, and so they can move freely as their concentration gradients allow.
- Charged substances such as K+, Na+, Cl– ions, cannot easily diffuse through the cell membrane due to its internal hydrophobic structure. Hence, to cross the cell membrane charged substances will utilise specialised, water-filled pores known as ion channels.
What are the 3 factors that can induce the movement of the ions through ion channels ?
- the concentration gradient
- the electrical gradient
- active transport
-The concentration gradient – a difference in concentration of the ion on the two sides of the membrane. Ions would cross the membrane from a compartment with a higher concentration to the compartment with a lower concentration.
-The electrical gradient – an electrical potential difference across the membrane defined as the electrical potential value inside the cell relative to the extracellular environment. Positive ions will be attracted to negative electrical potential and repelled from positive electric potential, and vice versa.
Potassium ?
To better understand how the concentration gradient and the electrical gradient influence the movement of ions across the cellular membrane, let’s analyse the movements of potassium (K+) ions:
-The concentration gradient – The intracellular concentration of potassium greatly exceeds the extracellular concentration (~130mmol/L vs ~4mmol/L). Thus, potassium ions will tend to exit the cell according to the concentration gradient.
-The electrical gradient – As positively charged K+ ions are released, the charge of the intracellular space becomes relatively negative. Hence, some K+ ions are attracted back towards the intracellular space, despite the concentration gradient leading them in the opposite direction.
Thus, two “streams” containing K+ ions are created; one that expels potassium as per its concentration gradient, and one which attracts potassium as per the increasing negative intracellular electrical environment.
What is the Na+/ K+ pump? Active transport ?
Outside of the cell:
[K+] = 20mM
[Na+] = 450mM
Inside the cell:
[K+] =400mM
[ Na+] = 50mM
Electrochemical gradient ?
Calculating the equilibrium potential ?
Equilibrium potential:
-At the equilibrium potential, the rate at which ions leave by concentration gradient is equal to the rate at which ions enter via the electrochemical gradient.
-Importantly, in a cell where only one type of ion can cross the membrane, the resting membrane potential will equal the equilibrium potential for that particular ion.
What is the Nernst equation ?
The Nernst equation is used to calculate the value of the equilibrium potential of a particular cell for a particular ion:
where Vm = equilibrium potential for any ion [V];
z = valence of the ion
[C]0 = concentration of ion X outside of the cell [mol]
[C]i = concentration of ion X inside the cell [mol]
So, assuming only potassium ions could cross the membrane and knowing common values for the intracellular and extracellular concentrations of potassium, one can calculate the approximate resting potential of a cell. In the example below, K0=4 and Ki=126 are used as common values:
The movement of Na+ in the cell ?
Alongside the flux of potassium ions towards the extracellular space, sodium, chloride and other ions also cross the membrane. For example, the positively charged sodium ions enter the neurones down the concentration gradient but they are also attracted by a negative electrical potential inside the neurone.
Hence, this movement will make the resting potential less negative. Overall, the resting potential accounts for the movements of all ions across the membrane.
The table below summarises the main direction of movement for various ions and the overall impact this has on the resting membrane potential of a neurone:
Whilst all of these contribute to the resting membrane potential, the cell is most permeable to sodium and potassium ions and so these will have the greatest impact. As the cell membrane of neurones are most permeable to potassium, the resting membrane potential will be closest to the equilibrium potential for potassium ions, with the impact of sodium ion influx making it slightly less negative (i.e. -75mV as opposed to -92mV).
If there was to be any change in the permeability of the cell membrane to ions (via channels opening or closing) then the membrane potential would be altered – this is how action potentials are generated.
Maintaining the resting membrane potential ?
Without something to maintain the ionic concentration gradients, the resting membrane potential would dissipate, and so therefore would the membrane potential. The sodium-potassium pump (Na+ K+ ATPase) prevent this and maintains the ionic differences across the membrane.
This pump actively transports potassium and sodium ions against their electrochemical gradients (i.e. potassium moves intracellularly and sodium moves extracellularly). This allows the concentration gradient that these ions travel down to be maintained and therefore, for the resting membrane potential to be maintained.
Describe Hyperkalaemia ?
What if extracellular [K+] increases ?
Assisted suicide ?
Dr Death ?
Resting membrane potential ?
Action potential ?
Multiple Sclerosis ?
Tetrodotoxin (TTX) ?
