20. Sensory Systems Flashcards
Describe the basics of sensory perception.
Define somatoensory.
- Relating to or denoting a sensation (such as pressure, or warmth) which can occur anywhere in the body, in contrast to one localized at a sense organ (such as sight, balance, or taste).
- Pain is usually considered an extreme form of somatosensory stimulation, and it is often considered separately from somatosensory systems.
What are the main modalities encompassed by somatosensory systems and what nerve fibres are they carried by?
[IMPORTANT]
- Proprioception (Aα) -> Muscle stretching, tension
- Touch (Aβ) -> Pressure, vibration
- Thermal sensation (Aδ and C)
- Itch (C)
Pain is also modulated by Aδ and C fibres
For all of the types of sensory fibres involved in somatosensory and mechanoreception, state:
- Names
- Myelination
- Diameter
- Speed
- Function
Aα:
- Myelinated
- 12-20μm
- 70-120m/s
- Proprioception of skeletal muscle
Aβ:
- Myelinated
- 5-12μm
- 30-70m/s
- Mechanoreceptors of skin
Aδ:
- Myelinated
- 1-5μm
- 5-30m/s
- Pain, temperature
C:
- Unmyelinated
- 0.2-1.5μm
- 0.5-2m/s
- Pain, temperature, itch
Explain how the names of sensory fibre types varies depending on their origin.
- If the fibres originate from the skin, they are the Aα, Aβ, Aδ and C fibres
- If the fibres originate from the muscle, they are instead called group I, II, III and IV respectively
These are just differences in nomeclature -> They refer to the same thing.
Do somatosensory systems respond to change or rate of change?
They detect change, which is a binary event, and then look at the rate of change (the rate of firing of action potentials).
How do receptors allow for graded response to a touch stimulus, for example?
- Force applied determines the number of channels opening, which determines the depolarisation
- Increased depolarisation leads to increased action potential firing per second (i.e. increased rate of change)
- This increases the likelihood of onward signal propagation via the spinal cord and up to conscious perception
Explain the principle of different sensitivity of different sensory receptors.
Some receptors must be more sensitive to stimuli than others. For example, nociceptors cannot be activated by very light stimuli, since this would cause unnecessary pain.
Describe the importance of diversity within somatosensory receptors.
- Not only do we require different receptors for different modalities (e.g. touch and heat), but we also require diversity within each receptor type
- This allows for more accurate perception of the stimulus
What is the receptive field?
The area of skin (or other organ) on which a somatosensory receptor can detect stimuli.
i.e. It is like the range of the receptor
Name some ways in which receptors can allow for diverse responses within, for example, mechanoception.
- Different receptive fields
- Different thresholds
- Adaptation vs non-adaptation
Describe the concept of receptor adaptation.
- In the presence of a continued stimulus, an adaptive receptor produces action potentials with decreasing frequency
- A non-adaptive receptor produces action potentials with continued frequency
Describe the connective tissues within a spinal nerve.
Where do afferent neurons originating from somatosensory receptors have their cell bodies?
In dorsal root ganglia (DRG).
Where is pain from the viscera referred to?
The midline, in areas that are not sharply defined.
Explain the concept of 2 point discrimination.
[EXTRA]
- Sensory receptors are not evenly distributed across the body
- Some locations have a higher density than others
- This means that in places with a high density of receptors, you can distinguish touch between two points a shorter distance apart
Compare the receptive fields of superficial and deep receptors in the skin.
Superficial receptors have a smaller receptive field than deep receptors.
How can increased sensitivity of receptors be achieved?
Convergence of receptive fields:
- Multiple individual receptors clustered into small hotspot can send signals to a single DRG afferent
- This means that the sensitivity is increased, since there is summation
- An example of this is with Meissner’s corpuscle mechanoreceptors
What is proprioception?
A sense of the body’s position in space, essential for locomotion and balance.
What fibres carry proprioceptive information?
Aα fibres (and to a lesser extent Aβ)
What are the two proprioprioceptors?
- Muscle spindles
- Golgi tendon organs
What are muscle spindles and Golgi tendon organs?
They are stretch receptors that detect tension in muscles.
