W10- Neuro: Somatosensation I Flashcards
What does the somatosensory system convey sensations from?
- Touch
- Proprioception - the sensation of knowing where body parts are
- Heat, cold
- Pain, itch
What role does the PNS play?
Peripheral nerves: spinal and cranial
The CNS (brain and spinal cord) is connected to the body via spinal (31 pairs) and cranial nerves (12 pairs).
The actual signals must be conveyed from the body parts to the CNS - this is done via PNS.
What are the PNS structures?
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 does the spinal cord and spinal nerves do?
Each section of the spinal cord gives rise to dorsal and ventral roots. A little away from the spinal cord, they anastomose to form a mixed spinal nerve. There are also dorsal root ganglion, which consist of cell bodies, which gives rise to axons travelling in both directions.
What does the dorsal root ganglion cell do?
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 does the quality of sensation depend on?
Quality of sensation depends on afferent fibre type
Specificity
e.g., mechanosensitive fibre insensitive to thermal stimulation
Thermosensitive fibres sensitive to warming or cooling
Example of cold receptor responding to skin cooling from 34 to 26 °C (A) and warming back to 34°C (B)
What are the receptors of the somatosensory system?
Proprioception
* A-α afferents: large diameter, myelinated, fastest conducting (≤100 m/s)
Muscle spindles
Tactile afferents (discriminative touch)
* A-β afferents: large diameter, myelinated, 2nd fastest conducting (30-70 m/s).
They include:
* superficial Meissner’s corpuscles Merkel’s discs
* deep
Ruffinni corpuscles
Pacinian corpuscles
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 of the somatosensory system?
- Meissner corpuscle - close to ejection between dermis and epidermis.
- Pacinian corpuscle
- Ruffini’s corpsucles
- Merkel’s disks
- Free nerve endings
Tactile sensation depends on the quality of stretch obtained from the activation of the a/B afferents associated with each of these.
How do you encode tactile information in different mechanosensory afferent classes?
When reading brail, we can observe the patterns of action potential firing as the finger is moved gives the general sense of the symbol.
Superficial to deep:
- Merkle’s disc - captures actual patterns
- Meissner’s corpuscle - good at detecting changes because of action potentials ceasing quickly.
- Ruffini’s ending
-Pacinian corpuscle
The last two gives sense of skin stretch, assessing grip, vibration, texture etc.
What are the receptive fields?
Ability to localize depends on sensory receptive fields.
For fine tasks such as brail, we need small receptive fields. Smallest on fingertips, larger on palm and larger still at the back of the hand etc.
What are the two major central pathways of the somatosensory system?
- Dorsal column – medial lemniscal system (DCML)
- mediates discriminative touch, vibration, proprioception
- Inputs from A-β and A-α afferent fibres
- Spinothalamic tract (STT, also known as
anterolateral system)
-coarse touch, temperature, pain - Inputs from A-δ and C fibres
What are the Central pathways of the
somatosensory system?
Axon from peripheral origin comes back to the spinal cord via the dorsal root ganglion cells, where it terminates at the dorsal grey matter, making contact with dorsal/interneurones. The axon then immediately crosses to the opposite side of the spinal cord into the white matter. Goes up to the brain and terminates in the thalamus.
What are the cortical representations of tactile information?
The Thalamus has Ventral Posterior medial nuclues (VPM) and Ventral posterior lateral nucleus (VPL).
VPM = sensory info from face
VPL = sensory info from rest of the body
The cerebral cortex is packed with neurones. The sheet has w-shape with the central sulcus and the postcentral gyrus.
What are the regional variation in 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 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 is a somatotrophic map?
Each of the Brodmann areas contains a complete representation of the body surface. This map is duplicated 4 times - one for each area.
Where there are lots of small receptors, the cortical representation expands.
This is a cartoon like illustration of where tactile senses are high - like huge hands and lips.
