Proprioceptors Flashcards
How does muscle spindle distribution differ in body?
Muscles necessary for fine movements contain more than muscles used for coarse movements.
Muscle spindles X to contractile fibres. And what shaped?
Parallel. Fusiform shaped.
What numbers of intrafusual fibres are in a typical muscle spindle?
1 dynamic NB, 1 static NB, around 5 NC.
What is the structure of nuclear chain fibres and what information do they provide?
Nuclei aligned in single row. Static length.
What is the structure of static and dynamic nuclear bag fibres and what information do they provide?
Static NB: nuclei in middle bundle, static length.
Dynamic NB: nuclei in middle bundle, rate of change of length.
What intrafusal fibres do Ia afferents connect to? What endings do they have? What do they encode?
All types. Wrap around centre.
Annulospiral endings.
Stretch and velocity of stretch (for dynamic NB).
Fires at high rate during stretch to encode velocity. When not changing firing decreases, encodes new length.
What intrafusal fibres do II afferents connect to? What endings do they have? What do they encode? What are the dynamics of their FR?
Nuclear chain and static NB. Innervate their ends.
Flowerspray endings.
Muscle length.
FR increases steadily as stretched.
Why is a hyper/hypoactive stretch reflex an important clinical sign to localise neurological damage?
Because they can be modulated by descending tracts.
What may lack of stretch reflex suggest? (2)
Peripheral nerve damage, can’t transmit, as in Guillain-Barre syndrome- autoimmune attack on peripheral nerves.
Spinal cord injury, alpha MNs destroyed or compressed, muscle doesn’t receive.
What may UMN lesion lead to? Why? When may this happen?
Hyperreflexia. Lack of descending inhibition from brainstem through CST. Stroke.
Describe the mechanism of the stretch reflex.
Pattelar tendon hit, quadriceps stretches. Muscle spindles stretch, activating mechanically-gated Ia afferent neurons. Sends APs to dorsal root of spinal cord, monosynaptic connection to alpha MNs which synapse with same (agonist) muscle. Excites extrafusal fibres. Muscle contracts to resist further stretching.
How did Lloyd (1946) show that the reflex is monosynaptic.
Used intracellular recoridngs to measure the time taken for the reflex to occur. 0.7msec to reach extensor afferent, time for one synaptic connection.
In what sense can the myotatic reflex aid posture?
If standing upright and begins to sway left, leg and torso muscles stretched, activating reflex to counteract sway.
What is protective role of stretch reflex?
Prevents injury by preventing overstretching. Reduces risk of muscle tears.
Why must opposing muscle be inhibited?
To prevent it working against the resulting contraction of the homonymous muscle.
Without, both groups may contract simultaneously.
How does reciprocal inhibition work?
Inhibitory interneuron in SC. Ia afferent bifurcates, one branch innervates alpha while other innervates Ia inhibitory interneuron. Interneuron innervates alpha neuron that synapses onto antagonistic muscle.
What happens without recurrent inhibition? What is some evidence for this?
Continuous high frequency firing could lead to muscle fatigue. Maintains efficient motor function over time.
Hyperreflexia.
Amyotrophic lateral sclerosis, loss of lower and upper MNs -> muscle weakness and paralysis. Loss of muscle control. Wootz et al (2014) found loss of presynaptic input to Renshaw cells occurs at early stages of ALS, disconnecting inhibitory circuit, diminshed control of MN firing.
How do descending supraspinal pathways inhibit the stretch reflex?
Suppress activity of gamma MNs, reducing spindle sensitivity.
How does CST inhibit stretch reflex synaptically vs postsynaptically?
PreS, modulates Ia afferent terminals reducing NT release.
Posts, exciting Renshaw cells.
What evidence shows effect of supraspinal inhibition of the stretch reflex?
Decerebrate cats, Sherrington, brainstem sectioned between superior and inferior colliculus, removes cortical influences. Limbs showed increased reflexes and spastic paralysis, hypertonus.
How can gamma MN activation selectively increase sensitivity of stretch reflex?
Gamma activation contracts intrafusal fibres. This prevents them slackening, maintains sensitivity to unexpected changes in stretch.
What descending tract aside from CST (voluntary movement) can affect the stretch reflex and how?
Reticulospinal tract (arousal) can excite gamma MNs, increasing spindle sensitivity.
GTO is located X with the muscle and sends information about … It is a what?
In series. The load/force being applied to the muscle. Capsule containing collagen fibres.
What are GTOs innervated by? What happens when GTO stretched? What are the dynamics of their FR?
Group Ib fibres, specialised endings which weave inbetween collagen fibres.
Stretch causes collagen fibres to squeeze, distorting membranes of primary afferent endings.
Fire APs proportional to amount of force.
How does the autogenic inhibition reflex work?
Ib afferents project to Ib inhibitory interneurons in spinal cord. Inhibits alpha MN that innervates homonymous muscle. Activation of Ib afferent thus ceases contraction.
What is the function of the autogenic inhibition reflex?
Protect muscle from excessive force? This suggested because early work showed GTO only activated with large amounts.
But more recent evidence suggests GTO more sensitive to lower.
Thus maybe more involved in motor control under normal conditions.
Maybe acts to spread amount of work evenly across muscle.
What reflex is initiated by non-proprioceptors?
Flexor reflex. Initiated by cutaneous receptors and pain receptors. Withdrawal before pain, hot stove.
How does flexor reflex work? Foot example.
Sharp object touching foot activates group III pain afferents. Branch innervates excitatory interneuron in lumbar region of SC. Excites alpha MN to contract thigh flexor muscle, another branch excites interneuron to MN exciting hip flexor muscle. Draws whole leg away from stimulus. Co-ordinated activity of two muscle groups.
What does the flexor reflex show?
That spinal reflexes don’t just work on a single joint, co-ordinate activity of multiple joints simultaneously.
Why is crossed-extensor reflex necessary?
Left leg must simultaneously extend when flexor reflex activated in right leg to support the body weight that would have been supported by the right. Aids balance.
How does the crossed extensor reflex work?
Branch of III afferent innervates excitatory interneuron that sends axon across midline to contralateral SC. Excites alpha MNs that innervate extensor muscle of opposite leg.
What is the plantar reflex?
Run stick along base of foot, flexion of toes towards stick, protective, pushing away from stimulus.
What is the abnormal plantar reflex? What does it indicate (in adults).
Extension of big toe and fanning out. CST lesion because lack of descending control.
Why do infants exhibit positive Babinski sign?
Immaturity of descending tracts.