Local anesthetics ?
Label the brain ?
There are 3 spaces within the brain. The Subdural space, subarachnoid space and epidural space
Describe the anatomical course of the spinal cord ?
The spinal cord is a cylindrical structure, greyish-white in colour. It has a relatively simple anatomical course:
-The spinal cord arises cranially as a continuation of the medulla oblongata (part of the brainstem).
-It then travels inferiorly within the vertebral canal, surrounded by the spinal meninges containing cerebrospinal fluid.
-At the L2 vertebral level the spinal cord tapers off, forming the conus medullaris.
- spinal cord is the nerves. The vertebral column is the bones.
Describe the spinal meninges ?
- The spinal meninges are three membranes that surround the spinal cord – the dura mater, arachnoid mater, and pia mater.
- cerebrospinal fluid travels in the sub-arachnoid space.
Dura mater:
- external of the meninges
- extends from the foramen magnum to the filum terminale
Arachnoid mater:
- subarachnoid space (before the pia mater) contains cerebrospinal fluid
- Distal to the conus medullaris, the subarachnoid space expands, forming the lumbar cistern. This space accessed during a lumbar puncture (to obtain CSF fluid) and spinal anaesthesia.
Pia mater:
- is the innermost of the meninges
- It is a thin membrane that covers the spinal cord, nerve roots and their blood vessels
- Inferiorly, the spinal pia mater fuses with the filum terminale
- As a result of the termination of the spinal cord at L2, it occupies around two thirds of the vertebral canal.
- The spinal nerves that arise from the end of the spinal cord are bundled together forming a structure known as the cauda equina.
-Compression of these nerves produces a range of signs and symptoms collectively termed cauda equina syndrome. There are many causes of compression, including intervertebral disc prolapse, extrinsic or primary cord tumours, spinal stenosis, trauma and abscess formation.
During the course of the spinal cord there are two points of enlargement:
Cervical enlargement:
- The cervical enlargement is located proximally, at the C4-T1 level.
- It represents the origin of the brachial plexus.
Lumbar enlargement:
- Between T11 and L1 is the lumbar enlargement, representing the origin of the lumbar and sacral plexi.
Cervical and lumbar enlargement
- The motor neuron leaves from the anterior/ventral side
- The sensory neuron leaves from the posterior/dorsal side.
- Each spinal nerve begins as an anterior (motor) and a posterior (sensory) nerve root. These roots arise from the spinal cord, and unite at the intervertebral foramina, forming a single spinal nerve
Formation of the spinal nerves ?
Each spinal nerve begins as an anterior (motor) and a posterior (sensory) nerve root. These roots arise from the spinal cord, and unite at the intervertebral foramina, forming a single spinal nerve.
The spinal nerve then leaves the vertebral canal via the intervertebral foramina, and then divides into two:
Posterior rami – supplies nerve fibres to the synovial joints of the vertebral column, deep muscles of the back, and the overlying skin.
Anterior rami – supplies nerve fibres to much of the remaining area of the body, both motor and sensory.
The nerve roots L2-S5 arise from the distal end of the spinal cord, forming a bundle of nerves known as the cauda equina
There are 31 spinal nerves:
Cervical: 8
Thoracic: 12
Lumbar: 5
Sacral:5
Cocygeal: 1
- The brainstem is the structure that connects the cerebrum of the brain to the spinal cord and cerebellum. It is composed of three sections in descending order: the midbrain, pons, and medulla oblongata.
Midbrain:
- originates from mesencephalon
-The paired cerebral peduncles extend from the cerebral hemispheres to converge as they meet the pons. They are separated anteriorly in the midline by the interpeduncular fossa, the floor of which is termed the posterior perforated substance (as many perforating blood vessels can be identified).
The oculomotor nerve (CNIII) is seen exiting from between the peduncles
Pons:
- originates from the metencephalon
- It is a group of nerves that function as a connection between the cerebrum and cerebellum
- several cranial nerves arise from the ventral surface of pons
Medulla:
- The posterior end of the medulla is when the first pair of cervical spinal nerves originate.
- Medulla exits the skull through the foramen magnum.
-
Which cranial nerves originate from the ventral surface of pons ?
