Somatic sensory systems Flashcards
Sensory systems
In general, what are the 2 ways in which a signal can be transmitted in response to a sensory stimulus?
Tell me about the sensory systems of the senses?
What are the sensory and motor components?
What part of the nervous system are they a part of?
Where does the sensory stimulus that effects them come from?
Where does the signal go to?
Whats a first order neuron?
First order neurons – These are pseudounipolar neurons 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 spinal cord/brainstem).
Tell me about the dorsal root ganglion (DRG) neuron
- Where it enters
- branches and their destination
- where their cell bodies are located
- myelination and diameter
Dorsal root ganglion (DRG) neurons
- Mechanosensory. Enters dorsal root and joins dorsal column
- Pain and temperature fiber makes a connection upon entrance
- Two axon branches depart from the soma.
- One branch runs to the periphery (PNS) and the other to the spinal cord (CNS)
- The cell bodies (somas) of all somatosensory fibres from the body are located in the dorsal root ganglia (DRG).
- Signals from face: somas in trigeminal ganglion.sa
- First order sensory neurons are pseudo unipolar.
- Two axon branches
- Mechanosensory neurons are myelinated and have large diameters
Tell me about mechanosensory neurons
- myelinated
- have large diameters
- touch
- Merkel, meissner, pacinian, ruffini cells
- 6-12 µm
- 35-75 m/s
somatic sensory system organisation
What receptors does the skin contain?
mechanoreceptors, nociceptors and thermoceptors
Tell me about the different compoents of the skin and whether they are located in the epidermis or dermis
The major classes of somatic sensory receptors
Classes of somatosensory afferents
tell me…
- Their sensory function
- Receptor type
- axon type
- diameter
- conduction velocity
The sensory transduction cascade, where the energy of a stimulus is converted into an electrical signal consists of several key steps. What are these key steps?
What is the sensory system stimulus strength encoded by?
The stimulus strength is encoded by the amplitude of the generator potential and the frequency of action potentials
What are the two types of sensory system receptors?
Tell me about eah
Tonic and phasic receptors
Tonic= static
Phasic= dynamic
Tell me the Sensory system receptors subtypes?
- Non encapsulated fibres with touch and injurious force
- Touch first respond but then quiet down
- With force they are persistent and don’t turn off. These fibres also response do temperature. Pain receptor will be continuously on
Tell me the key features of the following sub-types…
- Hair follicles
- Meissner corpuscel
- Pacinian corpuscle
- Merkel cell-neurite complex
- Ruffini corpuscle
- Encapsulated receptors
- Low-threshold (or high-sensitivity)
- Innervated by relatively large myelinated axons (type Aβ)
- Rapid central transmission of tactile information
Tell me the key features and characteristics of the Meissner’s corpuscle…
- Connective tissue capsule.
- Several lamellae of Schwann cells.
- 40% of the sensory innervation of the human hand (glabrous skin).
- Dynamic deformation.
- Movement of textured objects are moved across the skin (~50 Hz).
Tell me the key features and characteristics of the pacinian corpuscle…
- 10–15% of the cutaneous receptors in the hand.
- Present in subcutaneous tissue (bone and gut).
- Onion-like capsule / lamella separated by a fluid-filled space.
- Discrimination of fine surface textures / high-frequency vibration of the skin (250–350 Hz).
- Dynamic qualities of mechanical stimuli
- less abundant than the Meissners corpuscle
How does mechano- transduction work?
- stretch-sensitive cation channels
- Depolarisation of afferent neuron
- If you want to read more on this look at: Delmas et al 2011 Molecular mechanisms of mechanotransduction
What makes up 25% of the mechanoreceptors in the hand (fingertips)
Merkel cell- neurite complex
Tell me about the Merkel-cell neurite complex and the Ruffini complex…
- Static discrimination of shapes, edges, and rough textures. Light pressure.
- Ruffini’s corpuscles, 20% of the receptors in the human hand. also in ligaments and tendons.
What is meant by a receptive field?
The ability to distinguish 2 simultaneously applied stimuli as distinct is called 2-point discrimination and it is a measure of spatial acuity
Whats meant by two-point discrimination?
Two-point discrimination (2PD) is the ability to discern that two nearby objects touching the skin are truly two distinct points, not one. It is often tested with two sharp points during a neurological examination and is assumed to reflect how finely innervated an area of skin is.
Tell me about the conscious and unconscious ascending pathways for somatiosensation?
Unconscious
- Proprioception
- Spinocerebellar tract
Conscious
- Mechanosensation (fine touch, pressure, vibration, proprioception)
- Dorsal column-medial lemniscal pathway
- Pain and temperature
- Spinothalamic tract
Tell me about the two somatic sensory systems when conscious?
