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Neurology > Somatosensation I > Flashcards

Somatosensation I Flashcards

1
Q

What is the role of the somatosensory system?

A

The somatosensory system conveys sensations from the body:

  • Touch (pressure, texture, etc.)
  • Proprioception - one’s sense of body position
  • Heat, cold
  • Pain, itch
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2
Q

How is the CNS connected to the somatic system?

A 
The CNS (brain and spinal cord) is connected to the body via spinal (31 pairs) and cranial nerves
There are 7 cervical vertebrae, and the spinal nerve emerges between them = 8 cervical spinal nerves
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3
Q

What is a nerve?

A

Nerve is a bundle of axons carrying motor and sensory neurons of the PNS

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4
Q

Describe the structure of a nerve

A

The entire nerve is encased in a connective tissue called epineurium

Fascicles are contained within the perineurium of each nerve

Each fascicle consists of individual axons - myelinated axons are encased within individual endoneurium

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5
Q

What are the different types of connective tissue found in a nerve?

A

Connective tissue elements

  • Epineurium
  • Perineurium
  • Endoneurium
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6
Q

How does the structure of a nerve change along its route?

A

As the nerve travels along the periphery it branches and so some of the fascicles combine to give finer branches

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7
Q

Where do nerve axons terminate?

A

The axons terminate within the somatic system in skin, muscles or joints

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8
Q

Where do spinal nerves emerge from?

A

Spinal nerves emerge from each segment of the spinal cord

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9
Q

Which important structures are innervated by spinal nerves?

A

The brachial and cervical plexus’ are formed from the 8 cervical nerves and top thoracic nerves

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10
Q

What are the defining regions of the spinal cord?

A

The front portion (ventral) consists of the vertical discs and the back is the dorsal side

The spinal cord gives rise to dorsal and ventral roots which arise from every spinal segment forming the series of 31 pairs of spinal nerves

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11
Q

What are the roles of the 2 halves of the spinal cord?

A

The spinal cord is roughly divided into 2:

  • Incoming sensory information in dorsal half
  • Output motor commands in ventral half
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12
Q

Which of the spinal roots is the spinal nerve composed of?

A

The dorsal and ventral roots unite to form the spinal nerve - rami branch and coalesce in complex segments to give rise to the brachial plexus and the cervical plexus (and lumbar plexus of leg)

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13
Q

Where do the autonomic fibres of the spinal cord lead to?

A

Autonomic fibres are also present - some are autonomic efferent fibres leaving via dorsal roots and then via the rami to become preganglionic fibres of the autonomic system `

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14
Q

What is the spinal cord comprised of?

A

The cross section of the spinal cord consists of white matter (ascending / descending axon tracts) and grey matter (cell bodies, synapses etc.)

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15
Q

What is the role of the dorsal horns of the spinal cord?

A

Dorsal horns of the spinal cord is the site of incoming sensory axons

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16
Q

What is the function of the ventral roots of the spinal cord?

A

The Ventral horn gives rise to motor neurons sending outgoing axons

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17
Q

How many dorsal root ganglion are there?

A

There is a pair of dorsal root ganglia for each spinal segmental nerve (31)

These occur as the dorsal and ventral roots fuse located in the intervertebral foramen

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18
Q

What are the dorsal root ganglion?

A

These are the sensory receptors of the somatosensory system

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19
Q

Describe the structure of dorsal root ganglion

A

Pseudounipolar neurons - single process from cell body giving rise to an axon in both directions
The peripheral section terminal of the axon will terminate in the sensory target of the neuron

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20
Q

Outline the route travelled by an axon tract terminating in the skin of a finger

A

Axon terminals in the skin of the finger →
Axon travels centrally back via the dorsal ganglion where it continues and enters the spinal cord via the dorsal root and the axon may terminate in the section of the spinal cord or ascend to the medulla

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21
Q

Explain why dorsal root ganglion are so long

A

These are very long axons e.g. an axon innervating the skin of a toe:
- An axon would travel a meter in the periphery towards
the toe
- The sensory terminals would be in the skin of the toe
- The central portion would travel along a leg nerve
- The cell body would be located in a dorsal root ganglion
- Would enter the spinal cord via a lumbar segment via the
dorsal root
- As the axon enters the dorsal horn of the spinal cord it
would ascend multiple spinal cord segments towards
medulla

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22
Q

What are the 2 broad, anatomical and functional systems of dorsal root ganglion?

