Muscle receptors and spinal reflexes Flashcards
What is proprioception
The sense of body position in space based on specialised receptors in the muscles and tendons
What are the 2 types of proprioception
Static (joint-position) and dynamic (kinaesthesia, limb movement)
What 3 ways is info from muscle receptors integrated by the nervous system
Input for spinal reflexes, subconscious control of movement via the cerebellum, conscious proprioception via the dorsal column system and cerebeal cortex
What are the 2 components of the stretch reflex
Short latency component M1
Long latency component M2
What is the knee jerk stretch reflex called
The quadriceps patellar tendon reflex
What are the 4 types of mechanoreceptors involved in mechanoreceptors
Muscle spindles and Golgi tendon organs, joint mechanoreceptors, skin mechanoreceptors
What are the 2 main types of muscle receptor
Muscle spindles and Golgi tendon organs
What do the muscle receptors tell the CNS
The relative positions of body parts
What do muscle spindles signal
Stretch
What do Golgi tendon organs signal
Tension produced by muscle contraction
What are 2 types of joint mechanoreceptors
Larger fibres signal joint position
Smaller Aδ fibres are most active at the extremes of movement and are protective
What do skin mechanoreceptors signal (proprioception)
Postural information, speech/facial expression
What does muscle contain along with muscle spindles and GTOs
Nociceptors involved in pain perception
Where are muscle spindles located
In the fleshy part of muscles, in parallel with extrafusal fibres and attached to muscle connective tissue
Where are Golgi tendon organs located
In tendons at the ends of a muscle, in series of extrafusal fibres
What is the shape/size of muscle spindles
Small(2-4mm long), encapsulated, spindle-shaped
What are the 3 main components of a muscle spindle
Intrafusal muscle fibres
Sensory nerve fibres
Gamma motor nerve fibres (axons)
What sections of intrafusal muscle fibres are non-contractile vs contractile
Central part- non-contractile, contains the nuclei
Ends- contractile
Describe the sensory nerve fibres in the muscle spindle
Large diameter, myelinated, wrap around the non-contractile centre of intrafusal fibres, terminals sensitive to stretch of the intrafusal fibre
Describe the gamma motor nerve fibres in the muscle spindle
Small diamater, innervate the contractile ends of intrafusal fibres
What are the 2 types of intrafusal fibres in muscle spindles
Nuclear chain fibres
Nuclear bag fibres
How many nuclear chain fibres vs nuclear bag fibres are in each spindle
NC- variable no per spindle
NB- 2-3 per spindle
What are nuclear chain fibres
Nuclei aligned in a single row in the centre of the fibre
What are nuclear bag fibres
Nuclei are collected in a bundle in the middle of the fibre
What are the two FUNCTIONAL types of intrafusal fibres in muscle spindles
Static aka non-adapting
Dynamic aka rapidly-adapting
What type of intrafusal fibres are static
ALL NUCLEAR CHAIN FIBRES ARE STATIC
Nuclear bag fibres CAN be static
What type of intrafusal fibres are dynamic
ONLY NUCLEAR BAG FIBRES- their contractile ends are more viscous, so stretching occurs in the central nuclear part
What do static intrafusal fibres measure
LENGTH of the spindle at any instant
What do dynamic intrafusal fibres meaure
RATE OF CHANGE of length of spindle
What are the 2 types of sensory afferent nerves at the muscle spindles
Aa afferent, group II afferent
What do the Aa afferent nerves do at muscle spindles
Record from the centre of ALL fibres, so report dyamic and static info
Fastest fibres!
What do the group II afferent nerves do at muscle spindles
Record from static bag and chain fibres only, so report primarily static info ab spindle/muscle length
Gamma motor nerves- dynamic or static?
