Somatosensory System Flashcards
What is the exception of sensory receptors of various sorts
Distributed throughout almost all of the tissues of the body, with nervous tissues as a notable exception
Somatosensory receptor
Any mechanoreceptors, their … or nocireceptor found embedded in the skin, fatty tissue beneath that skin, muscle tissue of musculoskeletal connective tissue (tendon, ligament or joint capsule tissue)
Fibre groups 1,2,3,4 and their conduction speed
Large myelinated 80-120m/s
Medium myelinated 35-75m/s
Small myelinated 5-30m/s
Unmyelinated 0.5-2m/s
Cutaneous mechanoreceptors adaptations
Mechanoreceptors show diff firing patterns to code duration
2 receptors that are slow adapting fibres
2 - Merkel disk receptors, Ruffini end organ
3 fast adapting mechanoreceptors
2 - Meissner corpuscle, Pacinian corpuscle, hair follicle receptor
Slow adapting vs fast adapting mechanoreceptors
Continue to fire as long as stimulus is maintained vs fall silent even when stimulus is maintained
Where are muscle proprioceptors abundant?
Skeletal muscles
What do muscle proprioceptors convey info regarding? 1-2
State of muscle
- muslce spindles - length of muscle and velocity of stretch
- Golgi tendon organs - changes in tension
GTOs act as? Innervated by Structure -2 Location Innervated by Intertwined with?
Golgi tendon organs act as force sensors
IB afferents - intertwined within collagen fibres of tendon
Slender encapsuled structures 0.2-1mm
Junction of muscle and tendon: in series
Ib afferent demylinated
IB afferent is intertwined with collagen fibres of tendon
When do intertwined Ib afferents compress?
In response to muscle force - Ib afferents increases AP firing frequency when muscle force increases
Compressed of the IB afferents
-
Increased muscle force detected by Ib afferent is through 2
Muscle stretch - eccentric contraction/passive stream - when its stretched, most length taken by muslce fibre (more elastic)
Muscle shortening - concentric contraction - force acts directly on tendon when muscle actively contracts - increased tension on collagen fibres compresses sensory receptors of Ib afferent
In what muscle force generation do you see a higher firing of IB?
Shortening contraction
2 responses to Ib afferent responses to muscle stretch.
Increased firing frequency upon tension increase
Sustained firing at new tension level
2 Ib afferent response to muslce shortening
Increased firing frequency upon tension release
Sustained firing at tension level
muscle spindle structures
location
responds to
encapsulated 2-10mm
parallel to extrafusal muscle fibres
stretch and velocity of muslce stretch
muslce spindles contain
oriented
connected to
stretch in accordance with
muscle fibres - intrafusal
parallel to extrafusal fibres
connected at each end to extrafusal fibres
extrafusal fibre stretch
3 main components of muscle spindle
specialized muscle fibres - intrafusal
sensory axons - Ia
motor axons - II - intrafusal fibres have capacity for contraction
are intrafusal muslce fibres smaller or bigger than extrafusal fibres?
smaller
two types of intrafusal fibres and how they are differenciated
nuclear bag and nuclear chain - organization of central nuclei - clustered bags vs in series chains
what are the 2 categories of nuclear bags?
static (BFST)
Dynamic (BFDYN)
how do the 2 categories of nuclear bags differ ?
motor innervation
sensory innervation
contractile properties
sensory innervation of muscle spindles - what are they innervated by? where are they?
Ia and II afferents
spiral around or near central region
Ia - all intrafusal fibres at central regions
II - nuclear chain and static bag only - not dynamic bag
effect of muscle stretch
intrafusal stretch which leads to spindle loading
Spindle loading ?
majority of length change occurs in?
increased AP firing frequency - elongation of afferent ending depolorizes membranes and generates AP in both sensory afferents (Ia and II)
equatorial region so more robust effect (higher firing frequency) in Ia
spindle unloading
concentric contraction - decreased firing frequency - slacking of all activity and eventually stops - dramatically diff afferent (Ia, II) firing pattern compared to stretch
The release of stretch or extrafusal shortening causes intrafusal to slacken
diff in response in GTO with spindle loading and unloading
when muslce stretches, the spindle stretches - more response in spindles, when muslce shortens - the spindles are unladed but sensory neurons in GTO increases
AP firing during spindle unloading
decreases in both sensory afferents - Ia stops firing completely for a period of time until muslce returns to its normal length
Ia firing frequency changes with?