A
  • Large fibres

- Small fibres

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23
Q

Outline the structure and functions of large diameter fibres

A

Large fibres (large diameter, myelinated, fast conduction): tactile and proprioceptive (Red) - enter from all over body and ascend to the medulla → second order neurons

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24
Q

Describe the structure of small fibres

A

Small fibres are small diameter, thinly-myelinated or unmyelinated, medium or slow conducting fibres

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25
Q

What is the function of small fibres?

A

Respond to temperature, pain, itch, crude touch (Blue) central portion terminates in the dorsal horn of the spinal cord → second order neurons decussate and ascend in a separate white matter tract

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26
Q

Which somatosensory info is carried via the dorsal spinal cords?

A

Ascending tracts from the dorsal spinal cords carry tactile and proprioceptive info

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27
Q

What sensory info is carried by the ventral spinal cords?

A

Ascending axon tracts from the anterior lateral white matter of the spinal cord carry pain etc.

28
Q

What determines the quality of sensation?

A

Quality of sensation depends on afferent fibre type: specificity

29
Q

What determines the specificity of sensation?

A

Specificity of sensation depends upon the type of afferent (e.g. either large → tactile /small → crude touch

30
Q

What are the 4 major classes of receptors of the larger diameter afferents?

A

Superficial cutaneous:

  • Meissner Corpuscles
  • Merkel’s disks

Deeper classes:

  • Pacinaian Corpuscles
  • Ruffini’s corpuscles
31
Q

Where are the cutaneous receptors sensory endings located?

A

These sensory endings are at border between dermis and epidermis, innervate between dermal papillae

These are tactile afferents, sensitive to minute displacements of the skin (light touch)

32
Q

Where in the skin are the deeper classes of large diameter afferent receptors located?

A

These are located deeper in the dermis or between the dermis & hypodermis

33
Q

Describe the sensory nerve endings of the 4 classes of receptors

A

The sensory endings are the axon terminals of the relevant dorsal root ganglion neurons

34
Q

What is the role of the sensory nerve endings?

A

These contain mechanically sensitive ion channels which open allowing a current of Na+/K+ ⇒ net effect depolarisation ; acts as a receptor potential relating to the amplitude of the stimulus

35
Q

How does the corpuscle structure aid their function?

A

The 4 classes of receptors respond slightly differently

The corpuscles structure filters between mechanical stimulation of the neural membrane and mechanical pressure

36
Q

What is meant by slow adapting receptors?

A

Slow adaptation means that the receptor is slow to give a response to a stimuli, and when a stimulus is removed, the response falls slowly

37
Q

What are fast adapting receptors?

A

Fast adaptation means that the receptor will respond quickly, but will not give a sustained response

38
Q

Which receptors are responsible for proprioception?

A

Muscle spindles

39
Q

Which receptors are tactile afferents?

A

Tactile afferents (discriminative touch):
- cutaneous
Meissner’s corpuscles (Rapidly Adapting)
Merkel’s discs (Slowly Adapting)
- deep
Ruffini corpuscles (Slowly Adapting)
Pacinian corpuscles (Rapidly Adapting)

40
Q

What are the 2 types of free nerve endings?

A

Free nerve endings (low-resolution tactile, temperature, pain)

  • A delta fibres: thin myelinated fibres
  • C fibres: thin, unmyelinated fibres (slow conduction)
41
Q

Outline the receptive field of the deeper skin receptors?

A

The deeper receptors’ area of skin sensitivity is less localised - broader receptor field, larger area of skin able to be stimulated

42
Q

What is the significance of muscle spindles?

A

The muscle spindles are sensory structures within striated muscle (proprioceptors of somatosensory system)

43
Q

How does muscle length change when a muscle contracts?