Separate gamma motor nerves supply dynamic and static intrafusal fibres to modulate their activity
What did Sherrington (1906) demonstrates about the stretch reflex by cuttin the dorsal and ventral roots
The stretch reflex is not an intrinsic property of the muscle, but requires sensory input and a motor path to the muscle
What are spinal reflexes
Reflexes with neural circuitry fully contained in the spinal cord, that receive direct sensory info from the muscles/joints/skin
What does the stretch reflex act to do
Counter stretching and change in muscle length
How do afferent sensory fibres sense stretch in the intrafusal fibres
Stretch channels in their terminals open, altering their ionic current
Cell is depolarised and generates APs, signalling stretch
Which type of sensory afferent fibre is involved in the stretch reflex
Group Ia myelinated afferent fibres
What are the 3 types of alpha motor neurons involved in the stretch reflex
Those supplying the homonymous muscle, those supplying synergist muscles, those supplying antagonist muscles
What joins Ia afferents to alpha motor neurons supplying antagonist muscles in the stretch reflex
An inhbitory interneuron
Stretch reflex- what happens to the quadriceps muscle when you tap the knee tendon
Deforms the tendon, causing limb extension that stretches the quadriceps muscle
Stretch reflex- what happens when the quadriceps muscle stretches
Intrafusal fibres within muscle spindles detect stretch
Ia afferent terminals detect this stretch so Ia afferents increase firing
Stretch reflex- what do Ia afferents branch to form excitatory synapses with in the spinal cord
ALL the motor neurons innervating homonymous muscle
60% of the motor neurons innervating synergist muscles
Inhibitory interneurons for antagonist motor neuron
Stretch reflex- what is the effect of the excited homonymous and synergist alpha motor neurons
Cause contraction of the quadricepts muscle and synergist muscles -> lower leg swings forward
Stretch reflex- what is the effect of inhibited antagonist alpha motor neurons
The antagonist hamstring muscle relaxes, so it doesn’t oppose the action of the quadriceps muscle, allowing leg extension
Stretch reflex- what is reciprocal inhibition
The simultaneous excitation of the stretched muscle (and its synergists) and inhibition of antagonist muscles
What type of loop is the stretch reflex
A negative feedback loop
What is stretch reflex component M1
Strong and fast, predominantly controlled by monosynaptic connection in spinal cord, involved with axial and proximal muscle control
What is stretch reflex component M2
Slower but longer lasting (shorter than reaction time), involves the cerebral cortex, involved with fine voluntary distal limb movements,
What descending tracts modulate the stretch reflex
Corticospinal tract, vestibulospinal tract, reticulospinal tract
How does the corticospinal tract modulate the stretch reflex
Inhibits stretch reflexes to allow voluntary movement
How does the vestibulospinal tract modulate the stretch reflex
Primarily engages extensor antigravity muscles
How does the reticulospinal tract modulate the stretch reflex
Modulates intensity of reflex activity via gamma motor neurons
What cells are engaged by recurrent processes
Renshaw inhibitory interneurons- inhibit a muscle that is firing repeatedly, preventing fatigue
What are functions of the stretch reflex resisting stretch
Posture, holding a heavy object, muscle tone
What maintains muscle spindle sensitivity during voluntary movement (primarily contraction)
Alpha-gamma motor neuron coactivation
This is the simultaneous activation of alpha and gamma motor neurons
How does dynamic vs static gamma motor neuron activity differ across movement type
Gamma static activity- slow predictable movements
Gamma dynamic activity- rapid unpredictable movements
What happens to the stretch reflex in Klipper-Feil syndrome
The M2 component also occurs on the contralateral side of the body
What is muscle tone
Resistance of the muscle to stretch
What is the result of lack of descending control of reflex pathways
Hyperactive stretch reflex, meaning hypertonic (spastic) muscle
Sherrington- what is the result of removing cortical but not brainstem locomotor influences in decerebrate cats
Limbs showed increased reflexes and spastic paralysis- hypertonus
How are Golgi tendon organs innervated by a sensory afferent
A single Ib afferent fibre interweaves with the collagen fibres inside the GTOs at the muscle-tendon junction
What is the effect of muscle contraction on Ib afferent fibres in golgi tendon organs
Muscle contraction causing tendon tension, straightening of the collagen bundles compresses and stimulates the Ib afferent nerve
Functinos of GTOs
Clasp-knife reflex, prevents overstretching
Graded response to small changes in tension means involvement in fine movement
Modulate reflex pathways
How do Golgi tendon organs afferent signals contribute to reflex pathways
GTO Ib afferents provide feedback inhibition onto homonymous motor neurons via a spinal interneuron
How is GTO function protective in providing feedback inhibition of reflex pathways
If excessive load is placed on a muscle, Golgi-tendon senses tension caused by muscle contraction- clasp-knife reflex causes relaxation, protecting muscle
What is state-dependent reflex reversal
Ib afferents inhibit extensor motor neurons during rest, but during locomotion they recruit an alternative EXCITATORY interneuron path to excite them
What do leg extensors vs flexors do during locomotion