Several cranial nerves originate from the ventral surface of the pons:
-Cranial nerve V:
trigeminal – originates from the lateral aspect of mid pons
-Cranial nerve VI:
abducens – originates from the pontomedullary junction, close to the midline
-Cranial nerve VII:
facial – originates from the cerebellopontine angle, the more lateral aspect of the pontomedullary junction.
-Cranial nerve VIII: vestibulocochlear – originates laterally to the facial nerve.
Describe the surface of the medulla ?
- In the midline of the medulla is the anterior median fissure, which is continuous along the length of the spinal cord.
- the pyramids are paired swellings found between the anterior median fissure and the ventrolateral sulcus.
- The olives are another pair of swellings located laterally to the pyramids – between the ventrolateral and posterolateral sulci.
- two sulci are visible – the ventrolateral sulcus and the posterolateral sulcus
Cross section of the medulla:
- At this level, the central portion of the medulla contains gray matter, while the outer portions consist of white matter.
-The posterior white matter contains the fasiculus gracilis and the more lateral fasiculus cuneatus.
-Corresponding portions of gray matter extend to these regions and are the nucleus gracilis and nucleus cuneatus respectively.
-Unchanged from the spinal cord, the spinocerebellar tracts (posterior and anterior) are located laterally, with the lateral spinothalamic tract situated between them. The large trigeminal nucleus and tracts can be found posterior to these tracts. This is a continuation of the substantia gelatinosa of the spinal cord.
Where do the cranial nerves originate from ?
Cranial nerves that originate from the:
midbrain:
- oculomotor nerve (CN III)
-trochlear nerve (CN IV)
Pons:
- trigeminal nerve (CN V)
-abducens nerve (CN VI)
-facial nerve (CN VII)
-vestibulocochlear nerve (CN VIII)
Medulla:
- glossopharyngeal nerve (CN IX)
-vagus nerve (CN X)
-accessory nerve (CN XI)
-hypoglossal nerve (CN XII)
In neuroanatomy, a nucleus (plural form: nuclei) is a cluster of neurons in the central nervous system, located deep within the cerebral hemispheres and brainstem.
All cranial nerves originate from nuclei in the brain. Two originate from the forebrain (Olfactory and Optic), one has a nucleus in the spinal cord (Accessory) while the remainder originate from the brainstem
What happens if we have brainstem death ?
Cranial nerves function ?
Through which holes in the skull do the cranial nerves arise from ?
Describe the conscious and unconscious tracts ?
Functionally, the ascending tracts can be divided into the type of information they transmit – conscious or unconscious:
Conscious tracts – comprised of the dorsal column-medial lemniscal pathway and the anterolateral system.
Unconscious tracts – comprised of the spinocerebellar tracts.
The Anterolateral System
The anterolateral system consists of two separate tracts:
Anterior spinothalamic tract – carries the sensory modalities of crude touch and pressure.
Lateral spinothalamic tract – carries the sensory modalities of pain and temperature.
How does the name dorsal column-medial lemniscal pathway (DCML) help with understanding the pathway ?
Its name arises from the two major structures that comprise the DCML. In the spinal cord, information travels via the dorsal (posterior) columns. In the brainstem, it is transmitted through the medial lemniscus.
Dorsal column pathway ?
First order neurons:
- The first order neurones carry sensory information regarding touch, proprioception or vibration from the peripheral nerves to the medulla oblongata. There are two different pathways which the first order neurones take:
-Signals from the upper limb (T6 and above) – travel in the fasciculus cuneatus (the lateral part of the dorsal column).
They then synapse in the nucleus cuneatus of the medulla oblongata.
-Signals from the lower limb (below T6) – travel in the fasciculus gracilis (the medial part of the dorsal column).
They then synapse in the nucleus gracilis of the medulla oblongata.
Second order neurons:
- The second order neurones begin in the cuneate nucleus or gracilis. The fibres receive the information from the preceding neurones, and delivers it to the third order neurones in the thalamus.
-Within the medulla oblongata, these fibres decussate (cross to the other side of the CNS). They then travel in the contralateral medial lemniscus to reach the thalamus.
Third Order neurons:
- Lastly, the third order neurones transmit the sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain.
-They ascend from the ventral posterolateral nucleus of the thalamus, travel through the internal capsule and terminate at the sensory cortex.