A

When a muscle (e.g. bicep) contracts it shortens

When its antagonist (tricep) contracts, bicep stretches

44
Q

What is the role of muscle spindles?

A

Muscle spindles detect changes in muscle length - sensory neurons from large fibre system (group I and II afferent axons) detect changes in length

45
Q

What is the function of golgi tendon organs?

A

Golgi tendon organs sense muscle tension and joint receptors also detect muscle length change

46
Q

What is the significance of receptive fields?

A

Ability to localize depends on sensory receptive fields

47
Q

How does receptive field size affect sensory accuracy?

A

Many small receptive fields allow accuracy of localisation

A large receptive field enables the localisation of the stimulus within the entire receptive field - not able to precisely locate

48
Q

What are the 2 major central pathways of the somatosensory system?

A
  • Dorsal column – medial lemniscal system (DCML)

- Spinothalamic tract (STT, also known as anterolateral system)

49
Q

What is the role of the STT somatosensory system?

A

Senses coarse touch, temperature, pain

50
Q

What is the role of the DCML somatosensory system?

A

It mediates discriminative touch, vibration, proprioception

51
Q

Describe the inputs to the DCML

A

Inputs from wide diameter A-β and A-α afferent fibres

52
Q

Outline the neural input to the STT

A

Inputs from A-δ and C thin diameter fibres (blue)

53
Q

In both the DCML and STT describe the structure of their nerve cells

A

In both cases their cell bodies are in dorsal root ganglion and the central portion of their axon enter the spinal cord

54
Q

Where do DCML A-β axons supply?

A

A-β the axon can send out local branches within grey matter, but the main branch supplies the white matter dorsal columns

55
Q

Which region of the brain receives input from the A-β axons?

A

Cuneate nucleus receives input from the A-β axons from the hand, upper body (via cranial nerves → trigeminal) and gracile of the legs and lower body

56
Q

What are the 3 synapses of the DCML pathway?

A

DCML is a 3 synaptic pathway

  1. Dorsal column nuclei of medulla
  2. Ventral posterior complex (Thalamus)
  3. Primary somatosensory cortex (cerebral cortex)
57
Q

How do the A-β axons relay info to the first synapse of the DCML pathway?

A

The central portion of the axon travels up to the brainstem medulla → dorsal column nuclei are the synaptic relays of the medulla

58
Q

Explain the role of the second order nuclei involved in the DCML pathway

A

The axons of the second order nuclei originate from the dorsal column nuclei and cross the midline to ascend along a fibre tract line (medial meniscus) on the contralateral side

They go along the midbrain and terminate in the ventral posterior complex of the thalamus

59
Q

Outline the third order neurons of the DCML pathway

A

The thalamic neurons are the third order neurons in the DCML pathway, and their axons project to the cerebral cortex (primary somatosensory cortex) - first reception of tactile info

60
Q

What is the cerebral cortex?

A

Cerebral cortex is the outer surface of the cerebrum (~1-2mm thick)
Densely packed sheets of cells

61
Q

Where is the primary somatosensory cortex located?

A

The primary somatosensory cortex is the cortical area shaded red, anterior to the parietal lobe partly buried in the sulcus (sylvian fissure → dorsal cortex)
Motor cortex situated in frontal lobe towards the centre
Within sulcus, over the gyra and onto the next sulcus is the primary somatosensory cortex

62
Q

How does cell structure vary in different brain regions?

A

Different areas of cortex have the same basic cell types organized in layers, with the same basic organization

Regional differences can be identified on the basis of relative thickness of the different layers, cell size and density

Viewed using Brodmann’s areas (cytoarchitectonic areas)

63
Q

What do we use to represent the somatosensation in the body?

A

Somatotopic Maps

4 different complete representations of the body’s surface

64
Q

Why are somatotopic maps distorted?

A

The map is distorted as area given to digits is the same as the face → homunculus

Tactile resolution - small densely packed receptive fields for high resolution

Lots of small receptive fields in digits (cortical territory) compared to fewer but larger receptive fields in less sensitive areas like the back

65
Q

How are receptive fields used to organise cells?

A

Cells with similar receptive fields are organized in vertical columns

Neurology (20 decks)

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