Somatosensation I Flashcards
What is somatosensation and what sensations does it include?
The term somatosensation refers to all sensations felt from the body
The somatosensory system conveys sensations from the body including:
- Touch
- Proprioception- our sense of body position
- Heat and cold
- Pain and itch
What is the structure of a peripheral nerve like?
Think of a nerve as a bundle of axons ensheathed in connective tissue
Epineurium is the connective tissue ensheathing the whole nerve
Within the nerve axon bundles may be in separate fascicles surrounded by perineurium connective tissue sheath
What is a spinal cord/nerve like?
Each segment of the spinal cord give rise to dorsal and ventral roots
Outside the spinal cord they join to form a mixed spinal nerve
We also have the dorsal root ganglia, which contain cell bodies which give rise to axons traveling to both directions; into the spinal cord via the dorsal root and out towards the peripheral receptive field
What are the two different types of dorsal root ganglion cells?
Dorsal root ganglion cells are the sensory receptors of the somatosensory system Broadly, two anatomically and functionally distinct systems Large fibres (large diameter, myelinated, fast conduction): tactile and proprioceptive Small fibres (small diameter, thinly-myelinated or unmyelinated, medium or slow conducting): temperature, pain, itch, crude touch
What is the difference between A- alpha afferents and A-beta afferents?
Receptors of the somatosensory system
Proprioception
A-α afferents: large diameter, myelinated, fastest conducting (≤100 m/s)
Afferents from muscle spindles (also called group 1a)
Tactile afferents (discriminative touch)
A-β afferents: large diameter, myelinated, 2nd fastest conducting (30-70 m/s). They include:
- Superficial (close to the surface of the skin):
Meissner’s corpuscles
Merkel’s discs
- Deep:
Ruffini corpuscles
Pacinian corpuscles
How do proprioceptors work?
Muscle spindles are specialised muscle fibres embedded within contractile muscle fibres of voluntary muscles
Muscle spindle is weakly contractile
Afferents, group 1a, are embedded within the muscle spindle
As the muscle contracts or lengthens the muscle changes in length and consequently the muscle spindle changes in length and the sensory endings are highly sensitive to changes in length
So that’s how we know where our limbs are and if they are flexed or not
Another type of proprioceptor is the Golgi tendon organ, they are specialised sensory endings near the joint of a muscle with its tendon
They are sensitive to tension within a muscle
What are the two types of free nerve endings?
Free nerve endings (low-resolution tactile, temperature, pain)
- A delta fibres: small diameter, thinly myelinated, moderate conduction velocity (≤30 m/s) - C fibres: small diameter, unmyelinated, slow conducting (≤1 m/s)
What are the cutaneous receptors like?
Encoding tactile information in different mechanosensory afferent classes
These are low threshold mechanosensitive afferents
How they respond is determined by their mechanical properties, locations and encapsulations
For example braille reading
What type of mechanosensory afferent works for braille reading?
The experiment carried out showed that the pattern of action potential firing gives the rough pattern of the braille through Merkel’s disc
Meissner’s corpuscle also shows this but less well
The deeper receptors are less useful as they work mostly to sense skin stretch, vibration etc.
What are the two major central pathways of the somatosensory system?
Dorsal column – medial lemniscal system (DCML)
- mediates discriminative touch, vibration, proprioception
- Receives inputs from A-β and A-α afferent fibres
Spinothalamic tract (STT, also known as anterolateral system)
- coarse touch, temperature, pain
- Receives inputs from A-δ and C fibres
What is the central pathway of somatosensation from the peripheral axon?
The axon from the peripheral origin courses back to the spinal cord passing the dorsal ganglion cell and terminates in the grey mater of the spinal cord termed the dorsal horn
It makes synaptic contact with the second order interneuron called the dorsal horn interneurons with its cell body in the dorsal horn of the spinal cord
The axon of this neuron crosses to the opposite side of the spinal cord where it ascend in the white matter passing up through the brain stem and this eventually terminates in the thalamus
Some may send branches along the way
The dorsal column white matter tract is the way the DCML axons take but then run parallel further up
What is the central pathway of the somatosensory system from the face or head?
From the face or head come through the trigeminal ganglion which them forms a synapse with second order neurons in the principal nucleus of trigeminal complex
This crosses and ascends in the same white matter tract as the DCML, targeting thalamic nuclei and third order neurons
What is the cortical representation of tactile information?
The VP complex is the way that tactile afferents reach the cortex, divided into the medial and the lateral parts
The VPM receives input from the face and head and the VPL from the rest of the body
Both of these then target specific regions within the cortex namely the primary somatic sensory cortex (SI)
Remember that the cerebral cortex is a sheet, like a map
Different areas within the cerebral cortex are given different numbers
Areas 1, 2 and 3 are the primary somatic sensory cortex
Area 4 is the primary motor cortex
These numbers have been assigned to this cortical sheet based on variation of their cytoarchitecture
These are called Brodmann areas
check diagram if confused
What is the regional variation of cortical cytoarchitecture?
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 the relative thickness of the different layers, cell size, and density (cytoarchitectural differences)
Brodmann defined and numbered over 50 areas in human cortex based on subtle cytoarchitectural differences
Many Brodmann areas now associated with function: Brodmann areas 1,2&3 comprise somatosensory cortex
What are somatotopic maps?
Each of these Brodmann areas contains a complete representation of the body’s surface
Within area two we have representation of the hand, arm, shoulder and neck etc.
This map is duplicated four times, so we have a somatotopic map for each Brodmann area
Note that the map is highly distorted
Where receptive fields are many and small the cortical representation expands
The sensory homunculus shows a version of a human depicted by the somatotopic map
What are the cortical connections like?
The cortical representation is essential for any conscious perception of touch
The primary somatosensory cortex projects to a secondary somatosensory cortex where cell response becomes more complex as well as other parietal areas
And eventually onto the amygdala and hippocampus, for emotional processing
And motor and premotor