Brain stem lesion: at the point of the brainstem both the spinothalmic pathway and dorsal medial leminiscus pathway have both decussated. So a lesion in the brainstem will result in contra lateral sensory loss. If you have a lesion on the left side of your brainstem you will feel pain on the right hand side
Spinal cord lesion: the DCMLP decussates at the medulla and the spinothalmic at the spinal cord.
So a lesion in the spinal cord will cause pain and temperature sensory loss the contra lateral side and and vibration and touch sensory loss on the ipsilateral side
Describe the descending tracts ?
-The descending tracts are the pathways by which motor signals are sent from the brain to lower motor neurones.
- The lower motor neurones then directly innervate muscles to produce movement.
- Information synapses in the ventral horn of the grey matter.
- The motor neurone begins in the precentral gyrus which is the primary motor cortex. Information travels infreriorly.
What counts as an upper motor neuron and a lower motor neuron ?
There are no synapses within the descending pathways. At the termination of the descending tracts, the neurones synapse with a lower motor neurone. Thus, all the neurones within the descending motor system are classed as upper motor neurones. Their cell bodies are found in the cerebral cortex or the brain stem, with their axons remaining within the CNS.
Describe pyramidal tracts aka the lateral descending spinal pathway ?
-The pyramidal tracts derive their name from the medullary pyramids of the medulla oblongata, which they pass through.
-These pathways are responsible for the voluntary control of the musculature of the body and face.
Functionally, these tracts can be subdivided into two:
-Corticospinal tracts – supplies the musculature of the body.
-Corticobulbar tracts – supplies the musculature of the head and neck.
In the inferior part of the medulla what does the corticospinal tract divide into ?
In the most inferior (caudal) part of the medulla, the tract divides into two:
The fibres within the lateral corticospinal tract decussate (cross over to the other side of the CNS). They then descend into the spinal cord, terminating in the ventral horn (at all segmental levels). From the ventral horn, the lower motor neurones go on to supply the muscles of the body.
The anterior corticospinal tract remains ipsilateral, descending into the spinal cord. They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels.
Motor pathways
- Sensory information is passed along 3 neurones however motor information travels along a two neurone pathway.
- We split the two neurones into the upper motor neurone and lower motor neurone.
Upper motor neurone:
- The upper motor neuron is almost entirely within the CNS.
- They can start in the cerebral cortex, cerebellum and or brainstem depending on which spinal tract we are talking about.
-The upper motor neuron form the descending tracts. This is because when we are talking about descending pathways we are talking about the ones which are in the spinal cord and these are all UPN.
Once the upper motor neuron synapses with a lower motor neuron in the ventral horn of the spinal cord or brainstem, this is when it becomes a spinal nerve and is heading out towards the peripheral nervous system.
- If the lower motor neuron is exiting from the brainstem then its a cranial nerve.
- If the lower motor neuron is exiting from the spinal cord and from the ventral horn then it will be a spinal nerve.
- For these slides we are focusing on the upper motor neurones.
Describe the upper motor neuron descending pathways ?
We can divide the UMN descending pathways into two broad groups:
- Dorsolateral pathway
- Ventromedial pathway
Dorsolateral pathway:
- supplies the distal muscles ( muscles which are from the elbow down and knee down)
- these muscles control fine movement
Ventromedial pathway:
- they supply everything else so from shoulder to elbow and knee to hip
- supplies the proximal limbs and trunk muscles
- controls posture and movement correction
These tracts are divided based upon:
- anatomic region of the tract
- position of the synapse with LMN in the ventral horn
- The dorsal lateral tracts will synapse more laterally
- The ventromedial will synapse more medially
These are a small number of pathways we divide into dorsolateral and ventromedial, there are more.
DORSOLATERAL:
- Lateral corticospinal
- Rubrospinal
VENTROMEDIAL:
- Anterior corticospinal
- Reticulospinal
- Tectospinal
- Vestibulospinal
Ventromedial pathways ?
Reticulospinal tract goes from the reticulum to the spinal cord.
- Reticulospinal tract is used for alertness and consciousness and other things
- 1st order neurones arise from pons and medullary reticular formation.
- Remain ipsilateral ( no decussation)
- Polysegmental ( means they give nerves at every single spinal level and they run the length of the spinal cord)
- synapse with the LMN in the medial aspect of the ventral horn.
- LMN will supply the trunk and proximal UL and LL.