Somatic sensory systems (conscious)
- Body (and posterior portion of the head)
- Mechanical stimuli:
Dorsal column medial lemniscal system <– pathway for the mechanical stimuli
- Pain and temperature:
Spinothalamic tract a.k.a. anterolateral system
- Face and anterior portion of the head
Mechanical stimuli:
- Trigeminal somatic sensory system through the principal sensory nucleus of the trigeminal complex
- Pain and temperature: Through the spinal nucleolus of the trigeminal complex
Tell me about the dorsal column lemniscal pathway
The Trigeminal somatic sensory system
Low-threshold mechanoreception in the face mediated by first-order neurons in the trigeminal (cranial nerve V) ganglion
Whats a Dermatome and what is it helpful for?
What does the thalamus contain?
Tell me about shingles and the distribution of rash (Herpes Zoster)
Tell me about cortical maps of sensory surfaces
Tell me about cortical integration and signalling
- Thalamic input predominantly layer IV
- Cortex sends projections in turn to limbic structures such as the amygdala and hippocampus.
- Cortex also sends descending signals (thalamus, brainstem, and spinal cord)
Summary of lecture 16
Summary
- Ascending pathways (first, second, third order neurons) are topographically arranged throughout the system, the amount of cortical and subcortical space allocated to various body parts being proportional to the density of peripheral receptors.
- Complex interaction is the unified perceptual representation of the body and its ongoing interaction with the environment.
Tell me about cortical integration and signalling
What can be used to measure physiological variables?
- sugar levels
- blood pressure
- cardiac function
What do doctors use to measure pain?
Doctors still use this to judge pain as pain is a complex phenomenon
What is the definition of pain according to the international association for the study of pain?
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
What are some aspects of pain that could have an affect on how one feels pain depending on how they are feeling?
Aspects of pain: sensory, affective, autonomic or motor. All these factors can affect how we feel pain and how greatly it has an impact on us
- Sensory
- Affective
- Affective (emotional)
- Secondary affect (impact of chronic pain in emotional life)
- Psychosocial context
- Autonomic
- Motor
How are harmful stimuli detected?
Whats the definition for this?
Nociception: the neural process of encoding noxious stimuli (IASP definition)
Where are Nociceptors found?
Nociceptors lie near blood vessels between epithelial layers of the skin, the cornea, muscle joints, viscera, the alimentary tract, and in connective tissues.
Essentially innervate all tissue except brain!
What are Nociceptors activated by?
What can Nociceptor properties change?
- Activated by stimuli that are damaging or potentially damaging.
- Noxious mechanical, thermal, chemical stimuli.
- Nociceptor properties can change inflammation, damage, pathology.
Tell me about the TRPV1 channels?
- Painful heat >43˚C causes TRPV1 channels to open in C fiber endings. Ca and Na ions enter and depolarize the cell, causing AP
- Ions flow through channels, eg. Ca2+ and Na+ enter depolarising the cell, causing action potentials if threshold is reached
- TPRV is found everywhere more concentrated in the free nerve endings i.e., fingertips, mouth
- TPRV detect temperatures
Action potentials in C-fibres in response to pinch stimuli in anesthetised rate?
Unravelling the mystery of capsaicin: A toll to understand and treat pain
Tell me how action potentials from pain are generated?
- Activation of ion channels in free nerve endings can cause action potentials if sufficient depolarization in the nerve fiber occurs.
- Action potentials in DRG neuron in culture after application of capsaicin Action potentials in DRG neuron in culture after application of capsaicin
More on Nociceptors…
Tell me about Adelta and C-fibres?
- Aδ: mechanosensitive or mechanothermal nociceptors
- C fibres: polymodal nociceptors
How do Nociceptors compare to other sensory fibres?
They have a small diameter and are slowly conducting
Whats a Fascicle?
A bundle of nerve fibres (a few thousand)
Peripheral nerves
How does pain intensity differ between different Nociceptor fibres?
How are harmful stimuli detected?
What types of fibers detect what?
- Transcutaneous recordings using microneurography show that there are specialised nerve fibres that detect noxious stimulation
When the pain signal travels to the brain, where exactly is it travelling to?
- There is not a single “pain centre” or “pain nucleus” in the brain.
- Activity from different brain structures is integrated into a “conscious experience of pain”.
- The discriminative component: pathways that target the traditional somatosensory areas of cortex,
- The affective-motivational component is thought to depend on additional cortical and brainstem pathways. (Paleospinothalamic)
As is the case of the mechanosensory pathway, information about noxious and thermal stimulation of the face follows a separate route to the thalamus
Diagram of the spinothalamic tract which carries the sensory modalities of crude touch, pressure, pain and temperature
Somatic sensory systems (conscious)
The affective-motivational aspect of pain is mediated by what?
Draw out the anterolateral system and how it splits up into the sensory-discrimative and the affective-motivational
Tell me about the Nociceptors, Adelta?
- Small, thinly myelinated.
- 10 % sensory nociceptive fibres.
- Sensations of localised, sharp, pricking pain.
- Mechanical and thermal stimuli.
- Conduct at 5-40 m/sec.
