Somatosensory system Flashcards
What are the 3 divisions of the somatosensory system?
Exteroceptive, proprioceptive, enteroceptive
The somatosensory system mediates all sensations other than those of the special senses, such as…?
Fine discriminatory touch, stretch, joint and muscle position sense, temperature, pain and itch
What does the exteroceptive division do?
Registers information from the surface of the body by numerous receptors (cutaneous receptors)
What does the proprioceptive division do?
Monitors posture and movement (sensors in muscle and tendons and joints)
What does the enteroceptive division do?
Reports upon the internal state of the body and is closely related to autonomic function
Describe the somatosensory pathway in terms of where the neurons are in the body
1st order neuron/primary sensory afferent (in PNS) -> 2nd order neuron/projection neuron (in CNS) -> 3rd order neuron/projection neuron (in CNS) -> somatosensory cortex (in CNS)
What is the cell body location of the 1st order neuron in the somatosensory pathway?
Dorsal root ganglia (innervation of limbs, trunk, posterior head) or cranial ganglia (innervation of anterior head)
What is the cell body location of the 2nd order neuron in the somatosensory pathway?
Dorsal horn of spinal cord or brainstem nuclei
What is the cell body location of the 3rd order neuron in the somatosensory pathway?
Thalamic nuclei
Sensory neurone terminals transduce a stimulus into electrical activity. How does the stimulus produce a signal?
Stimulus opens cation selective ion channels in peripheral terminal of primary sensory afferent eliciting a depolarising receptor potential. Amplitude of receptor potential is graded and proportional to stimulus intensity. A supra-threshold receptor potential triggers ‘all or none’ potentials, conducted by the axon, at a frequency proportional to its amplitude. Action potentials arriving at the central terminal cause the graded release of neurotransmitter on to 2nd order neurons
Describe the modality of sensory units
Primary afferent neurons are especially tuned to respond to a specific type of energy that normally excites them underlying the sensation that they subserve
Describe the threshold of sensory units
Relates to the intensity of a stimulus required to excite a sensory unit Low threshold units respond to low intensity stimuli e.g. fine discriminatory touch, cold, warm and hot High threshold units (nociceptors) respond to high, but not low, intensity stimuli e.g. high intensity mechanical stimuli, extremes of heat or cold, chemicals
Describe the adaptation of sensory units
Adaptation is a feature of sensory units that determines whether they change their firing rate only in response to a stimulus of changing intensity, or fire continuously throughout a constant stimulus
Describe a slow adapting response/tonic/static response
Continuous information to CNS while terminal deformed. Provides information about position, degree of stretch, or force e.g. stretch receptors
Describe a fast adapting response/phasic/dynamic response
Detects changes in stimulus strength e.g. rate of movement. Number of impulses proportional to rate of change of stimulus. Some muscle spindle afferents, hair follicle afferents
Describe very fast adapting/very phasic/dynamic response
Responds only to very fast movement, such as rapid vibration e.g. Pacinian corpuscle
Describe the conduction velocity of sensory units
Primary sensory afferent fibres differ greatly in their axon diameter, extent of myelination, conduction velocity and associated sensory receptor
Describe the receptive field (site of termination) of sensory units
The peripheral terminal of cutaneous afferent fibres branch into many fine processes the tips of which can be free nerve endings or associated with specialised structures. A patch of skin may contain overlapping RFs of sensory units with a different modality
Describe two-point discrimination
Two-point discrimination is an important measure of somatosensory function. Clinically tested by applying simultaneously 2 sharp point stimuli, separated by a variable distance, at different sites on the body surface. Subject reports on whether 1 point, or 2, are sensed and a threshold distance between the two is established. Two-point thresholds match the diameter of the corresponding RF. Regions with the highest discriminitive capacity have the smallest RFs
What are the subdivisions of skin low threshold mechanoreceptors?
FA1 (fast adapting small receptive field), FA2, SA1, SA2 (slow adapting, wide receptive field)
Describe Merkel cell-neurite complexes in terms of type of mechanoreceptor and location
SA1 - formed from many expanded nerve terminals,each closely associated with a Merkel cell as a synapse-like structure. Located in basal epithelium of skin areas with high sensory acuity e.g. fingers. Signal sustained pressure, texture and are extremely sensitive to the edges of objects
Describe Ruffini endings
SA2 - collagenous core in which axons branch. Located in dermis of both glabrous and hairy skin. Sensitive to drag/shearing forces
Describe Pacinian corpuscles
FA2 - large onion-like structure. Subcutaneous location close to periosteum of bone. Very sensitive to vibration over frequency range higher than Meissner corpuscle. Act as event detectors during manipulation of objects
Describe Meissner corpuscles
FA1 - formed from a capsule in which several axons zigzag between modified Schwann cells (teloglia). Located close to basal epithelium of skin areas with high sensory acuity. NOT present in hairy skin. Sensitive to stroking, flutter, low frequency vibration.
Which sensory receptors allow for the detection of textured surfaces?
The location of Merkel cell neurite complexes and Meissner corpuscles at, or just below, base of epidermis, together with their small RF and high density, allows for the detection of textured surfaces
Which sensory receptors are associated with hairy skin?
Merkel cell neurite complex (SA1), Ruffini endings (SA2) and Pacinian corpuscles (FA2) but not Meissner corpuscles (FA1) that are instead functionally replaced by hair units
What is a hair unit?
Comprise follicular (free) nerve endings that either wrap around the follicle (circumferential fibres) or parallel to it (palisade fibres)
What is a dermatome?
An area of skin that is innervated by the left and right dorsal roots of a single spinal segment. When mapped, the dermatomes delineate a set of bands on the body surface
Which condition highlights dermatomes?
Shingles - due to infection of dorsal root ganglion neurones by Varicella Zoster virus (causes chickenpox in childhood). In adulthood the virus may reactivate revealing, anatomically, as inflamed and blistered skin, the dermatome innervated by that ganlgion
The grey matter is divided into dorsal and ventral horns and 10 distinct …?
Laminae of Rexed
What is the fibre class and termination site (in the dorsal horn of grey matter) of nociceptors?
Fibre class = Adelta/C Termination site = laminae I and II
What is the fibre class and termination site (in the dorsal horn of grey matter) of low threshold mechanoreceptors?
Fibre class - Abeta Termination site - laminae III to VI
What is the fibre class and termination site (in the dorsal horn of grey matter) of proprioceptors?
Fibre class - Aalpha Termination site - laminae VII to IX