Compare the position muscle spindles and Golgi tendon organs.
Muscle spindles lie in parallel to muscle fibres, whereas Golgi tendon organs lie in series.
What important process are muscle spindles involved in? How does this work?
- The muscle stretch reflex (muscle contraction in response to stretching within the muscle).
- This involves activation of synergistic muscles and inhibition of antagonistic muscles (vi inhibitory interneurons)
(Note: There will be more flashcards on this later).
What fibres types do muscle spindles and Golgi tendons send information via?
- Muscle spindles -> Type Ia (Aα) and II (Aβ)
- Golgi apparatus -> Type Ib (Aα)
(Remember how the I system corresponds to the Aα when in muscle. Aα fibres include both Ia and Ib fibres.)
Therefore, note how Aβ fibres are also minorly involved in proprioception despite not always being mentioned.
What are the two fibre types that take afferent information from muscle spindles? What is the function of each?
- Ia (Aα) -> Adapting, fast-responding discharge, which allows immediate reflex response to changes in length
- II (Aβ) -> Non-adapting, slower discharge, which allows constant tension depending on current stretch
Ia fibres arise from dynamic and static fibres, while II fibres arise from static fibres only.
What are the targets of fibres carrying information from muscle spindles?
- Synergistic α-motorneurons
- Ia inhibitory interneurons that synapse onto antagonistic α-motor neurons
How do Aα neurons enable rapid reflexes?
- They synapse not only on motor neurons that innervate the muscle that needs to contract, but also on interneurons that are inhibitory to the antagonistic muscle.
- Since Aα fibres are responsible for propriception, when there is stretch of the muscle, the agonist will contract slightly, while the antagonist will relax.
What important process are Golgi tendon organs involved in? How does this work?
- Negative feedback during muscle contraction
- This is done via inhibitory interneurons that prevent the muscle from over-contracting
What are the fibre types that take afferent information from Golgi tendon organs?
Ib (Aα) fibres
What are the targets of fibres carrying information from Golgi tendon organs?
Ib inhibitory interneurons in the ventral horn -> These then inhibit synergistic α-motor neuron activity, allowing negative feedback of muscle contraction.
What is another name for touch?
Somatosensation
What are the main receptor types in the skin?
- Meissner corpuscle
- Ruffini corpuscle
- Merkel cells
- Pacinian corpuscle
- Free nerve endings
These are essentially the major mechanoreceptors.
Give a summary of the main receptor types in the skin, in terms of:
- Stimuli detected
- Location in skin
- Afferent response
- Stimulus shape
- Receptive field
- Perceptual functions
What is glabrous skin and how are the mechanoreceptors in it distributed?
- Skin that does not contain hair follicles, such as that over the palms and soles.
- On the ridges of your fingerprint, there are Merkel cells and Meissner corpuscles
- This means that when you run your finger along a surface, the ridges vibrate and the stimulus can be detected
What fibre types carry afferent information from mechanoreceptors?
Aβ
(And also some Aδ fibres from free nerve endings)
Compare the receptive field for:
- Meissner’s corpuscles
- Merkel cells
- Pacinian corpuscles
- Ruffini endings
- Meissner’s corpuscles and Merkel cells have a small receptive field since they are quite superficial
- Pacinian corpuscles and Ruffini endings have a larger receptive field since they are deeper
Compare whether these are rapidly or slow-adapting:
- Meissner’s corpuscles
- Merkel cells
- Pacinian corpuscles
- Ruffini endings
- Meissner’s corpuscles -> Rapidly-adapting
- Merkel cells -> Slow-adapting
- Pacinian corpuscles -> Fast-adapting
- Ruffini endings -> Slow-adapting
Describe how mechanoreceptor diversity allows a range of responses to touch.
- There are two more superficial receptors (Meissner’s and Merkel), with a smaller receptive field, and two more deep receptors (Pacinian and Ruffini), with a large receptive field
- Of each category, one of the receptors is rapidly-adapting (Meissner’s and Pacinian), while the other is slow-adapting (Merkel and Ruffini)
- This means that a range of responses is possible
What are the different types of ion channels in the membrane of mechanoreceptors?