Afferent portion of the reflex arc (quick reactionto noxious stimuli)
- A delta fibers carry pricking/sharp pain
Tell me about the Nociceptors, C fibres?
- Small, unmyelinated fibres.
- 90% of afferent sensory fibres.
- Conduct at 0.5-2.0 m/sec.
- Mechanical thermal, chemical.
- Potential for long-term sensitisation.
- Types of pain associated with c-fibres
- Deep, visceral, dull, diffuse burning, aching pain
Tell me about local anesthetic?
Local anesthetic, also known as nerve block eg. lidocaine, Novocain
- Inhibit sodium channel molecules directly and prevents the generation and propagation of action potentials.
- Nitrous oxide
- Adjunct (manage the cognitive aspect (eg anxiety) diazepam, antidepressants)
Give some examples of painkillers?
- local anesthetics
- Non-steroidal anti-inflammatory e.g. paracetamol
- Opioid analgesics
Tell me about Opioid analgesics
Ketamine (acute management of very painful conditions: e.g., after a car crash to reposition bones)
Information aboutt he Mechano-nociceptor and polymodal nociceptor
what do inflammatory chemicals/ mediators cause an increase in?
Cause an increase in neuronal excitability
Give three examples of common inflammatory mediators
- Prostaglandins
- Bradykinin
- Substance P
Give an example of tissue damage and inflammation?
What happens in this case?
Sunburn
- Nociceptors are bathed in inflammatory mediators –> increase sensitivity of nociceptors, lower the depolarisation threshold
Painkillers: insight into pain modulation
Where does the first level of modulation in the processing of nociceptive signals occur?
What happens in this stage?
Dorsal horn and spinal cord
- C fibers. Substantia gelatinosa layers II and III
- Some C fibers go directly to reticular formation (sleep), or limbic areas of the brain (mood, emotion)
Tell me about first level modulation at the spinal cord (called: segmental control of spinal origin)
- See gate control theory on BB
- ‘A gating mechanism in the spinal cord (substantia gelatinosa of dorsal horn) can be opened or closed in varying degrees thereby modulating incoming signals before they reach the brain’
Tell me about how signals are integrated in the context of other ongoin stimuli
- Touch: segmental controls of non-pain peripheral origin
- Another pain: descending pathways. Diffuse inhibitory controls induced by nociceptive stimuli
- Acute stress: descending controls of from the brain associated with psychological factors
Diffuse inhibitory controls are induced by what?
Nocicptive stimuli (one pain masks another) brain stem origin
Tell me about diffuse inhibitory controls
- Ascending nociceptors make connections within the brainstem, e.g., in the periaqueductal gray matter.
- These brain stem structures can return descending signals to inhibit nociceptors.
- Thus, a first pain would mask a second pain
Tell about the pain killers of Opioid analgestics
- The body has its own analgesic system: it uses endogenous opioids.
- Exogenous opioids (morphine, diamorphine, codeine) are used for the treatment of pain
Descending control from the brain associated with psychological factors
Give an example of an endogenous opioid
Enkephalin
How does inhibition by enkephalin work?
Pre-synaptically
Makes the action potential narrower –> limits neurotransmitter release
Post-synaptically
generates an inhibitory postsynaptic potential driving the cell away from the action potential firing threshold
Tell me some stages of modulation for nociceptive signals
- Segmental controls of non-pain peripheral origin (see also gate control theory).
- Descending pathways
- Diffuse inhibitory controls induced by nociceptive stimuli (one pain masks another). Brain stem origin.
- Descending controls of from the brain associated with psychological factors. Endogenous opioids (production depends on complex psychological phenomena, artificial stimulation of the periaqueductal grey produces analgesia)
What is referred pain and tell me about it?
- Referred pain: pain perceived at a location other than the site of nociception
- Very few projection neurons specialised for transmission of visceral pain.
- Cutaneous and visceral nociceptive afferents converge on projection neurons.
- Activation of heart nociceptors leads to activation of projection neurons that signal for left arm pain.
- Knowledge of the sites where visceral pains are commonly referred to is important in diagnosis.
Whats allodynia?
Pain due to a stimulus that does not normally provoke pain
Whats neuropathic pain?
Pain caused by a lesion or disease of the somatosensory nervous system
Tell me about where the Mechanosensory pathway ascends ?
Mechanosensory pathway ascends ipsilaterally (same side), decussates at lower medulla.
What does the unilateral spinal cord produce sensory loss?
Unilateral spinal lesion will produce sensory loss below the lesion on the same side.
Tell me about the pathways for pain and temperature?
- The pathways for pain and temperature, however, cross the midline to ascend on the opposite side of the cord.
- –> Diminished sensation of pain below the lesion will be observed on the side opposite the mechanosensory loss (and the lesion). This pattern is referred to as a dissociated sensory loss (used in clinical diagnosis)
Further reading
Purves, Neuroscience: https://www.hse.ru/data/2011/06/22/1215686482/Neuroscience.pdf