muslce length
muslce spindles are sensitive to?
changes in length -
AP firing Ia and II neurons differ during …
static and dynamic phases of muscle length
AP firing during static phase
when muscle gets stretched or released, both Ia and II afferents reach steady state firing rate that reflexts new muscle length
AP firing during dynamic phase
Ia and II respond differently
Ia more robust during dynamic phase than steady state phase
II firing frequency increases slightly, but without the same vigor as Ia
who has a greater dynamic sensitivity
Ia because it is more sensitive to transient changes in length
Diff in Ia and II firing patterns is due to
innervation sites of these afferents, because the majority of length change happens at the equatorial region, which means most centrally located afferents (Ia) have greater opportunity for detecting length change
what else are Ia afferents sensitive to?
stretch velocity - velocity sensitivity
As velocity increases
Firing frequency increases - dynamic response
steady state firing firing in Ia
reflects resting length
Transient increase in Ia firing state
more sensitive to small length change (<0.1mm) - magnitude of transient increase in firing frequency reflects stretch velocity
Ia encodes 3
static length, dynamic length, rate of length change
position, movement, and velocity of movement
where do motor neurons innervate on intrafusal fibers?
polar regions - fusimotor system
gamma motor system
regulate sensitivity of muslce spindles for detecting stretch
2 types of gamma MN
increased steady state firing - static gamma MN
increased firing state during dynamic phase of stretch - dynamic gamma MN
spindle efferents innervate diff types of intrafusal fibres
dynamic gamma MN - dynamic nuclear bag
static gamma MN - static nuclear bag and nuclear chain
Diff gamma mn provide opportunity to differentially regulate
dynamic or static sensitivity of muslce spindle
innervation of dynamic nuclear bags
junction of polar and central
Region of Ia afferent
equatorial of all fibres
Region of II afferent
near equatorial, nuclear chain and static bag only
gamma dynamic innervates
dynamic nuclear bag
gamma static innervates
static nuclear bag and nuclear chain
how does CNS ensure that info regarding muscle length/rate of length change will be received during muslce contraction if muslce shortening means spindle unloading? - 3
activation of gamma motor neuron
fusimotor system counteracts intrafusal slackening during muslce shortening
simultaneous activation of alpha and motor MN prevents spindle from unloading during muslce shortening
mechanistic foundation of gamma activation - what happens?
gamma activation leads to contraction (shortening) of polar regions which stretches the equatorial ends from both ends.
Vallbo, 1981 - 3
spindle (Ia) discharge is maintained during slow concentric contractions
if gamma MN were not firing, Ia afferent would cease alpha MN
Gamma MN activited in con
Why can gamma MN activity be adjusted independently of alpha MN activity? 2
CNS capable of contracting intrafusal fibres independently of extrafusal fibres
alpha gamma coactivation: MN activated synchronously
gamma activation prior to alpha activation
prestretch central regions of intrafusal fibres and alter their sensitivity
Who regulates the sensitivity of spindles to stretch
gamma MN
influence of gamma MN activity known as 3
gamma bias
gamma gain
fusimotor set
Prochazka, 1988 - regulation of?
How is fusimotor activity set?
spindle sensitivity, fusimotor activity is set according to complexity of behaviour
Prochaska: if activity necessitates slow and predictable changes in muslce length, what MNs are activated?
static gamma
Prochaska: if activity neccesitates rapid and unpredictable changes in muslce length, what MNs are activated?
static gamma and dynamic gamma
Prochaka and Hulliger, 1988
fusimotor set theory
activation of gamma MN increases as requirements for
stretch sensitivity increases
i.e. walking on ice
CNS uses fusimotor system to
prime impending stretch
Gamma bias can be adjusted by 2
upper motor neurons and lower neuron circuits
to maintain constant force -
You can recruit fewer MU and have them fire at a higher frequency, or recruit more MUs and have them fire at a lower frequency
receptive fields can vary in
size and definition
increase of stim intensity
increase AP firing frequency which increases neurotransmitter output - compounded by increasing stim duration
