Somatosensation I Flashcards
What is the role of the somatosensory system?
The somatosensory system conveys sensations from the body:
- Touch (pressure, texture, etc.)
- Proprioception - one’s sense of body position
- Heat, cold
- Pain, itch
How is the CNS connected to the somatic system?
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
What is a nerve?
Nerve is a bundle of axons carrying motor and sensory neurons of the PNS
Describe the structure of a nerve
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
What are the different types of connective tissue found in a nerve?
Connective tissue elements
- Epineurium
- Perineurium
- Endoneurium
How does the structure of a nerve change along its route?
As the nerve travels along the periphery it branches and so some of the fascicles combine to give finer branches
Where do nerve axons terminate?
The axons terminate within the somatic system in skin, muscles or joints
Where do spinal nerves emerge from?
Spinal nerves emerge from each segment of the spinal cord
Which important structures are innervated by spinal nerves?
The brachial and cervical plexus’ are formed from the 8 cervical nerves and top thoracic nerves
What are the defining regions of the spinal cord?
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
What are the roles of the 2 halves of the spinal cord?
The spinal cord is roughly divided into 2:
- Incoming sensory information in dorsal half
- Output motor commands in ventral half
Which of the spinal roots is the spinal nerve composed of?
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)
Where do the autonomic fibres of the spinal cord lead to?
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 `
What is the spinal cord comprised of?
The cross section of the spinal cord consists of white matter (ascending / descending axon tracts) and grey matter (cell bodies, synapses etc.)
What is the role of the dorsal horns of the spinal cord?
Dorsal horns of the spinal cord is the site of incoming sensory axons
What is the function of the ventral roots of the spinal cord?
The Ventral horn gives rise to motor neurons sending outgoing axons
How many dorsal root ganglion are there?
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
What are the dorsal root ganglion?
These are the sensory receptors of the somatosensory system
Describe the structure of dorsal root ganglion
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
Outline the route travelled by an axon tract terminating in the skin of a finger
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
Explain why dorsal root ganglion are so long
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
What are the 2 broad, anatomical and functional systems of dorsal root ganglion?
- Large fibres
- Small fibres
Outline the structure and functions of large diameter fibres
Large fibres (large diameter, myelinated, fast conduction): tactile and proprioceptive (Red) - enter from all over body and ascend to the medulla → second order neurons
Describe the structure of small fibres
Small fibres are small diameter, thinly-myelinated or unmyelinated, medium or slow conducting fibres
What is the function of small fibres?
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
Which somatosensory info is carried via the dorsal spinal cords?
Ascending tracts from the dorsal spinal cords carry tactile and proprioceptive info
What sensory info is carried by the ventral spinal cords?
Ascending axon tracts from the anterior lateral white matter of the spinal cord carry pain etc.
What determines the quality of sensation?
Quality of sensation depends on afferent fibre type: specificity
What determines the specificity of sensation?
Specificity of sensation depends upon the type of afferent (e.g. either large → tactile /small → crude touch
What are the 4 major classes of receptors of the larger diameter afferents?
Superficial cutaneous:
- Meissner Corpuscles
- Merkel’s disks
Deeper classes:
- Pacinaian Corpuscles
- Ruffini’s corpuscles
Where are the cutaneous receptors sensory endings located?
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)
Where in the skin are the deeper classes of large diameter afferent receptors located?
These are located deeper in the dermis or between the dermis & hypodermis
Describe the sensory nerve endings of the 4 classes of receptors
The sensory endings are the axon terminals of the relevant dorsal root ganglion neurons
What is the role of the sensory nerve endings?
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
How does the corpuscle structure aid their function?
The 4 classes of receptors respond slightly differently
The corpuscles structure filters between mechanical stimulation of the neural membrane and mechanical pressure
What is meant by slow adapting receptors?
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
What are fast adapting receptors?
Fast adaptation means that the receptor will respond quickly, but will not give a sustained response
Which receptors are responsible for proprioception?
Muscle spindles
Which receptors are tactile afferents?
Tactile afferents (discriminative touch):
- cutaneous
Meissner’s corpuscles (Rapidly Adapting)
Merkel’s discs (Slowly Adapting)
- deep
Ruffini corpuscles (Slowly Adapting)
Pacinian corpuscles (Rapidly Adapting)
What are the 2 types of free nerve endings?
Free nerve endings (low-resolution tactile, temperature, pain)
- A delta fibres: thin myelinated fibres
- C fibres: thin, unmyelinated fibres (slow conduction)
Outline the receptive field of the deeper skin receptors?
The deeper receptors’ area of skin sensitivity is less localised - broader receptor field, larger area of skin able to be stimulated
What is the significance of muscle spindles?
The muscle spindles are sensory structures within striated muscle (proprioceptors of somatosensory system)
How does muscle length change when a muscle contracts?
When a muscle (e.g. bicep) contracts it shortens
When its antagonist (tricep) contracts, bicep stretches
What is the role of muscle spindles?
Muscle spindles detect changes in muscle length - sensory neurons from large fibre system (group I and II afferent axons) detect changes in length
What is the function of golgi tendon organs?
Golgi tendon organs sense muscle tension and joint receptors also detect muscle length change
What is the significance of receptive fields?
Ability to localize depends on sensory receptive fields
How does receptive field size affect sensory accuracy?
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
What are the 2 major central pathways of the somatosensory system?
- Dorsal column – medial lemniscal system (DCML)
- Spinothalamic tract (STT, also known as anterolateral system)
What is the role of the STT somatosensory system?
Senses coarse touch, temperature, pain
What is the role of the DCML somatosensory system?
It mediates discriminative touch, vibration, proprioception
Describe the inputs to the DCML
Inputs from wide diameter A-β and A-α afferent fibres
Outline the neural input to the STT
Inputs from A-δ and C thin diameter fibres (blue)
In both the DCML and STT describe the structure of their nerve cells
In both cases their cell bodies are in dorsal root ganglion and the central portion of their axon enter the spinal cord
Where do DCML A-β axons supply?
A-β the axon can send out local branches within grey matter, but the main branch supplies the white matter dorsal columns
Which region of the brain receives input from the A-β axons?
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
What are the 3 synapses of the DCML pathway?
DCML is a 3 synaptic pathway
- Dorsal column nuclei of medulla
- Ventral posterior complex (Thalamus)
- Primary somatosensory cortex (cerebral cortex)
How do the A-β axons relay info to the first synapse of the DCML pathway?
The central portion of the axon travels up to the brainstem medulla → dorsal column nuclei are the synaptic relays of the medulla
Explain the role of the second order nuclei involved in the DCML pathway
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
Outline the third order neurons of the DCML pathway
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
What is the cerebral cortex?
Cerebral cortex is the outer surface of the cerebrum (~1-2mm thick)
Densely packed sheets of cells
Where is the primary somatosensory cortex located?
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
How does cell structure vary in different brain regions?
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)
What do we use to represent the somatosensation in the body?
Somatotopic Maps
4 different complete representations of the body’s surface
Why are somatotopic maps distorted?
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
How are receptive fields used to organise cells?
Cells with similar receptive fields are organized in vertical columns