[EXTRA?]
- 1 and 2 are opened by physical distortion
- 3 is opened by an indirect process
For a Meissner’s corpuscle, state:
- Function
- Location
- Receptive field
- Rapidly/Slow adapting
- Number of receptors per DRG cell
- Function -> Light touch
- Location -> Superficial epidermis, High density in fingertips
- Receptive field -> Small
- Rapidly/Slow adapting -> Rapidly adapting
- Number of receptors per DRG cell -> 10-25
Draw the structure of a Meissner’s corpuscle.
What makes a Meissner’s corpuscle special?
- It has the lowest highest sensitivity to low frequency vibrations.
- This is due to a low threshold and 10-25 Meissner’s corpuscles per DRG cell.
For a Merkel disc, state:
- Function
- Location
- Receptive field
- Rapidly/Slow adapting
- Number of receptors per DRG cell
- Function -> Deep touch, Pressure
- Location -> Superficial epidermis, High density in fingertips
- Receptive field -> Small
- Rapidly/Slow adapting -> Slowly adapting
- Number of receptors per DRG cell -> 10-25
Give some experimental evidence relating to the function of Merkel discs.
(Maricich, 2009):
- Identified that a gene involved in Merkel disc development is Atoh1
- Atoh1 knockout mice allow us to see the importance of Merkel discs
For a Pacinian corpuscle, state:
- Function
- Location
- Receptive field
- Rapidly/Slow adapting
- Number of receptors per DRG cell
- Function -> Vibration
- Location -> Deep in the dermis, High density in finger tips
- Receptive field -> Large
- Rapidly/Slow adapting -> Rapidly adapting
- Number of receptors per DRG cell -> 1
Draw the structure of a Pacinian corpuscle.
Give some experimental evidence relating to the structure and function of the Pacinian corpuscle.
[EXTRA]
(Lowenstein, 1959):
- Experiment 1 showed that the lamellae are not required, just the nerve ending inside
- Experiment 2 showed that interruption of the action potential transmission causes no response, but does not interrupt the generator voltage
- Experiment 3 showed that partial degeneration of the nerve ending prevents the generator voltage
Therefore, the mechanoreceptive component is the nerve ending within the Pacinian corpuscle.
Give some experimental evidence relating to the function of the lamellae in a Pacinian corpuscle.
This showed that the lamellae are important for rapid adaptation, so that the receptor can respond to a high frequency of signal.
For a Ruffini ending, state:
- Function
- Location
- Receptive field
- Rapidly/Slow adapting
- Number of receptors per DRG cell
- Function -> Stretch (involved in hand position)
- Location -> Deep dermis, Fewer in fingers
- Receptive field -> Large
- Rapidly/Slow adapting -> Slow adapting
- Number of receptors per DRG cell -> 1
What receptor type is this?
Pacinian corpuscle
What receptor type is this?
Merkel disc
What receptor type is this?
Ruffini ending
What receptor type is this?
Meissner’s corpuscle
Describe the arrangement of Ruffini corpuscles.
They are arranged along tissue planes on the hand.
What are thermoreceptors?
[IMPORTANT]
Free nerve endings
What are the proteins on free nerve endings that allow perception of heat?
Transient Receptor Potential (TRP) family of proteins often located on free nerve endings (Aδ and C fibres) in mammals.
What are the two types of thermreceptors?
- Cold fibres
- Warm fibres
These have different firing rates at different temperatures. They both fire equally at around 37 degrees (the desired body temperature).
What is the diameter of cold and warm thermoreceptors?
- Cold -> 5-10mm
- Warm -> 15mm
Name some different TRP proteins on free nerve endings and what they respond to.
Describe how spicy foods cause a strong response via TRP receptors.
- Spicy foods (and heat) open TRPV1, giving it slight permeability to cations
- However, prolonged exposure to spicy foods or heat causes phosphorylation, which induces a transition to a dilated state, in which permeability to large cations is increased
What TRP protein is responsible for extreme hot thermoreception?
TRPV2 -> It responds to stimuli over 52*C. Unlike the capsaicin receptor TRPV1, TRPV2 has no response to capsaicin or acid.
