Primary Sensory Neurons Flashcards
Where are the different receptors of the somatosensory system located?
They’re distributed throughout the body
What modalities does the somatosensory system mediate?
Fine discriminatory touch = light touch, pressure, vibration, flutter, stretch
Joint and muscle position sense = proprioception
Temperature, pain and itch
What are the three categories of the somatosensory system?
Exteroceptive = cutaneous senses
Proprioceptive = monitors posture and movement
Enteroceptive
What does the exteroceptive somatosensory system monitor?
Registers info from the surface of the body by numerous receptor types
What does the enteroceptive somatosensory system monitor?
Reports upon internal state of the body and is closely related to autonomic function
Where is the first order neuron located?
Primary sensory afferent of PNS = cell body located in dorsal root ganglia or cranial ganglia
Where is the second order neuron located?
Projection neuron of CNS = cell body located in dorsal horn of spinal cord or brainstem nuclei
Where is the third order neuron located?
Projection neuron of CNS = cell body located in thalamic nuclei
How many neurons make up the series of the somatosensory system?
Three neurons in series = end in the somatosensory cortex
What is the function of the sensory neuron terminals?
Transduce a stimulus into electrical activity
How does a sensory neuron terminal transduce a stimulus into electrical signals?
Stimulus opens cation selective ion channel in peripheral terminal of primary sensory afferent = elicits a depolarising receptor potential
What is the amplitude of the receptor potential graded and proportional to?
Stimulus intensity
What does a supra-threshold receptor potential generate?
An all or nothing AP = conducted by the axon at a frequency proportional to its amplitude
What does APs arriving at the central terminal cause?
The graded release of neurotransmitter on to a second order neuron
What are the different properties of sensory units?
Modality, threshold, adaption rate, conduction velocity and the site and extent of peripheral termination (receptive field)
What does threshold relate to?
The intensity of a stimulus required to excite a sensory unit
What are some examples of low threshold receptors?
Low threshold units = respond to low intensity stimuli
Low threshold mechanoreceptor = mediate fine discriminatory touch
Low threshold thermoreceptors = mediate cold through to hot
What are some examples of high threshold receptors?
High threshold units = respond to high intensity (damaging) stimuli
High threshold mechanoreceptors = respond to high intensity mechanical stimuli
What do thermal nociceptors respond to?
Extremes of hot (>45) or cold (<10-15)
What do chemical nociceptors respond to?
Substances in tissues (e.g during inflammatory response)
What do polymodal receptors respond to?
At least two modalities
What does adaption determine?
Whether they change their firing rate only in response to a stimulus of changing intensity or fire continuously throughout a constant stimulus
What are the features of the slow adapting response?
Continuous info to CNS while terminal deformed = provides info about position, degree of stretch and force
What are the features of the fast adapting response?
Detects changes in stimulus strength = number of impulses proportional to rate of change of stimulus
What are the features of the very fast adapting response?
Responds only to very fast movements (e.g rapid vibration)
What are some features of Group I fibres?
13-20 micrometer diameter
80-120 m/s conduction velocity
Sensory receptors for proprioceptors for skeletal msucle
Thick myelination
What are some features of Group II fibres?
6-12 micrometer diameter
Moderate myelination
35-75 m/s conduction velocity
Sensory receptor for mechanoreceptor of skin
What are some features of Group III fibres?
1-5 micrometer diameter
Thin myelination
5-30 m/s conduction velocity
Sensory receptor for pain and temperature
What are some features of Group IV fibres?
0.2-1.5 micrometer diameter
No myelination
0.2-0.5 m/s conduction velocity
Sensory receptor for temperature, pain and itch
What happens to the peripheral terminal of cutaneous afferent fibres?
Branch into many fine processes = tips of which can be free nerve endings or associated with specialised structures
What is the receptive field?
Target territory from which a sensory unit can be excited = size varies greatly over body surface
What is the relationship between the receptive field and innervation density?
Receptive field is inversely related to innervation density
How does sensory acuity correlate with receptive field size?
Sensory acuity is inversely related with receptive field size
What are some features of high density and low density innervation?