Extensors- recruited during stance phase, ‘anti-gravity’ function
Flexors- recruited during swing phase, voluntary movement
What is the flexor withdrawal reflex
Protective reflex pathway that pulls a damaged limb away from a painful stimulus
How does the flexor withdrawal reflex work
If we stand on a painful thing, sensory neuron collaterals in the spinal cord activate ascending pathways for pain and postural adjustment, and cause contraction of flexors / inhibition of extensors that move the foot away from the stimulus
What reflex occurs at the same time as the flexor withdrawal reflex
A contralateral ‘crossed-extensor’ supporting reflex- contralateral extensors contract and flexors are inhibited as weight shifts to contralateral leg, supporting the body
What is the Babinski response
Flexor withdrawal reflex, elicited by stroking the sole of the foot forward along its lateral aspect and across the ball, causing flexion of the toes to push away the stimulus
What is an abnormal response to the Babinski response
Extension of the big toe and fanning out of the other toes, a sign of an upper motor neuron lesion
What is an exception to the Babinski response being a sign of upper motor neuron lesion
Newborns- extension is the norm due to immaturity of descending tracts
What is the effect of lower motor neuron lesion
Weakness (paresis) or actual paralysis, loss of reflexes, reduced muscle tone and muscle wasting
What is a lower motor neuron lesion
Anything that damages the sensory afferent or lower motor neuron or motor neuron in the spinal cord
What is an upper motor neuron lesion
Anything that damages the corticospinal tract
What is the result of an upper motor neuron lesion
Paralysis, increased stretch reflexes and muscle tone are increased (spastic), less wasting
What is the most characteristic lesion producing paralysis, hyperreflexia and spasticity
Stroke- only occurs in its full form when the cereberal cortex or internal capsule is damaged
What are reflex responses
Rapid, stereotyped motor behaviours (can be modulated )
What are motor neuron pools
A cluster of motor nuclei of all the motor neurons that innervate a muscle
What is the proximal-distal rule
motor neurons innervating the most proximal (axial) muscles are located most medially in the spinal cord VS motor neurons innervating the most distal muscles are located more laterally
What is the flexor-extensor rule
The motor neurons that innervate extensor muscles lie ventral to those innervating flexor muscles
What is a motor unit
The smallest functional unit in the nervous system- a single motor neuron and the several muscle fibres it innervates
What are the 3 types of motor units
Fast fatigable, slow fatigue resistant, fast fatigue ressitant
What are fast fatigable motor units
Fibres contract and relax repeatedly, but fatigue rapidly when stimulated repeatedly
Generate the greatest force during contraction
What are slow fatigue resistant motor units
FIbres have a much longer contraction time, highly fatigue resistant, generate 1-10% of the force of fast fatigable fibres
What are fast fatigue resistant motor units
Intermediate properties, contract slower than fast fatigable fibres but almost as fatigue resistant as slow fatigue-resistant fibres
What is the effect on afferent sensory fibres during muscle contraction
Extrafusal fibres slacken and shorten, so intrafusal fibres are no longer stretched, firing rate in afferent endings decreases
Why do GTOs respond best to muscle contraction
Durnig contraction, muscle fibres pull directly on collagen fibres, mean small stretches of the tendon can deform the nerve endings
What 2 phases make up the change of length o a muscle
Dynamic (length changes) and steady-state (new muscle length is stabilised)
What is the firing rate of Ia and II afferent sensory nerves during the dynamic phase of stretch
Ia endings fire at a much higher rate than during the steady state phase
Group II endings increase their firing gradually, but it is not much higher than in steady-state phase
What is the firing rate of Ia and II afferent sensory nerves durnig the steady state phase of stretch
When a muscle lengthens, both Ia and II endings increase their firing to a higher steady state rate
What type of stimuli do Ia endings respond best to
Sensitive to small changes in muscle length and transient stimuli eg taps or vibration
How do Ia fibres show velocity sensitivity as a muscle shortens also
Ia endings stop firing, then resume at a lower rate when shortening stops
How do Ia fibres respond as muscle length increases
As muscle length increases, firing rate increases- increased firing rate reflects the rate of change aka VELOCITY SENSITIVITY
What is the effect of short transient stimuli on group II sensory afferent endings
No effect- changes in muscle length occur too quickly to alter their steady state discharge
What is the importance of alpha-gamma coactivation
If the spindles stopped firing when muscles contracted, they would not convey any info about length changes when it was critical- gamma motor neurons ensure this info is transmitted
What do gamma motor neurons do in alpha-gamma coactivation
Gamma motor neurons innervate intrafusal muscle fibres while alpha motor neurons innervate extrafusal fibres
Both neurons are stimulated during muscle contraction, meaning the activity of the spindle afferents continues
How does gamma motor neurons stimulating the ends of intrafusal fibres have an effect
The polar contractile regions shorten, which stretches the central region from both ends, stretching the spindle afferent terminals so they can continue firing