What TRP protein is responsible for extreme cold thermoreception?
TRPM8
What are some agonists of hot and cold thermoceptors?
- Cold (TRPM8) agonists -> Menthol, Eucalyptol
- Hot (TRPV1) agonists -> Capsaicin
Give some experimental evidence for the importance of the TRPM8 receptor in cold thermoreception.
(Bautista, 2007):
- Showed that mutant TRPM8 mice have difficulty differentiating between a hot and cold space
- However, below around 10*C, they can detect a cold space, which shows that there is likely to be another thermoreceptive protein for low temperatures
What ultimately determines the function of any given free nerve ending?
The receptors embedded in the membrane.
Describe what stimuli can activate free nerve endings and what fibre type each of these travel down.
- Thermoreceptive-> Aδ and C fibres
- Mechanoreceptive -> Aδ fibres
- Specific nociceptive -> Aδ and C fibres
- Polymodal nociceptive -> C fibres
Compare the sort of pain that Aδ and C fibres mediate.
- Aδ -> Sharper pain with reflex withdrawal, Fast-adapting (coding change)
- C -> Spreading pain, Slow-adapting
What are some chemicals that can act on nociceptors?
(More on this later, hopefully)
What is nociception?
- It is pain
- It is essentially the perception of extreme thermal, chemical and mechnical stimuli that could be damaging
What are the main types of nociceptor?
- Mechanical nociceptors -> Respond to strong stimuli and mediate sharp pain
- Thermal nociceptors -> Respond to cold and warm stimuli
- Polymodal nociceptors -> Respond to a variety of stimuli and evoke slow burning pain
What are the functions of the spinal cord?
- Receives sensory information from sensory neurons
- Provides motor information to motor neurons
What does the spinal cord receive sensory information from and where does it enter?
From the central axons of dorsal root ganglion neurons -> Into the grey matter of the dorsal horn
On this diagram of the spinal cord, what are the grey and white sections?
- Central H-shaped grey matter -> Neuron cell bodies
- White matter -> Myelinated axon tracts
There is also the central CSF-filled canal.
Label the GREY MATTER of spinal cord with the main sensory and motor areas.
- In the dorsal half, there are the sensory interneurons
- In the ventral half, there are the:
- Lateral motor column (a.k.a. pool)
- Medial motor column (a.k.a. pool)
- Motor interneurons
Describe how the grey and white matter of the spinal cord work together.
Horn (grey matter):
- The dorsal horn receives sensory information
- This can then be taken within the grey matter along a reflex arc or through segmental interneurons
- Alternatively, it can be passed into the white matter
White matter:
- The white matter is made of axons and it is divided into tracts that carry information up the spinal cord and back down from the spinal cord
Draw the position of the main ascending and descending white matter columns.
Ascending (sensory):
- Dorsal column
- Spinocerebellar tracts
- Anterolateral system
Descending (motor):
- Corticospinal tracts (lateral + anterior)
[YOU NEED TO KNOW THIS IS MUCH MORE DETAIL. MAKE SURE YOU’VE MADE MORE NOTES ON IT.]
Draw the main nuclei and laminae in the grey matter of the spinal cord. [IMPORTANT]
Dorsal horn:
- Marginal zone
- Substantia gelatinosa
- Nucleus proprius
Lateral horn (in thoracic region?):
- Thoracic nucleus (Clarke’s column)
- Intermediolateral nucleus
Ventral horn:
- Lateral motor pool
- Medial motor pool
What sort of sensory fibres enter the dorsal horn of the spinal cord? From what direction? What is their function?
Large diameter axons:
- Enter medially and possess ascending collaterals
- Aα -> Proprioception
- Aβ -> Mechanoceptors
Fine diameter axons:
- Enter laterally and project across several segments -> Lissauer’s tract (LT)
- Aδ -> Nociception, Thermal
- C -> Nociception, Thermal, Itch
For Aα fibres, describe where they originate and terminate.
- Originate in muscle spindles, from where they carry sensory proprioception information
- Terminate at lamina 6 to 9, which includes interneurons in the dorsal horn and motor neurons in the ventral horn
For Aβ fibres, describe where they originate and terminate.
