Somatosensory System I Flashcards by María Rodríguez Lebrón

Includes the sense of

Touch
Proprioceptive sensation
Positional sensation
Vibratory sensation
Fine touch or epicritic touch
Crude touch or epicritic touch
Temperature
Pain

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PC-ML system perceives

Vibration, kinesthesia, fine touch and spatial discrimination through mechanoreceptors and proprioceptors

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Anterolateral system perceives

Temperature, pain (fast and slow), and crude touch

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Sensory info

Reaches the thalamus - synapses w/ next neuron - project to contralateral cortex (somatotopic organization)

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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

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Sensations and types of fibers that it uses

Golgi tendons (proprioception) —> Ib
Muscle spindles —> Ia / II
Pain and temperature (ALS) —> III, IV

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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

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Types of muscles fibers in muscle spindles

Extrafusal
Intrafusal

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Extrafusal muscle fibers

Make force

Slow: type I
Fast: type IIa, IIb

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Intrafusal muscle fibers

Part of sensory organs
Proprioperception in muscles (house muscle spindles)

Nuclear bag intrafusal fiber
Nuclear chain intrafusal fiber

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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

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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.

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The muscle spindle involves

terminals surrounding actual muscle fibres

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The muscle spindle involves terminals surrounding actual muscle fibres, and therefore:

Muscle stretched —> spindle stretched
Muscle contracted —> spindle contracts (can be detected)

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Why are the nerve terminals surrounding muscle fibres?

So we can know when the muscle is being stretched involuntarily

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Little intrafusal fibers are innervated by

Gamma-motor neurons

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Normal muscle fibers (extrafusal) are innervated by

A-alpha fibers

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Annulospiral terminals detect

Speed of stretch
(Ia fibers are fired when the change is fast)

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Flower spray terminals detect

Degree of stretch
(type II are fired mostly when stretch degree changes a lot)

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Golgi tendon organ - location, detects

Located in the tendons
Detects the degree of tension (not stretching)

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Golgi tendon organ - fibers

Type Ib fibers

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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)

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Golgi tendon organ - functions

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Proprioceptive
Protective

Joint receptors - detect

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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

Somatosensory System I Flashcards

(55 cards)