Sensory receptors and neurones - the Somatosensory system. Flashcards
The somatosensory mediates all sensations other than what?
Special senses.
Give an example of a special sense.
- Mechanosensation (fine discriminatory touch e.g. light touch, pressure, vibration, flutter and stretch).
- Proprioception (joint and muscle position sense).
- Thermosensation (temp.).
- Nociception (pain).
- Pruriception (itch).
What are the three broad categories into which the Somatosensory system can be divided?
- Exteroceptive division.
- Proprioceptive division.
- Enteroceptive division.
What is the function of the exteroceptive division of the Somatosensory system?
Cutaneous senses - i.e. registering information from body surface by numerous receptors.
What is the function of the proprioceptive division of the Somatosensory system?
Monitors posture and movements via sensors in muscles and tendons and joints.
What is the function of the enteroceptive division of the Somatosensory system?
Reports on internal body state in close relation to autonomic function.
How are stimuli transduced into electrical activity by sensory neurone terminals?
Stimulus opens a cation selective ion channel in the peripheral terminal of a primary sensory afferent to elicit a depolarising receptor potential.
The amplitude of receptor potential is graded and proportional to what?
Stimulus intensity.
Action potentials arriving at the central terminal cause graded release of what to where?
Neurotransmitter on to second order neurones.
What type of sensory unit would sense a mechanical force acting on the skin?
Skin mechanoreceptors
What type of sensory unit would sense a mechanical force acting on joints and muscles?
Joint and muscle mechanoreceptors.
What type of sensory unit would sense heat?
Cold and warm thermoreceptors.
What type of sensory unit would sense pain?
- e.g. strong mechanical force on skin or viscera, high heat on skin/mucous/membranes/viscera?
Mechanical, thermal and polymodal nociceptors.
Threshold is the intensity of a stimulus required to what?
Excite a sensory unit.
Low threshold units respond to what?
Low intensity stimuli.
Low intensity stimuli tend to be what?
Non-damaging/innocuous.
High threshold units respond to what?
High intensity stimuli.
High intensity stimuli tend to be what?
Noxious, potentially damaging.
Thermal nociceptors respond to what?
Extreme degrees of heat (>45oC) or cold (<10-15oC).
Polymocal nociceptors respond to what?
At least TWO of the following:
- High intensity mechanical stimuli.
- Extreme degrees of heat or cold.
- Substances in tissue.
What type of nociceptors respond to substances in tissue?
Chemical nociceptors.
This feature of sensory units determines whether firing rate is changed in response to stimuli of changing intensity or remains consistent throughout a constant stimulus.
Adaptation.
Give an example of a slowly adapting receptor.
Stretch receptors.
Give an example of a fast adapting receptor.
Some muscle spindle afferents, hair follicle afferents.
Give an example of a very fast adapting receptor.
Pascinian corpuscle.
Slowly adapting receptors e.g. stretch receptors provide what information?
Position, degree of stretch or force.
Fast adapting receptors e.g. some muscle spindle afferents, hair follicle afferents detect what?
Changes in stimulus strength e.g. rate of movement.
Very fast adapting receptors e.g. Pacinian corpuscles respond to what?
Only to very fast movement e.g. rapid vibration.
What type of response is associated with slowly adapting receptors?
Tonic/static response.
What type of response is associated with fast adapting receptors?
Phasic/dynamic response.
What type of response is associated with very fast adapting receptors?
Very phasic/dynamic response.
Peripheral terminals of cutaneous afferent fibres branch into many fine processes, the tips of which may be what?
- Free nerve endings.
- Associated with specialised structures.
Receptive field size is inversely related to what?
Innervation density.
How does receptive field size correlate with sensory acuity?
Sensory acuity correlates inversely with receptive field size.
Two-point discrimination is an important measure of what?
Somatosensory function.
How is two-point discrimination tested?
Simultaneously applying two sharp point stimuli separated by a variable distance at different sites on the body surface.
Regions with the highest discriminative capacity have what receptive field size?
The smallest.
What are the sensory receptors of glabrous (hairless) skin?
- Merkel cell-neurite complexes.
- Meissner corpuscle.
- Ruffini endings.
- Pacinian corpuscle.
What are Merkel cell-neurite complexes formed of?
Expanded nerve terminals closely associated with a merkel cell as a synapse like structure.
Where are Merkel cell-neurite complexes located?
Basal epithelium (pegs and ridges) of skin with high sensory acuity e.g. fingers.
What collectively accounts for the SA1 characteristic of the Merkel cell-neurite complexes?
The mechanosensitivity of the merkel cell AND the afferent fibre terminal.
What is a Meissner corpuscle formed of?
A capsule in which several axons zigzag between modified Schwann cells (teloglia).
Where are Meissner corpuscles located?
Close to basal epithelium (intermediate ridges) of skin with high sensory acuity.
Meissner corpuscles are not found in what type of skin?
Hairy skin.
Meissner corpuscles are sensitive to what?
Stroking, flutter, low frequency vibration.
What allows for the detection of textured surfaces?
- Merkel cell neurite complexes.
- Meissner corpuscles.
+ small RF and high density.
Sensory receptors of glabrous skin with a collagenous core in which axons branch.
Ruffini endings.
Where are Ruffini endings located?
In the dermis of both glabrous and hairy skin.
Ruffini endings are sensitive to what?
Drag i.e. shearing forces.
These sensory receptors of glabrous skin are described as having a large onion-like structure.
Pacinian corpuscle.
Where are Pacinian corpuscles found?
Subcutaneously close to the periosteum of bone.
Pacinian corpuscles are very sensitive to what?
Vibration over a frequency range higher than that of a Meissner corpuscle.
Pacinian corpuscles act as what during the manipulation of objects?
Event detectors.
What do hair units functionally replace?
Meissner corpuscles.
Hair units are comprised of what?
Follicular nerve endings that either:
- wrap around the follicle.
- run parallel to the follicle.
Follicular nerve endings that wrap around the follicle are known as what?
Circumferential fibres.
Follicular nerve endings that run parallel to the follicle are known as what?
Palisade fibres.
Areas of skin innervated by the left and right dorsal roots of a single segment is referred to was what?
A dermatome.
Shingles is due to what?
Dorsal root ganglion infection by Varicella Zoster Virus.
Grey matter is divided into ten distinct what?
Laminae of Rexed.
