Somatosensory System I Flashcards
Includes the sense of
Touch
Proprioceptive sensation
Positional sensation
Vibratory sensation
Fine touch or epicritic touch
Crude touch or epicritic touch
Temperature
Pain
PC-ML system perceives
Vibration, kinesthesia, fine touch and spatial discrimination through mechanoreceptors and proprioceptors
Anterolateral system perceives
Temperature, pain (fast and slow), and crude touch
Sensory info
Reaches the thalamus - synapses w/ next neuron - project to contralateral cortex (somatotopic organization)
Classification of sensory fibers
Lloyd’s classification
- I: greater degree of myelination, greater diameter= Faster
- II: smaller diameter, less myelin = fast, but less than type I
- III: narrow diameter, little myelin = slower AP conduct v
- IV: very very narrow diameter, amyelinic = slowest fibres
Gasser classification (A, B, C)
- A-alpha: equivalent to I
- A-beta: equivalent to II
- A-delta: equivalent to III
- C: equivalent to IV
Sensations and types of fibers that it uses
Golgi tendons (proprioception) —> Ib
Muscle spindles —> Ia / II
Pain and temperature (ALS) —> III, IV
Classification motor fibers
Alpha and gamma (fast and slow) for somatic efferents.
B and C fibers for autonomic nervous system:
- B = preganglionic, little myelination, faster
- C = postganglionic, no myelin, slower
Types of muscles fibers in muscle spindles
Extrafusal
Intrafusal
Extrafusal muscle fibers
Make force
- Slow: type I
- Fast: type IIa, IIb
Intrafusal muscle fibers
Part of sensory organs
Proprioperception in muscles (house muscle spindles)
- Nuclear bag intrafusal fiber
- Nuclear chain intrafusal fiber
Neuromuscular spindles - location, composition
Within muscles
Composed of muscle fiber + a sensory fiber surrounding it
Motor fiber can be surrounded by:
- Annulospiral endings: travel in type Ia axons (fastest)
- Flower spray endings: travel in type II axons
How do neuromuscular spindles work?
They have mechanoreceptors:
Muscle stretched = annulospiral terminals stretched -> activate mechanoreceptors (detect degree of muscle stretching)
Stretch = stimulus for reflex: activate motor fibre to generate reflex + send fibers to cortex (reflex = conscious) and to cerebellum for proprioception.
The muscle spindle involves
terminals surrounding actual muscle fibres
The muscle spindle involves terminals surrounding actual muscle fibres, and therefore:
Muscle stretched —> spindle stretched
Muscle contracted —> spindle contracts (can be detected)
Why are the nerve terminals surrounding muscle fibres?
So we can know when the muscle is being stretched involuntarily
Little intrafusal fibers are innervated by
Gamma-motor neurons
Normal muscle fibers (extrafusal) are innervated by
A-alpha fibers
Annulospiral terminals detect
Speed of stretch
(Ia fibers are fired when the change is fast)
Flower spray terminals detect
Degree of stretch
(type II are fired mostly when stretch degree changes a lot)
Golgi tendon organ - location, detects
Located in the tendons
Detects the degree of tension (not stretching)
Golgi tendon organ - fibers
Type Ib fibers
Golgi tendon organ - mechanism of action
Stimulus conveyed by Ib fibers = inhibitory reflex -> protect us from breaking tendons.
Too much tension = muscle relaxation (inhibitory reflex)
Golgi tendon organ - functions
Proprioceptive
Protective
Joint receptors - detect
Detect position of the joint: whether the joint is extended, flexed,..
Joint receptors - fibers
Type II, III and IV fibres
Proprioperception is sensed by
Muscle spindles
Golgi-tendons
Joint receptors
Skin receptors
Are neuronal endings (= neuron that enters spinal cord)
Soma is always located in the dorsal root ganglion
Receptor field
Area of skin covered by a single neuron. However, there is some overlap between the receptive fields of different neurons = safety mechanism
Spatial resolution is
Degree of precision when detecting where we are being touched.
Higher resolution =
+ neurons with smaller receptive fields (ex: in fingertips)
Less resolution =
Lower nº of neurons that cover bigger receptive fields (ex: back)
Mechanoreceptors in glabrous skin
Meissner corpuscles
Mechanoreceptors in skin with hair
Hair follicle receptors, detect movement and direction
Mechanoreceptors in glabrous skin and skin with hair
Pacinian corpuscles
Ruffini corpuscles
Merkel’s receptors (glabrous) / Tactile discs (hairy skin)
Pacinian corpuscles - detect, location
Detect vibration
Located very deep in skin
Ruffini corpuscles - detect, location
Detect degree of stretch of the skin over the joint
Located in joints
Merkel’s receptors / Tactile discs detect
Sustained pressure
Hair follicles and Meissner detect
Fast pressure
Classification of mechanoreceptors - relation nº receptors and receptor field size
Large receptive field
Small receptive field
Large receptive field mechanorecept - stimuli, receptors + nº
Stimuli we don’t need to precisely localize
- Vibration —> Pacinian corpuscles
- Stretch —> Ruffini corpuscles
Requires little nº of receptors
Small receptive field mechanorecept - stimuli, receptors + nº
Stimuli for which we need precision
- Fast pressure —> Meisner / hair follicle receptor
- Sustained pressure —> Merkel’s receptor / Tactile discs
Requires large nº of receptors
Classification of mechanoreceptors - speed of adaption
Fast adaption (phasic)
Slow adaption (tonic)
Fast adaption (phasic) mechanoreceptors - stimuli
Fast pressure (Meissner)
Vibration (Pacinian corpuscles)
Slow adaption (tonic) mechanoreceptors - stimuli
Sustained pressure (Merkel’s / Tactile discs)
Stretch (Ruffini corpuscles)
Small receptive field (I), fast adapting (RA) mechanoreceptors
RA I
Meissner
Hair-follicle receptors
Small receptive field (I), slow adaption (SA) mechanoreceptors
SA I
Merkel’s / Tactile discs
Large receptive field (II), fast adapting (RA) mechanoreceptors
RA II
Pacinian corpuscles
Large receptive field (II), slow adaptation (SA) mechanoreceptors
SA II
Ruffini’s corpuscles
RA I detect
Superficial pressures
SA I detect
Shape
RA II detect
Vibration
SA II detect
Stretch
Thermoreceptors
Cold receptors: 5-30/34 ºC
Hot receptors: 32-45ºC (+ 45ºC = pain, burns)
Both tonic (constant Tª) and phasic (changes in Tª)
Drastic change = physically - after few seconds = firing rate normalizes
Nociceptors (pain receptors)
Tª receptors —> A-delta fibers, C fibers
Mechanical stimuli —> A-delta fibers
Polymodal fibres —> C fibers
Silent (visc) receptors —> nerve endings (not pain, feel sick)
Tº + pain (+ subst) —> transient recept potential (TRP fam)
Pain is
A perception, NOT A STIMULUS