High density = small receptive field, high acuity
Low density = large receptive field, low acuity
What is two point discrimination?
Measure of somatosensory function = also called spatial acuity
How is two point discrimination tested?
By applying two sharp point stimuli simultaneously separated by a variable distance at different sites on the body surface
How is threshold distance determined from two point discrimination testing?
Subject reports on whether one point or two or sensed and a threshold distance is established between the two
What do two point thresholds match?
The diameter of the corresponding receptive field = regions with the highest discriminative capacity have the smallest receptive fields
What are some examples of cutaneous receptors?
Free nerve endings, Meissner’s corpuscles, Merkel’s discs, hair end organs, Krause end bulbs, Ruffini endings (encapsulated), Pacinian corpuscles (encapsulated)
What kind of innervation do free nerve endings provide?
Ubiquitous innervation
What are some features of Meissner’s corpuscles?
Abundant in skin locations where two point discrimination is highest, not present in hairy skin
What are some features of Merkel’s discs?
Distribution same as Meissner’s corpuscles but also present in moderate numbers in hairy skin, often grouped in Iggo domes
Where are Krause end bulbs located?
At border of dry skin and mucous membranes
Where are Ruffini endings located?
Within dermis and also present in joint capsules
Where are Pacinian corpuscles located?
Within dermis and fascia
What occurs in the Iggo dome?
Multiple Merkel cells are innervated by a single myelinated fibre = both are mechanosensitive
What are the physiological divisions of skin low threshold mechanoreceptors?
Rate of adapting = fast adapting, slow adapting
Size of receptive field = small (type 1 unit), wide (type 2 unit)
What do skin low threshold mechanoreceptors consist of histologically?
A receptor and its parent fibre type
What are some examples of skin low threshold mechanoreceptors?
Free nerve endings, follicular nerve endings, Merkel cell-neurite complexes, encapsulated nerve endings
What are some features of the receptive fields of Pacinian corpuscles?
Corpuscle responds when the stimulus strength increases sufficiently anywhere within the area
Larger fields than Meissner’s corpuscles
What does the threshold for detecting vibration correspond to?
The tuning threshold of the mechanoreceptor
What is grey matter subdivided into?
Dorsal and ventral horns, and ten distinct laminae of Rexed
What are the sensory receptors of hairless skin?
Merkel cell-neurite complexes, Meissner’s corpuscles, Ruffini endings, Pacinian corpuscles
What forms Merkel cell-neurite complexes?
Formed from many expanded nerve terminals, each closely associated with a Merkel cell as a synapse like structure
What are some features of Merkel cell-neurite complexes?
Located in basal epithelium of skin areas with high sensory acuity
Signal sustained pressure/texture and are extremely sensitive to edges of objects
What are Meissner’s corpuscles formed from?
Formed from a capsule in which several axons zigzag between modified Schwann cells
What are some features of Meissner’s corpuscles?
Located close to basal epithelium of skin in areas with high sensory acuity
Sensitive to stroking/flutter and low frequency vibration
What does the location of Merkel cell-neurite complex and Meissner’s corpuscles allow?
Detection of textured surfaces = also aided by their small receptive field and high density
What are some features of Ruffini endings?
Collagenous core in which axons branch
Located in dermis and sensitive to drag
Where are Pacinian corpuscles located?
Subcutaneously close to the periosteum of the bone
What are some features of Pacinian corpuscles?
Very sensitive to vibration over higher frequency range than Meissner’s corpuscles
Act as event detectors during manipulation of objects
What are the sensory receptors of hairy skin?
Merkel cell-neurite complexes, Ruffini endings, Pacinian corpuscles, hair units
What do hair cells functionally replace in hairy skin?
Meissner’s corpuscles
What do hair cells consist of?
Comprise follicular nerve endings that either wrap around the follicle (circumferential) or parallel with it (palisade)
What are some features of hair cells?
Single afferent fibre innervates many follicles
Sensory units may be slow or fast adapting
What are hair cells responsive to?
The bending of hairs = increases or decreases their firing when the hair is in motion but not when static
