5 - Somatosensory System

Question 1

What are the differences between afferent and efferent divisons?

Answer 1

Afferent (sensory): relay of info from peripheral tissues to CNS

Efferent: relay of info from CNS to peripheral tissues to perform an action/effect

Question 2

Function of afferent division

Answer 2

• Responsible for detecting stimuli that act on these tissues (from both internal and external environment)
• Relaying info about the location, intensity and quality of these stimuli to higher centres

Question 3

What is the somatosensory system

Answer 3

Somatosensory system: sensations that arise from tissues other than specialised sense organs (skin, viscera, muscles and joints)
1. Cutaneous sensation: sensations that arise from the skin
2. ******Interoreception:****** sensations arise from viscera, muscles and joints (NOT related to movement)
3. ******Proprioception:****** sensations that arise from skeletal muscle and joints (ARE related to movement)

Question 4

What are the special senses?

Answer 4

Senses that arise from dedicated sensory organs (e.g: eye, ear, nose)

Question 5

What are the basic structural components of the somatosensory system? where are these located in PNS and CNS?

Answer 5

• Peripheral tissues connected to cerebral cortex by 3 neurone pathway (disynaptic pathway)
• Activation of primary sensory neurone by a stimulus produces action potentials that travel through this pathway and reach cerebral cortex where stimulus is percieved

Question 6

Explain transduction

Answer 6

• Mechanism by which stimulation of a peripheral tissue induces action potentials in primary sensory neurone (receptor)
• Stimulus envokes a depolarising graded potential (generator potential) in primary sensory neurone
• If generator potential is large enough to reach threshold, then AP are produced in primary sensory neurone
• Generator potential appears to involve opening of stretch-gated ion channels for mechanical stimuli
  
  • And ligand-gated ion channels for chemical stimuli
• Some temperature-gated ion channels have been identified that appear to be responsible for generator potentials associated with changes in temp.

Question 7

Explain frequency encoding

Answer 7

• Somatosensory system also using frequency coding to communicate the size of peripheral stimuli
• Small stimulus → low frequency response in PSN and vice versa
• Frequency responses are relayed through remaining 2 neurones in the pathway
  
  • Interpreted by cerebral cortex as stimuli of differing intensities

Question 8

Explain receptive field

Answer 8

• ******Receptive field:****** region of tissue that produces action potentials in that neurone
  
  • If you record the membrane potential of a primary sensory neurone and then stimulate the peripheral tissue that it innervates you will eventually locate the region of that tissue that produces action potentials in that neurone.
• The receptive field of a neurone is quite closely related to the extent of the axon terminals of the primary sensory neurones.
• Interestingly however the size of receptive fields varies quite significantly in different parts of the body.

Question 9

Explain innervation density

Answer 9

• The extent to which a particular peripheral tissue is able to detect stimuli that affect it is directly related to the number of neurones that innervate it.
• ******Innervation density:****** The number of neurones that innervate a particular unit area
• The higher the innervation density the more neurones terminate in each cm2of that tissue.
• The innervation density of tissues varies quite significantly. For example there may be over 140 pressure receptors in each cm2of skin at the tips of our fingers and less than 1 receptor per cm2in the skin of the back.

Question 10

Explain adaptation

Answer 10

• ****Adaptation:**** the way in which a PSN responds to a sustained stimulus
• In response to a sustained stimulus (e.g. skin deformation) some neurones show little change in their action potential frequency until the stimulus is removed
  
  • ****Slowly adapting:**** these neurones exhibit very little adaptation
• ******Rapidly adapting:****** some neurones stop responding after a few seconds of a sustained response

Question 11

What are the 7 sensations that arise from skin stimulation?

Answer 11

touch, pressure, pain, temperature, position, movement, and vibration

Question 12

Describe low intensity mechanical stimuli

Answer 12

• Low intensity (non-painful) mechanical stimuli are responsible for 3 distinct sensations:
  
  • Pressure: degree of skin indentation
  • Touch: Rate at which a skin indentation is applied
  • Vibration: frequency of a vibratory stimulus
• These sensations are referred to collectively asmechanoreception.

Question 13

Describe low intensity thermal stimuli

Answer 13

• Cold and warm
• Don’t produce damage to skin
• These senses are known as: ******thermoreception******

Question 14

Describe high intensity (painful) stimuli

Answer 14

• Stimuli with sufficiently high intensity to produce damage to the skin produce pain sensation
• 2 distinct pain sensations
  
  • ****Span sharp:**** initial sharp pain after a mechanical injury
  • ****Burning:**** After the sharp pain, burning pain follows
    
    • Also associated with high temperatures and chemicals (acid)
• ****Nociception:**** pain sensations

Question 15

What sensations fall under mechanoreception?