How is alpha-gamma coactivatino useful during slow voluntary, precise movements
eg trajectory of finger movement during flexion reflects variation in the rate of contraction, meaning firing of Ia afferents mirrors any irregularities in this trajectory
This info can be used by CNS to compensate for irregularities and smooth movement, fine-tuning the info it receives
Why are stretch reflexes weaker and less stereotyped in intact animals that decerebrate animals
Descending pathways from higher brain areas are continuously modulating the strength of stretch reflexes to meet the requirements of ongoing action
What are synergist musclesq
Muscles that control the same joint and inhibit antagonist motor neurons
What are the components of muscle tone
Intrinsic muscle stiffness, neural, stretch reflex
Functions of muscle tone
Maintenance of posture, allows muscles to store energy like springs for later release eg in walking, smoothing movement
How does muscle tone smooth movement
Muscle elasticity smooths out any jerks in movement starts and stops, allowing the muscles to achieve equilibrium length more gradually
How are most reflexes able to be modified
Most reflex pathways are polysynaptic, with interneurons between the sensory and motor neurons that receive input from multiple sources
What is a myotatic unit
The group of muscles around a joint linked together by a system of reflex pathways
How is reciprocal inhibtion also invovled in voluntary movement
It prevents prime mover muscles having to act against the contraction of opposing muscles
How does the brain inhibit antagonist muscles during volutnary movement
Descending axons from the motor cortex make direct excitatory connectinos to spinal motor neurons AND collaterals to Ia inhibitory interneurons that inhibit antagonist motor neurons
What type of interneuron inhibits antagonist muscles during voluntary movement
Ia inhibitory interneurons in the spinal cord
What type of interneuron inhibits the homonymous msucle during voluntary movement (negative feedback)
Ib inhibitory interneurons
What do Ia inhibitory neurons in the spinal cord receive input from for modulating joint stiffness
Excitatory and inhibitory input from all major descending pathways, allowing the brain to change the balance to control the amount of joint stiffness needed
How do Ia inhibitory interneurons coordinate muscle action
Control co-contraction aka simltaneous contraction of the prime mover and antagonist muscle, stiffens and stabilises the joint
What different sources do Ib inhibitory interneurons receive input from
GTOs, Ia afferents from muscle spindles, cutaneous afferents, excitatory/inhibitory descending input
What sort of mechanism do the Ib inhibitory interneurons provide by receiving input from GTOs
Negative feedback mechanism for regulating muscle tension, parallel to the negative feedback from muscle spindles for regulating muscle length
Explain the negative feedback mechanism controlled by Ib inhibitory interneurons in the spinal cord
Increased muscle tension-> increased GTO activity-> Ib inhibitory interneurons excited->homonymous motor neurons inhibited-> decreased muscle tension
How do converging inputs on Ib inhibitory interneurons allow fine control of exploratory movements
eg when the hand contacts an object, muscle force is inhibited by combined activatino of tendon organs and cutaneous afferents, softening contact
Descending pathways can modulate eg enhancing inhibition for a fragile object
What is local sign
Fixed spatial relationship between the locus of a stimulus and the particular muscles that contract
Example of how stimulus quality affects the reflex effect of cutaneous eg foot
eg light pressure ->generalised extensor response in whole leg
eg painful stimulus- flexor withdrawal
Example of a rhythmic reflex
Scratch reflex in furry animals- leg is brought to stimulus for rhythmic scratching, crossed extension reflex maintains the animal’s standing posture
How is the scratch reflex unique
Fully manifested in animals where the spinal cord had been severed at the cervical level, and still kinda effective when dorsal roots were cut, showing sustained rhythmic alteration of movement doesn’t depend on supraspinal or peripheral input
What is the half-centre model
Describes how a central neural circuit is organised- interneurons controlling flexor and motor neurons have reciprocal inhibitory connections
What are central pattern generators in walking
Local spinal circuits that coordinate the contractino of the muscle groups needed to geenrate stepping, allowing automatic locomotion
Where are central pattern generators located
Innate to spinal circuitry- a cat can still walk on a treadmill if their spinal cord is severed at 1-2 weeks or at the lower thoracic level, suggesting descending input is not needed
What is the swing phase in walking
When foot is off the ground and flexing forwards
Contraction of flexor muscles
What is the stance phase in walking
When the foot is planted and the leg is extending relative to the body
Contraction of extensor muscles
What is the result of holding a cat’s hind leg on walking
The other continues stepping normally in a rhythmic cycle, showing each limb has its own independent pattern generator
What is the relationship between central pattern generators in different limbs
Pattern generators are coupled during normal locomotion- the movements of L and R hind legs in walking cat are exactly out of phase