- Originate from cutaneous receptors and static proprioceptors
- Terminate in the laminae 3 to 6 of the dorsal horn (a.k.a. the nucleus proprius)
- They synapse onto wide dynamic range neurons (interneurons involved in polysynaptic reflexes)
Where do Aδ and C fibres (nociceptive fibres) synapse?
- Aδ -> At the marginal zone of the dorsal horn (lamina 1) and nucleus proprius (laminae 3-5)
- C -> At the substantia gelatinosa of the dorsal horn (lamina 2)
Note: Each of these fibres can synapse at multiple heights in the spinal cord.
Compare the direction from which proprioceptive/mechanoreceptive (Aα and Aβ) and nociceptive (Aδ and C) enter the spinal cord.
- Proprioceptive/mechanoreceptive (Aα and Aβ) -> From medial side
- Nociceptive (Aδ and C) -> From lateral side
Which laminae do each of these fibres synapse at:
- Aα
- Aβ
- Aδ
- C
- Aα -> 6-9
- Aβ -> 3-6
- Aδ -> 1, 4 and 5
- C -> 2
Do large diameter afferent fibres (Aα and Aβ) supply just one point along the spinal cord?
- No, they can enter and course up and down the spinal cord, innervating more than one motor/interneuron. However, all the motor neurons innervated tend to have synergistic action.
- They do this via the ascending tracts
Where do Aα neurons synapse?
- On interneurons that are inhibitory to antagonistic muscles (in the dorsal horn)
- On motor neurons in the ventral horn (for stretch reflex)
They also send out branches without synapsing which ascend in the ascending dorsal columns to inform higher centres (conscious proprioception).
Draw a diagram to show where the collateral branches of large diameter sensory fibres originate. What do these form?
- These form the dorsal ascending tracts that carry proprioceptive and mechanoreceptive information up to the brain.
- This allows for conscious perception of body position and touch.
Describe transmission through the dorsal column tracts. [IMPORTANT]
It carries afferent proprioceptive and mechanoreceptive information:
- Dorsal root ganglion axons have ascending branches that travel cranially in the dorsal column
- These then synapse at the dorsal column nuclei in the medulla
- The fibres then decussate (cross to the other side of the body)
- They then travel up the medial lemniscus through the midbrain and to the thalamus
- Relay neurons take the information from the thalamus to the somatic sensory cortex
Explain the concept of orders of neurons.
- First-order neurons carry signals from the periphery to the spinal cord
- Second-order neurons carry signals from the spinal cord to the thalamus
- Third-order neurons carry signals from the thalamus to the primary sensory cortex
Compare the orders of neurons carried by the dorsal column and anterolateral systems.
- Dorsal column system carries 1st order neurons, since synapsing and decussation only happens in the medulla
- Anterolateral system carries 2nd order neurons, since synapsing and decussation occur within the spinal cord
What are the two fascicles of the dorsal column?
These correspond to the cuneate and gracile nuclei in the medulla, which these join up with.