Function of reticulospinal:
- posture & gait-related movements
- has a role in modulating sensation
Tectum= posterior aspect of the midbrain (superior/inferior colliculi)
- 1st order neurons arise from the superior colliculi
- Decussate in the midbrain
- Peripheral retinal input
- Synapse with LMN in cervical spinal cord
- LMN innervate shoulder girdle and neck
Function:
- head turning in response to visual stimulus
- coordination of head and eye movement
Ventromedial pathways ?
Vestibulospinal tract:
- 1st order neurons arise from the vestibular nuclei
- Remains ipsilateral (no decusasation)
- Medial VST - stops at Cervical; Lateral VST - travels through All spinal segments
- Synapse with LMN in medial aspect of ventral horn
- LMN innervate shoulder girdle and neck
Remember the lateral VST will travel the full length of the body.
Function:
- Positioning of head and neck
- Balance - movement in response to sudden postural change.
Ventromedial pathways:
- Unilateral axon synapses with interneuron and provides bilateral inneravtion.
- So if the motor neuron came down the left hand side when it reaches the place it is going to stop it synapses with the interneuron to supply both the left and right hand side of the the body.
- UNILATERAL LESION = no obvious defects
- So if there was a unilateral lesion there would be no obvious defects because the right side can still innervate the left hand side.
BILATERAL LESION = obvious defects
- however if you have a bilateral lesion and take out both ventromedial pathways you would end up with things like a:
- loss of posture correction reactions
- loss of navigational control ( collides with objects)
- loss of trunk mobility ( patient slumps forward)
- loss of ability to reach out ( e.g. for food)
- However you can flex elbow and digits
Motor Descending spinal pathways part 2
Dorsolateral pathways ?
Rubrospinal tract:
- relatively short tract does not pass cervical region of the spinal cord
- originates in the red nucleus of the midbrain
- decussates in the midbrain
- contralateral supply to the head neck and upper limb
Function:
- control of muscle tone in flexor muscle group
- coordination of movement ( with cerebellum)
Lesion (isolated):
if there was a legion in the rubrospinal tract you would see:
- impaired distal arm & hand movement
- ## Intention tremors ( related to cerebellar function)
Corticospinal tract ?
- the corticospinal tract is coming from the cortex towards the spinal cord.
Reminder: motor cortex is found in the precentral gyrus. This information is sent to the internal capsule
- Internal capsule forms a somatotropic map from head to toe.
- The internal capsule lies laterally to the thalamus
- In the most inferior (caudal) part of the medulla, the tract divides into two:
- The fibres within the lateral corticospinal tract decussate (cross over to the other side of the CNS). They then descend into the spinal cord, terminating in the ventral horn (at all segmental levels). From the ventral horn, the lower motor neurones go on to supply the muscles of the body.
-The anterior corticospinal tract remains ipsilateral, descending into the spinal cord. They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels.
Through which holes in the skull do the cranial nerves arise from ?
Describe vertebrae ? Vertebral body?
- All vertebrae share a basic common structure. They each consist of an anterior vertebral body, and a posterior vertebral arch.
- The vertebral body forms the anterior part of each vertebrae.
It is the weight-bearing component, and vertebrae in the lower portion of the column have larger bodies than those in the upper portion (to better support the increased weight).
-The superior and inferior aspects of the vertebral body are lined with hyaline cartilage. Adjacent vertebral bodies are separated by a fibrocartilaginous intervertebral disc.
Describe vertebrae ? vertebral arch
- The vertebral arch forms the lateral and posterior aspect of each vertebrae.
- In combination with the vertebral body, the vertebral arch forms an enclosed hole – the vertebral foramen. The foramina of all the vertebrae line up to form the vertebral canal, which encloses the spinal cord
- The vertebral arches have several bony prominences, which act as attachment sites for muscles and ligaments:
The vertebral arches have bony prominences which act as attachment sites for muscles and ligaments ?
-Spinous processes – each vertebra has a single spinous process, centred posteriorly at the point of the arch.
-Transverse processes – each vertebra has two transverse processes, which extend laterally and posteriorly from the vertebral body. In the thoracic vertebrae, the transverse processes articulate with the ribs.
-Pedicles – connect the vertebral body to the transverse processes.
-Lamina – connect the transverse and spinous processes.
-Articular processes – form joints between one vertebra and its superior and inferior counterparts. The articular processes are located at the intersection of the laminae and pedicles.
What is intervertebral disc herniation ?
Cervical vertebrae ?