Answer 15

Pressure, touch and vibration

Question 16

Explain how we sense pressure

Answer 16

• Action potential frequency is directly proportional to the magnitude of the stimulus
• • In these neurones, small indentationsproduce a low frequency of action potentials and progressively larger indentationsproduce a higher frequency response.
• Neurones: ************Slowly adapting mechanoreceptors************
• Slowly adapting mechanoreceptors have specialised receptor endings associated with each of the branches of their axons in the skin.
• There are two major classes of receptor endings associated with slowly adapting mechanoreceptors.
  
  • Merkel’s corpusclesandRuffini’s endingsafter the microscopists who first identified them.

Question 17

Explain how we detect touch sensations

Answer 17

• Slowly applied indentation produces low frequency of action potentials
  
  • Increases linearly as rate of application increases
• These neurones do not continue to respond throughout the skin indentation and so are referred to asrapidly adapting mechanoreceptors.
• Provide little information about duration of skin deformation (because action potentials cease before the stimulus is removed)
  
  • BUT well suited to encode the rate at which a mechanical stimulus is applied.
• Inglabrous(hairless) skin, rapidly adapting mechanoreceptors have specialised receptor endings known asMeissner’s corpuscles.
• In hairy skin, these neurones innervatehair folliclesand are responsible for detecting the rate of hair movement.

Question 18

Explain how we sense vibrations

Answer 18

• Most primates can discriminate between very small differences in vibration frequency from 50-300Hz
• Vibration is in effect a cyclical skin indentation and encoding of this stimulus is enabled by a population ofvery rapidly adapting mechanoreceptors.
• These neurones adapt so quickly that only one AP is produced in response to each skin indentation
  
  • Can distinguish between differentvibration frequencies quite readily.
• VRAM have specialised receptor endings called ********Pacinian corpuscles**********
  
  • 2mm long, 1m wide and consist of concentric layers of connective tissue surrounding the axonal ending

Question 19

What are the 3 major classes of low threshold mechanoreceptors?

Answer 19

Slowly adapting mechanoreceptors: pressure
Rapidly adapting receptors: touch
Very rapidly adapting mechanoreceptors: vibration

Question 20

What makes up the mechanoreception projection pathway?

Answer 20

Primary, second-order and third-order neurones

Question 21

Describe primary sensory neurones of mechanoreception pathway

Answer 21

• The large diameter myelinated axons of low threshold mechanoreceptors course through the peripheral nervous system and enter into the spinal cord through thedorsal roots
• These axons take up a position in the white matter on the dorsal (posterior) aspect of the spinal cord just lateral to the midline. These structures run the length of the spinal cord and are known as thedorsal columns
• The axons of these mechanoreceptors project through the spinal cord in the dorsal columns and synapse with the second-order neurones on the posterior surface of the medulla oblongata in structures known as thedorsal column nuclei

Question 22

Describe second order neurones of mechanoreceptive pathway

Answer 22

• Cell bodies are located in dorsal column nuclei
  
  • Their axons cross the midline at ******medial lemniscus******
• These axons go throughout the brain, and terminate in the ****thalamus -**** contralateral to their cell bodies
• Thalamus: part of ****diencephalon**** and is a large complex collection of sensory nuclei located lateral to 3rd ventricle
• Second order neurones synapse at **ventrobasal complex** (group of nuclei)

Question 23

Describe third-order neurones of mechanoreceptive pathway

Answer 23

• Cell Bodies: Ventrobasal complex of thalamus
• Axons: project up into ******parietal lobe******
• Site of termination: ********post central gyrus of the parietal lobe / Somatosensory cortex********
  
  • Thin strip of cortex immediately behind the central sulcus
  • Where the conscious perception of mechanoreception occurs

Question 24

What are von frey hairs? How do you use these to measure relative innervation density for different skin regions?

Answer 24

IN PRAC BOOK (add)

Question 25

What is the two-point discrimination test? explain how this test can be used to assess innervation density

Answer 25

ADD FROM PRAC BOOK

Question 26

Explain convergence of mechanoreceptive projection pathways

Answer 26

• Mechanoreceptive projection pathways have an extensive ****convergence**** of neurones at each of the synaptic relays
  
  • Convergence is between neurones that have adjacent receptive fields in the periphery
  • Neurones with adjacent receptive fields terminate in adjacent parts of somatosensory cortex.

Question 27

What does this convergence cause?

Answer 27

• within somatosensory cortex there is a precise map (****Homunculus)**** of the body surface
  
  • If you recorded from adjacent neurones in somatosensory cortex they would have adjacent receptive fields on the skin.
• In the homunculus, the representation of different body regions is not in proportion to their actual size.
• The amount of cortex related to a body part is completely determined by theinnervation density of the skin.
  