Describe transmission through the spino-cerebellar tracts. [IMPORTANT]
It carries afferent proprioceptive information from the lower limb:
- Dorsal root ganglion axons at the lumbar region, which receive information from the lower limb, have branches that ascend up to the thoracic region
- At the thoracic region, they terminate at Clarke’s column (a.k.a. thoracic nucleus)
- This sends out relay neurons to the dorsal spino-cerebellar tracts
- These take the information to the cerebellum on the same side of the body -> This is important for unconscious perception and response to the stimulus
Give a summary of proprioception and mechanoception. [IMPORTANT]
- Proprioception is mediated by Aα fibres (from muscle spindles), while mechanoception is mediated by Aβ fibres (from mechanoreceptors)
- These fibres then do two things:
- Take the information directly to the grey matter of the spinal cord to allow reflex response
- Aα fibres synapse at laminae 6 to 9, in both the dorsal and ventral horns
- They synapse with motor neurons in the ventral horn and interneurons in the dorsal horn (which are inhibitory to the antagonistic muscle) -> Allows reflex reactions so muscles to maintain position
- Aβ fibres synpase in laminae 3 to 6 of the dorsal horn (a.k.a. the nucleus proprius)
- These are projection neurons that decussate and enter the anterolateral system (in particular the anterior spinothalamic tract), which take the information to the thalamus (the anterolateral tracts carry CRUDE touch information)
- Send out branches before they reach the grey matter, which travel up to the brain to give conscious and unconscious perception of the stimulus (and also innervate muscles at different spinal levels e.g. if the biceps needs to contract then lots of muscle bundles need to be recruited)
- These form the dorsal column tracts (thes carry proprioception and FINE touch):
- Fibres synapse at the dorsal column nuclei in the medulla
- The fibres then decussate (cross to the other side of the body)
- They then travel up the medial lemniscus (a bundle of fibres) through the midbrain and to the thalamus
- Relay neurons take the information from the thalamus to the somatic sensory cortex -> CONSCIOUS PERCEPTION
- There are also the spino-cerebellar tracts, which carry afferent proprioceptive information from the lower limb:
- Fibres ascend up to the thoracic region
- At the thoracic region, they terminate at Clarke’s column (a.k.a. thoracic nucleus)
- This sends out relay neurons to the dorsal spino-cerebellar tracts
- These take the information to the cerebellum on the same side of the body -> UNCONSCIOUS PERCEPTION
- Note how there is no decussation in this case
- These form the dorsal column tracts (thes carry proprioception and FINE touch):
- Take the information directly to the grey matter of the spinal cord to allow reflex response
Note how crude touch is carried by the anterolateral system (anterior spinothalamic tract), while fine touch is carried by the dorsal column system along with proprioception. Proprioception is also carried to the cerebellum via the spinocerebellar tracts.
What enables nociceptive fibres (Aδ and C) to terminate at more than one height in the spinal cord?
- They are carried by Lissauer’s tract up and down, before they enter the grey matter.
- This allows ampification of the signal, which does not happen with proprioception/mechanoreception since these need to be localised
What is Lissauer’s tract?
An ascending tract to the lateral side of the dorsal horn that takes the axons of nociceptive fibres up to higher spinal levels for synapsing.
Describe what happens after nociceptive fibres (Aδ and C) synapse in the spinal cord.
The sensory fibres travel up the spinal cord via Lissauer’s tract, then they synapse at various levels in the spinal cord.
Aδ:
- Some synapse at the marginal zone of the dorsal horn (lamina 1) and nucleus proprius (laminae 3-5) with projection neurons:
- Projection neurons send an axon across to the contralateral side of the spinal cord
- These axons then join the anterolateral system, which takes the signal to the thalamus
- Relay neurons take the information from the thalamus to the somatic sensory cortex
C:
- Some synapse at the substantia gelatinosa of the dorsal horn (lamina 2) with interneurons:
- These send axons to the nucleus proprius, which contains projection neurons
- Projection neurons send an axon across to the contralateral side of the spinal cord
- These axons then join the anterolateral system, which takes the signal to the thalamus
- Relay neurons take the information from the thalamus to the somatic sensory cortex
How do axons of neurons from the nucleus proprius get to the other side of the body?
They pass via the anterior white commissure.
Describe transmission through the anterolateral column.
It carries afferent nociceptive (and some mechanoreceptive) information:
- Wide dynamic range neurons in the nucleus proprius send axons across the midline (via the anterior white commissure)
- The fibres then travel via the anterior and lateral spinothalamic tracts to the medulla, where the spinal leminiscus
- This carries the information to the thalamus
- Relay neurons take the information from the thalamus to the somatic sensory cortex
Give a summary of nociception. [IMPORTANT]
- Nociception is mediated by Aδ and C fibres
- These fibres then travel up to multiple levels in the spinal cord using Lissauer’s tract (this allows for amplification)
- Each each level, they enter the grey matter from the lateral side and synapse in the grey matter.