There are seven cervical vertebrae in the human body. They have three main distinguishing features:
- Bifid spinous process – the spinous process bifurcates at its distal end.
Exceptions to this are C1 (no spinous process) and C7 (spinous process is longer than that of C2-C6 and may not bifurcate).
-Transverse foramina – an opening in each transverse process, through which the vertebral arteries travel to the brain.
-Triangular vertebral foramen
Image below: vertebral artery is seen within the foramina of the cervical transverse processes
Two cervical vertebrae that are unique.
C1 and C2 (called the atlas and axis respectively), are specialised to allow for the movement of the head.
Atlas:
The atlas is the first cervical vertebra and joins with the occiput of the head and the axis (C2).
It differs from the other cervical vertebrae in that it has no vertebral body and no spinous process. Instead, the atlas has lateral masses which are connected by an anterior and posterior arch. Each lateral mass contains a superior articular facet (for articulation with occipital condyles), and an inferior articular facet (for articulation with C2).
The anterior arch contains a facet for articulation with the dens of the axis. This is secured by the transverse ligament of the atlas – which attaches to the lateral masses. The posterior arch has a groove for the vertebral artery and C1 spinal nerve
Axis:
The axis (C2) is easily identifiable due to its dens (odontoid process) which extends superiorly from the anterior portion of the vertebra.
The dens articulates with the anterior arch of the atlas, in doing so creating the medial atlanto-axial joint. This allows for rotation of the head independently of the torso.
The axis also contains superior articular facets, which articulate with the inferior articular facets of the atlas to form the two lateral atlanto-axial joints.
The joints of the cervical spine can be divided into two groups – those that are present throughout the vertebral column, and those unique to the cervical spine.
Present throughout Vertebral Column
There are two different joints present throughout the vertebral column:
Between vertebral bodies – adjacent vertebral bodies are joined by intervertebral discs, made of fibrocartilage. This is a type of
cartilaginous joint, known as a symphysis.
Between vertebral arches – formed by the articulation of superior and inferior articular processes from adjacent vertebrae. It is a synovial type joint.
Unique to Cervical Spine
There are two joints unique to the cervical spine – the atlanto-axial (x3) and atlanto-occipital joints (x2).
The atlanto-axial joints are formed by the articulation between the atlas and the axis:
Lateral atlanto-axial joints (x2) – formed by the articulation between the inferior facets of the lateral masses of C1 and the superior facets of C2. These are plane type synovial joints.
Medial atlanto-axial joint – formed by the articulation of the dens of C2 with the articular facet of C1. This is a pivot type synovial joint.
The atlanto-occipital joints consist of an articulation between the spine and the cranium. They occur between then superior facets of the lateral masses of the atlas and the occipital condyles at the base of the cranium. These are condyloid type synovial joints, and permit flexion at the head i.e. nodding.
Ligaments
There are six major ligaments to consider in the cervical spine. The majority of these ligaments are present throughout the entire vertebral column.
Present throughout Vertebral Column
-Anterior and posterior longitudinal ligaments – long ligaments that run the length of the vertebral column, covering the vertebral bodies and intervertebral discs.
-Ligamentum flavum – connects the laminae of adjacent vertebrae.
-Interspinous ligament – connects the spinous processes of adjacent vertebrae.
Uniique to Cervical Spine
-Nuchal ligament – a continuation of the supraspinous ligament. It attaches to the tips of the spinous processes from C1-C7 and provides the proximal attachment for the rhomboids and trapezius.
-Transverse ligament of the atlas – connects the lateral masses of the atlas, and in doing so anchors the dens in place.
Anatomical relationships ?
The cervical spine has a close relationship with several neurovascular structures in the neck.
The transverse foramina of the cervical vertebrae provide a passageway by which the vertebral artery, vein and sympathetic nerves can pass. The only exception to this is C7 – where the vertebral artery passes around the vertebra, instead of through the transverse foramen.
The spinal nerves are intimately related to the cervical vertebrae. They extend from above their respective vertebrae, through the intervertebral foramen created by the joints at the articular processes. Again, C7 is an exception – it has a set of spinal nerves extending from above (C7) and below (C8) the vertebra. Therefore, there are eight spinal nerves associated with seven cervical vertebrae.
Describe the thoracic vertebrae ?