  • Higher density of mechanoreceptors, the more second and third order neurones → More cerebral cortex is required to interpret info

Question 28

Describe thermoreception and thermoreceptors

Answer 28

• ******Thermoreception:****** the ability to detect small increases or decreases in environmental temperature, by the direct effect that these changes have on our skin
• ****Thermoreceptors:**** temperature-sensitive primary sensory neurones
  
  • Enable cold and warm sensations

Question 29

Describe cold receptors

Answer 29

• Spontaneously active at normal environmental skin temperatures
• Shows a linear increase in AP frequency as skin temperature decreases
• Small diameter myelinated axons
  
  • AP in range of 12-30 m/sec
• Able to reliably encode a decline in skin temperature
• Peripheral endings of these neurones don’t have specialised receptor endings
  
  • Unlike low threshold mechanoreceptors
• Instead have ********free nerve endings******** - axons terminate blindly amongst skin cells

Question 30

Describe warm receptors

Answer 30

• Are also spontaneously active but their AP frequency increases as skin temperature increases.
• Have small diameter unmyelinated axons
  
  • Conduct slow AP (0.5-2.5 m/sec)
• Encode information from increases in skin temperature in non-painful range

Question 31

Describe primary sensory neurones of thermoreception pathway

Answer 31

• The small diameter axons of thermoreceptors project through peripheral nerves and enter into the spinal cord through thedorsal roots
• These axons enter into the grey matter of thedorsal (posterior)horn of the spinal cord where they form axodendritic synapses with the second-order neurones.

Question 32

Describe second order neurones of thermoreception pathway

Answer 32

• Cell bodies: dorsal (posterior) horn of the grey matter of the spinal cord
• Axons: Project down and across midline, underneath ******central canal******, and enter the white matter on contralateral side of spinal cord **********(ventrolateral funiculus, anatomically************
• Physiologically these neurones form part of a projection pathway known as thespinothalamic tract
• Because the axon terminals of these second-order neurones project out of the spinal cord through thebrainstemand terminate with theventrobasal complexof thethalamuswhere they synapse with the third-order neurones.**********)************

Question 33

Describe third-order neurones of thermoreception pathway

Answer 33

• Cell bodies: ventrobasal complex of the thalamus
• Axons: project up into parietal lobe
• Termination: postcentral gyrus (somatosensory cortex)
  
  • Conscious perception occurs here

Question 34

What are the 2 types of pain sensation?

Answer 34

Burning and sharp

Question 35

Describe high threshold mechanoreceptors

Answer 35

• Activated by high intensity mechanical stimuli (pin prick, laceration, pinch)
• Not activated by high temp or pain-producing chemicals
• Have ********free nerve endings********** (no specialised receptors)
• Small diameter myelinated axons
  
  • Conduct AP in range of 12-30 m/sec
• Responsible for ****fast-sharp pain****

Question 36

Describe polymodal nociceptors

Answer 36

• Activated by high intensity thermal, chemical and mechanical stimuli
  
  • Temps above 45 degrees
  • Pain-producing chemicals (acid, histamine, bradykinin…)
• ******Small diameter unmyelinated axons******
  
  • Conduction velocity: 0.5-2.5 m/sec
• ********Free nerve endings********** in periphery
• Responsible for ****burning pain**** associated with thermal or chemical injury
• Also activated by high intensity mechanical stimuli but have conduction velocities that are much slower than high threshold mechanoreceptors
  
  • Reaches CNS slower
• Responsible for ******slow-burning pain****** associated with mechanical injury

Question 37

Describe the nociception projection pathway

Answer 37

nociceptors -> second order neurones -> third-order neurones

Question 38

Describe nociceptors in nociception projection pathway

Answer 38

• The small diameter axons of high threshold mechanoreceptors and polymodal nociceptors enter into the spinal cord through the dorsal roots.
• These axons enter into the grey matter of thedorsal(posterior)hornof the spinal cord where they form axodendritic synapses with the second-order neurones.

Question 39

Describe second-order neurones in nociception projection pathway

Answer 39

• Cell bodies: dorsal horn of the grey matter of the spinal cord
• Axons: project down and across midline underneath the central canal
  
  • Enter white matter on the contralateral **********ventrolateral funiculus**********
• Together with the neurones of the thermoreceptive pathway these neurones form thespinothalamic tract
• Axon terminals: Project out of the spinal cord and terminate with the ********ventrobasal complex********** of the thalamus where they synapse with third-order neurones

Question 40

Describe third-order neurones in nociception pathway

Answer 40

• Cell bodies: ventrobasal complex of the thalamus
• Axons: project up into ******parietal lobe******
• Termination: terminate within postcentral gyrus (somatosensory cortex)
  
  • Where the conscious perception of either sharp or burning pain occurs.