- Aδ fibres:
- Synapse at the marginal zone of the dorsal horn (lamina 1) and nucleus proprius (cell bodies in lamina 5 with dendrites in laminae 3-5) with projection neurons:
- Projection neurons arise from laminae 1 and 5, send an axon across to the contralateral side of the spinal cord
- These axons then join the anterolateral system, which takes the signal to the thalamus
- Relay neurons take the information from the thalamus to the somatic sensory cortex
- Synapse at the marginal zone of the dorsal horn (lamina 1) and nucleus proprius (cell bodies in lamina 5 with dendrites in laminae 3-5) with projection neurons:
- C:
- Synapse at the substantia gelatinosa of the dorsal horn (lamina 2) with interneurons:
- These send axons to the nucleus proprius, which contain projection neurons
- Projection neurons send an axon across to the contralateral side of the spinal cord
- These axons then join the anterolateral system, which takes the signal to the thalamus
- Relay neurons take the information from the thalamus to the somatic sensory cortex
- Synapse at the substantia gelatinosa of the dorsal horn (lamina 2) with interneurons:
Compare the dorsal column (proproception and mechanoception) and anterolateral (nociception) systems.
- In the dorsal column, the DRG afferents do not synapse, but send fibres straight up the tract, while in the anterolateral, the DRG afferents synapse and the wide dynamic range neurons send fibres up the tract (i.e. dorsal column contains 1st order axons, while the anterolateral system contains 2ns order axons)
- In the dorsal column, the decussation occurs in the medulla at the dorsal column nuclei, while in the anterolateral decussation occurs at the spinal level where synapsing occurs
Label this spinal cord cross-section.
What is Clarke’s column? [IMPORTANT]
- A bundles of nerve cells in the thoracic region of the spinal cord.
- It is in the LATERAL horn of the grey matter [IMPORTANT]
- It receives ascending fibres from lumbar dorsal root ganglion neurons, and then passes this information on to the dorsal spino-cerebellar tracts (then to the cerebellum)
- Therefore, it plays a role in UNCONSCIOUS perception of proprioceptive information
What are the important zones in the dorsal horn of the grey matter and what laminae do these include? [IMPORTANT]
- Lamina 1 -> Marginal zone (a.k.a. posteromarginal zone)
- Lamina 2 -> Substantia gelatinosa
- Laminae 3-5 -> Nucleus proprius (a.k.a main sensory nucleus)
Compare proprioceptive/mechanoreceptive (Aα and Aβ) and nociceptive (Aδ and C) fibres in terms of where how many heights they synapse.
- Proprioceptive/mechanoreceptive fibres -> These synapse at just one height in the spinal cord (and although they send out collateral fibres, these synapse in the brain, unless it is in Clarke’s column)
- Nociceptive fibres -> These can synapse at multiple heights in the spinal cord, with each axon terminating at more than one dorsal root
Describe the importance of the nucleus proprius.
- The nucleus proprius (laminae 3-5) receives combined input from Aβ (mechanoreceptive) and Aδ fibres (nociceptive)
- The wide dynamic range (WDR) neurons in the nucleus proprius cross the midline (via the anterior white commissure) and ascend in the anterolateral columns.
Where in the brain does sensory information pass to from the spinal cord? How?
- It goes to the thalamus, which relays it to the primary sensory cerebral cortex (all senses except smell).
- Some also goes to brainstem structures, which allows for reflex and unconscious behaviours,
What are the two main ascending spinal pathways you need to know about?
- Dorsal column system
- Anterolateral system
For proprioception, touch, thermoreception and nociception, summarise:
- Receptors
- Fibre types
- Termination in spinal cord
- Ascending pathway
- Higher nuclei
- Conscious perception
What is decussation and why is it important?
- It is the crossing of ascending and descending tracts to the contralateral side of the body.
- Knowing where tracts decussate is important because it allows us to understand the consequences of various lesions.
Do the cerebral and cerebellar hemispheres receive information from the ipsilateral or contralateral side of the body?
- Cerebral hemispheres -> Receive information from the contralateral side (therefore conscious perception is contralateral)
- Cerebellar hemispheres -> Receive information from the ipsilateral side (therefore unconscious perception is ipsilateral)
What is Brown-Séquard syndrome and what symptoms does it present with?