Thoracic Vertebrae:
-The twelve thoracic vertebrae are medium-sized, and increase in size from superior to inferior. Their specialised function is to articulate with ribs, producing the bony thorax.
-Each thoracic vertebra has two ‘demi facets,’ superiorly and inferiorly placed on either side of its vertebral body. The demi facets articulate with the heads of two different ribs.
On the transverse processes of the thoracic vertebrae, there is a costal facet for articulation with the shaft of a single rib.
For example, the head of Rib 2 articulates with the inferior demi facet of thoracic vertebra 1 (T1) and the superior demi facet of T2, while the shaft of Rib 2 articulates with the costal facets of T2.
The spinous processes of thoracic vertebrae are oriented obliquely inferiorly and posteriorly. In contrast to the cervical vertebrae, the vertebral foramen of thoracic vertebrae is circular.
Lumbar vertebrae ?
Lumbar Vertebrae
There are five lumbar vertebrae in most humans, which are the largest in the vertebral column. They are structurally specialised to support the weight of the torso.
Lumbar vertebrae have very large vertebral bodies, which are kidney shaped. They lack the characteristic features of other vertebrae, with no transverse foramina, costal facets, or bifid spinous processes.
However, like the cervical vertebrae, they have a triangular-shaped vertebral foramen. Their spinous processes are shorter than those of thoracic vertebrae and do not extend inferiorly below the level of the vertebral body.
Their size and orientation permits needle access to the spinal canal and spinal cord (which would not be possible between thoracic vertebrae). Examples include epidural anaesthesia administration and lumbar puncture.
Sacrum and Coccyx
The sacrum is a collection of five fused vertebrae. It is described as an inverted triangle, with the apex pointing inferiorly. On the lateral walls of the sacrum are facets for articulation with the pelvis at the sacroiliac joints.
The coccyx is a small bone which articulates with the apex of the sacrum. It is recognised by its lack of vertebral arches. Due to the lack of vertebral arches, there is no vertebral canal.
Joints between the articular facets ?
The joints between the articular facets, called facet joints, allow for some gliding motions between the vertebrae. They are strengthened by several ligaments:
Ligamentum flavum – extends between lamina of adjacent vertebrae.
Interspinous and supraspinous – join the spinous processes of adjacent vertebrae. The interspinous ligaments attach between processes, and the supraspinous ligaments attach to the tips.
Intertransverse ligaments – extends between transverse processes.
What is the purpose of anterior longitudinal ligaments ?
They prevent hyper extension of the body
What is the function of the intertranverse ligaments ?
Prevents lateral flex ion
What is the purpose of the ligamentum flavum ?
The ligamentum flavum connect to the laminae of the vertebrae above and below
Allow the spinal column to stretch and return to its original position
What are joints ?
A joint is where two bones (or cartilage) meet, or articulate. Movement occurs at joints, but not all joints are movable. Joints are commonly classified on the basis of the tissue that lies between the articulating surfaces, namely:
-fibrous
- cartilaginous (primary and secondary)
- synovial
Synovial joints themselves can be further categorized according to the shape of their articulating surfaces. Identify/locate an example of each of the following categories of synovial:
Muscle fibre orientation
Muscles come in three different forms: skeletal, smooth and cardiac. Skeletal muscle is typically
under voluntary control unlike cardiac and smooth muscle. Skeletal muscle is usually attached to
bone to facilitate movement, locomotion and posture. These attachments are achieved through
tendons. Tendons have several important properties and functions: they allow muscles to act at a
distance, they allow changes in the line of pull, and they store elastic energy. Tendons are subject
to friction and are therefore protected by several anatomical features
Skeletal muscle is commonly classified on the basis of the arrangement of its muscle fibres in
relation to the line of pull of the muscle.
What is a unipennate muscle ?
- The force produced by pennate muscles is greater than the force produced by parallel muscles
- Unipennate muscles are those where the muscle fibers are oriented at one fiber angle to the force-generating axis and are all on the same side of a tendon.
-The pennation angle in unipennate muscles has been measured at a variety of resting length and typically varies from 0° to 30°
- The lateral gastrocnemius is an example of this muscle architecture.
Bipennate muscle ?
Bipennate
Muscles that have fibers on two sides of a tendon are considered bipennate.
The stapedius in the middle ear of humans, as well as the rectus femoris of the quadriceps are examples of bipennate muscles.
Parallel (Fusiform) muscles ?
