Peter's Physiology 3 - Somatosensory System (1) Flashcards
What type of sensations is the somatosensory system not concerned with?
Visual Auditory Gustatory Olfactory Vestibular sense of equilibrium
What sensory modalities does the somatosensory system deal with?
Mechanosensation (touch, pressure and vibration)
Proprioception (joint and muscle position sense)
Thermosensation (temperature)
Nociception (pain)
Pruriception (itch)
How are primary sensory afferent fibres able to generate different sensations when they all transit impulses along their axons using the same physiological principles?
They connect to unique neurones in the CNS which are capable of decoding similar nerve signals in different ways (i.e. primary sensory afferent fibres confer modality by their site of termination in the CNS - labeled line principle)
4 classes of somatic sensation?
Exteroceptive sensations
Proprioceptive sensations
Visceral sensations
Deep sensations
What are exteroceptive sensations?
Cutaneous senses from the surface of the skin
What are proprioceptive sensations?
sensations concerning posture and movement (sensors in muscle and tendons and joints)
What are visceral sensations?
Sensations specifically from the internal organs
What are deep sensations?
Sensations from fasciae, muscles and bone
How does a stimulus elicit a depolarisation receptor (or generator) potential?
Stimulus opens cation selective on channels in nerve terminal
What is the amplitude of the generator potential proportional to?
Stimulus intesnity
What does local current flow in a sensory neurone terminal due to a stimulus trigger?
“All or none” action potentials at a frequency proportional to the amplitude of the receptor potential
What is muscle spindle?
A sensory structure in skeletal muscle activated by stretch
What is the name for sensory receptors which are activated by variable stimuli?
Polymodal
What is the name for the principle type of adequate stimulus that is traduced into an electrical signal by a primary afferent neurone?
Modality
What is the physiological receptors for touch, pressure and vibration primary afferent neurones?
Skin mechanoreceptors
What are the physiological receptors for proprioception?
Joint and muscle mechanoreceptors
What is the stimulus for proprioceptive primary afferent neurones?
Mechanical forces acting on joints and muscles
What are the physiological receptors for pain?
High threshold mechanoreceptors (aka mechanical nociceptors)
Thermal nociceptors
Chemical nociceptors
Polymodal nociceptors
What are the stimuli for pain primary afferent neurones?
Strong mechanical force on skin, viscera
Heat on skin, mucous membranes and viscera
What is the stimuli for itch sensation?
Irritant (e.g. chemical) on skin, or mucous membranes
What is the physiological receptor for itch sensation?
Itch receptors
What kind of stimuli do low threshold units respond to?
Low intensity i.e. non-damaging and non-painful
What sensations do low threshold mechanoreceptors mediate?
Touch, vibration, pressure
What sensations do low threshold thermoreceptors mediate?
Cold, cool, indifferent, warm and hot
What is another name for high threshold units?
Nociceptors
Describe the intensity of stimuli that high threshold units respond to?
High (Noxious, potentially damaging) but not (normally) low, intensity stimuli
Is information from high threshold units allows perceived as painful?
No, they are subject to both spinal segmental and supra spinal influences that “gate” input to second order neurones
What is another name for high threshold mechanoreceptors?
What do these respond to?
Mechanical nociceptors
High intensity mechanical stimuli
What do thermal nociceptors respond to?
Extreme degrees of heat (>45 degrees C) to cold (< 10-15 degrees C)
What do chemical nociceptors respond to?
Substances in tissue (as found in inflammation) e.g. prostaglandins, bradykinin, serotonin, histamine, K+, H+ and ATP
What is the name for the feature of primary sensory neurones that determines whether they change their firing rate only in response to a stimulus of changing intensity?
Adaption
What type of adaptive neurone response provides continuous information to CNS while terminal deformed?
Slowly adapting (SA) tonic/ static response
What type of information do slowly adapting tonic/ static response neurones give?
Examples?
Information about position, degree of stretch, or force
E.g. Stretch receptors
What type of adaptive neurone response detects changes in stimulus strength with the number of impulses proportional to the rate of change of stimulus?
Rapidly adapting (RA) phasic/ dynamic response
Examples of location of rapidly adapting (RA) phasic/ dynamic response neurones?
Some muscle spindle afferents
Hair follicle afferents
What type of adaptive neurone response responds only to very fast movement, such as rapid vibration?
Very rapidly adapting (very RA) very phasic/ dynamic response
Example of structure that displays a very rapidly acting (very RA) very phasic/ dynamic response?
Pacinian corpuscle
What is the nomenclature used to describe primary sensory afferents leaving the skin with a diameter of 13-20 micro metres?
A alpha
What is the nomenclature used to describe primary sensory afferents leaving the skin with a diameter of 6-12 micro metres?
A beta
What is the nomenclature used to describe primary sensory afferents leaving the skin with a diameter of 1-5 micro metres?
A delta
What is the nomenclature used to describe primary sensory afferents leaving the skin with a diameter of 0.2-1.5 micro metres?
Does this type of fibre have myelination?
C
No
As you go from group A alpha to group C primary sensory afferents, how does the conduction velocity differ/
Decreases
What sensory receptor do type A alpha primary sensory afferents function as?
Proprioceptors of skeletal muscle
What sensory receptor do type A beta primary sensory afferents function as?
Mechanoreceptors of skin
What sensory receptor do type A delta primary sensory afferents function as?
Pain, temperature
What sensory receptor do type C primary sensory afferents function as?
Temperature, pain, itch
What is the receptive field of an afferent neurone?
The region that when stimulated with an adequate stimulus causes a response in that neurone
Does receptive field vary a lot over the body surface?
Yes (evidenced by wide differences in 2 point discrimination)
Do regions with the highest discriminative capacity have the smallest or largest receptive fields?
Smallest
Where in the skin are Meissner’s corpuscles found abundantly?
In areas where 2 point discrimination is highest
What are Merkel’s discs often grouped in?
Iggo domes
What is the difference between the distribution of Meissner’s corpuscles and Merkel’s discs?
Meissner’s corpuscles aren’t present in hair skin where as Merkel’s discs are present in moderate numbers in hairy skin
Where are krause end bulbs found?
At the border of dry skin and mucous membrnaes
Where are Ruffini endings found?
Within dermis and also joint capsules
Where are Pacinian corpuscles located?
Within dermis and fascia
Fibre group, adaptability and type of sensation for Meissner’s Corpuscle?
A beta
Rapidly acting
Stroking, flutter
Fibre group, adaptability and type of sensation for merkel disk receptor?
A beta
Slowly adapting
Pressure, texture
Fibre group, adaptability and type of sensation for pacinian corpuscle?
A beta
Very rapidly adapting
Vibration
Fibre group, adaptability and type of sensation for Ruffini ending?
A beta
Slowly adapting
Skin strethc
Fibre group and type of sensation for hair (G) - guard (long)?
A beta
Stroking, flutter
Fibre group and type of sensation for hair (D) - down (short)?
A delta
Light stroking
Fibre group and type of sensation for C mechanoreceptor?
C
Stroking, social and erotic touch
What is the name for the 10 laminae the spinal cord is divided into?
Laminae of Rexed
Which laminae do the nociceptors terminate in?
Fibre class?
Laminae I and II
A delta/ C
Which laminae do the LTMs terminate in?
Fibre class?
Laminae III to VI
A beta
Which laminae do the proprioceptors terminate in?
Fibre class?
Laminae VII to IX
A alpha
