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