- Fusiform muscles are those in which all the muscle belly fibers are arranged parallel to each other. An example of the fusiform muscle is m. biceps brachi
Parallel (non - fusiform) muscles ?
Parallel muscles can be divided into fusiform and non-fusiform types based on their shape. Fusiform muscles are more spindle shaped (their diameter at the center is greater than at either end), whereas, non-fusiform muscles are more rectangular with a constant diameter.
What is a convergent muscle ?
-These occur where the base is much wider than the insertion, giving the muscle a triangular shape and enables the muscle to contract with great force. An example of this type of muscle is the deltoid (shoulder).
- The large muscle on the chest, the pectoralis major, is an example of a convergent muscle because it converges on the greater tubercle of the humerus via a tendon.
- The temporalis muscle of the cranium is another.
Circular muscle ?
- orbicularis oris
- obicularis oculi
Muscles of the back ?
The muscles of the back may be divided into those that act primarily on the vertebral column and back (intrinsic) and those whose primary action is on the pectoral girdle and
upper limb (extrinsic).
The anatomy of the extrinsic muscles of the back will be covered together with that of the upper limb in the second year. The intrinsic muscles (also referred to as the paraspinal muscles) mainly function to maintain posture and are organised into superficial, intermediate and deep layers.
The superficial layer is associated with the head and neck, the intermediate with the thorax and the deep with the movements of the vertebral column. There is no need to identify or memorise each individual muscle.
The intermediate and deep layers are more commonly referred to as the erector spinae (intermediate) and transversospinalis (deep) groups.
-Iliocostalis
-Longissimus
- Spinalis
Spinalis
Dermatomes of the lower limb ?
-L1: the inguinal region and the very top of the medial thigh.
-L2: the middle and lateral aspect of the anterior thigh.
-L3: the medial epicondyle of the femur.
-L4: the medial malleolus.
-L5: the dorsum of the foot at the third metatarsophalangeal joint.
-S1: the lateral aspect of the calcaneus.
-S2: at the midpoint of the popliteal fossa.
-S3: at the horizontal gluteal crease (the horizontal crease formed by the inferior aspect of the buttocks and the posterior upper thigh).
-S4/5: the perianal area.
What is a myotome ?
A myotome is a group of muscles innervated by a single spinal nerve
Grey and white matter ?
Grey matter:
-In terms of tissue, the CNS is divided into grey matter and white matter. Grey matter comprises neuron cell bodies and their dendrites, glial cells, and capillaries.
- in the brain the grey matter is found in the outer layers
- in the spinal cord the grey matter forms the core ‘butterfly’ shape.
White matter:
- White matter refers to the areas of the CNS which host the majority of axons, the long cords that extend from neurons. Most axons are coated in myelin - a white, fatty insulating cover that helps nerve signals travel quickly and reliably.
- In the brain, white matter is buried under the grey surface, carrying signals across different parts of the brain.
- In the spinal cord, white matter is the external layer surrounding the grey core.
Longitudinal ligament ?
The posterior longitudinal ligament is broad where it is firmly attached to the intervertebral discs but narrow and less for,oh attached to the veterinary bodies.
The anterior longitudinal ligament is uniformly broad and firmly attached to the discs and vertebral bodies
Dorsal rootlets of the spinal nerve
Zygaphophyseal joints ?
The joints of the vertebral arches are plane synovial joints known as the zygapophyseal joints, often called facet joints. These joints are formed between the superior and the inferior articular processes (zygapophyses) of adjoining vertebrae.
In cadavers the vertebral bodies are dark and the intervertebral discs are light
What are spinal roots ?
Spinal roots come in pairs—one of each pair on each side of the body. There are 31 pairs:
There are 8 pairs of sensory nerve roots for the 7 cervical vertebrae.
Each of the 12 thoracic, 5 lumbar, and 5 sacral vertebrae has one pair of spinal nerve roots.
In addition, at the end of the spinal cord, there is a pair of coccygeal nerve roots, which supply a small area of the skin around the tailbone (coccyx)
There are dermatomes for each of these nerve roots.
Sensory information from a specific dermatome is carried by sensory nerve fibers to the spinal nerve root of a specific vertebra. For example, sensory information from a strip of skin along the back of the thigh, is carried by sensory nerve fibers to the 2nd sacral vertebra (S2) nerve root.
