Somatostatin 1 Flashcards
What is somatosensation?
Somatosensation – all sensation from the body
What sensations does the somatosensory system convey?
- Touch
- Proprioception – body position
- Heat, cold
- Pain, itch
How is the CNS connected to the body?
The CNS (brain and spinal cord) is connected to the body via spinal (31 pairs) and cranial nerves
Describe the structure of the peripheral nerves
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
Describe the structure of the spinal cord and spinal nerves
- The spinal cord gives rise to dorsal roots and ventral roots. They then join to form a mixed spinal nerve
- Lying next to the dorsal root is the dorsal root ganglia – they contain cells bodies and the cell bodies of the sensory neurones of the spinal cord are located here that gives rise to axons
- Sensory information into the spinal cord into the grey matter (dorsal horn) and motor information arises from the ventral portion of spinal cord at the ventral horn where they exit via ventral roots and join after the dorsal roots to form a mixed spinal nerve
Dorsal root ganglion cells are the sensory receptors of the somatosensory system
How are they anatomically and functionally distinct?
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
How does the Quality of sensation depend on afferent fibre type?
Sensory neurones so 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 (sense muscle length) 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
Describe the structure of the Proprioceptors of the somatosensory system
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Give some more receptors of the somatosensory system
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)
Identify the Cutaneous receptors of the somatosensory system
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What are receptive fields
Ability to localize depends on sensory receptive fields
What are the Two major central pathways of the somatosensory system?
• Dorsal column – medial lemniscal system (DCML)
o mediates discriminative touch, vibration, proprioception
o Inputs from A-β and A-α afferent fibres
• Spinothalamic tract (STT, also known as anterolateral system)
o coarse touch, temperature, pain
• Inputs from A-δ and C fibres
Describe the Central pathways of the somatosensory system
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Describe 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