- A lesion of half of the spinal cord at a given level
- It can be caused by gunshot/knife wounds, tumours or spinal disc herniation
Symptoms:
- Loss of proprioception/mechanoreception on the ipsilateral side -> This is because the dorsal column system decussates in the medulla, so they have not decussated yet
- Loss of nociception, thermoreception and crude touch on the contralateral side, at a lower spinal level -> This is because the nociceptive, thermoreception and crude touch information decussates in the spinal cord and then travels up via the anterolateral system. The loss starts at a lower spinal level because Lissauer’s tract carries fibres up to several levels before decussation.
What information do the dorsal column and anterolateral systems carry? Why?
Dorsal column:
- Proprioception and fine touch
- This is due to the ascending collateral fibres (first order) that are Aα and Aβ fibres.
Anterolateral systems:
- Nociception, temperature and crude touch
- Nociception and temperature are due to Aẟ and C fibres that synapse in the nucleus proprius and supply second order fibres that arise in the. Crude touch is due to Aβ fibres that do the same.
Compare the speed of the dorsal column and anterolateral systems.
The dorsal column system is faster and less susceptible to modulation, since there are fewer synapses.
Explain the concept of the anterolateral system.
- Aẟ, C and Aβ fibres synapse in the dorsal horn at various locations (these are covered in earlier flashcards, but don’t worry too much since it is a complicated topic)
- All of these either synapse directly onto projection neurons or use interneurons to converge onto projection neurons.
- Projection neurons cross the midline via the anterior white commisure.
- The main projection neurons arise from laminae 1 (marginal zone) and 5 (part of the nucleus proprius), but there are also some on laminae 6 and 7
- The projection neurons form three main tracts that make up the anterolateral system -> Different tracts arise from different laminae
- The spinothalamic tract is the main tract, going directly to the thalamus for conscious perception of pain, while the other two tracts synapse at lower structures and are involved in altertness and pain-control mechanisms
What are the three tracts that form the anterolateral system? Where does each terminate?
- Spino-reticular
- Spino-mesencephalic
- Spino-thalamic
For the spino-reticular tract (part of the anterolateral system), where does it originate and terminate? What is the role?
- Origin: Medial laminae (6-8) of grey matter
- Terminates: Reticular formation (throughout the brainstem) -> This passes to the thalamus after this
Roles:
- Increases alertness in response to pain -> ‘Reticular activating system’ -> Projects to higher centres and alerts the cerebral cortex about stimuli.
- Increases reflex activity (e.g. in muscles) -> Via influencing the reticulospinal tracts
(Note: You can try and check where these fibres from laminae 6-8 arise from, but it probably doesn’t matter that much. Just assume that interneurons brought information there.)
For the spino-mesencephalic tract, where does it originate and terminate? What is the role?
- Origin: Laminae 1 and 5
- Terminates: The periaqueductal grey matter
Roles:
- Pain-control mechanisms -> Via the periaqueductal grey in the midbrain, which has pathways that come back down the spinal cord
- Unconscious body orientation (e.g. moving the eyes towards the painful stimulus) -> Via the superior collicitus in the midbrain, which influences the tectospinal tracts
For the spino-thalamic tract, where does it originate and terminate? What are the roles?
- Origin: Laminae 1 and 5 (also 6-7)
- Terminates: Thalamus (in the ventral posterior lateral nucleus and central lateral nucleus)
Roles:
- Conscious perception of pain and its location
- Also carries mechanoreceptive information (from Aβ fibres that synapse in the nucleus proprius)
Where do anterolateral projections originate from?
[IMPORTANT]
Main sensory nucleus (a.k.a. nucleus proprius) of the dorsal horn. Also from the marginal zone.
Where do fibres decussate to give rise to the anterolateral system?
[IMPORTANT]
Anterior white commissure
What are the two parts of the spinothalamic tract?
- Anterior spinothalamic tract -> Crude touch and pressure
- Lateral spinothalamic tract -> Pain and temperature
Where do the nociceptive fibres terminate in the brain?
[IMPORTANT]
- Central lateral (CL) nuclei -> a.k.a. Intralaminar nuclei of the thalamus
- Ventral posterior lateral (VPL) nuclei