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Flashcards in Muscle stretch reflex Deck (37):

What is proprioception

Sense of body position/movement compromising static (i.e. joint position) and dynamic (limb movement) sensing components


Why is proprioception important

Locomotion and balance


Input from receptors in tendons/muscles integrated by CNS for...

Spinal reflexes involving interneurones and motor neurones
Unconscious control of movement e.g. for posture by cerebellum
Conscious perception of location and movement (via dorsal column system) by cortex area 3a S1 --> projects to primary motor cortex for voluntary movements


Describe structure of muscle spindle

Small (2-4mm), encapsulated and spindle shaped. Compromise intrafusal fibres within fibrous capsules attached to connective tissue


Describe position of spindles relative to extrafusal fibres, why is this significant

In parallel, signal stretch


What are the 3 components of muscle spindle

Large diameter myelinated sensory neurone wrapped around intrafusal fibres --> compressed as intrafusal fibres stretch
small diameter gamma motor nerve fibres innervating contractile ends of intrafusal fibres
Central part of intrafusal fibres contains nuclei (non contractile)


What are two anatomical types of intrafusal fibres

Nuclear chain fibres (more common)--> variable number per spindle, nuclei arranged in single row
Nuclear bag fibres - nuclei collected in bundle in middle of fibre


How do anatomical intrafusal fibre types relate to two functional types of fibres

Static non/slowly adapting --> measure length of spindle at any moment --> constantly send AP int spinal cord. All nuclear chain fibres static, some nuclear bag fibres are
Dynamic rapidly adapting fibres : measure rate of change of length , nuclear bag fibres only


What are the two types of sensory axons wrapped around middle of all fibres

Primary type 1a
Secondary type 2


Why when muscle stretches does firing rate of sensory axons wrapped around them increase

Mechanoreceptors in axon endings open, generating depolarising receptor potential proportional to amplitude and velocity of stretch


Describe structure and function of type 1 a afferent axons

Largest diameter myelinated fibres with fastest conduction velocity (80-120m/s) in body--> convey static and dynamic information


Describe structure and function of type 2 sensory axons

Myelinated but thinner and slower (35-75 m/s) convey static info only i.e. spindle/muscle length


Describe what adapting and non adapting discharge are good for

Adapting discharge: signalling dynamic stretch
Non adapting: signalling current state of stretch


What is the stretch/tendon reflex, why is it necessarry

Muscle contractions in response to stretch, important when standing upright (tonic stretch reflex for posture) and holding heavy objects --> muscle contract t correct deviation from intended position


What was Sherrington's discovery in 1906

Cut dorsal roots to show sensory afferents from muscle spindles needed for sensory feedback in stretch reflex and without it muscle goes flaccid


How does tendon vibration lead to illusion of specific limb movement

Selectively activates spindle afferents leading to illusion of specific limb movement


Muscle spindle afferents enter dorsal horn and branch repeatedly,describe path of 1a afferents

Form excitatory monosynaptic connections with almost all alpha motor neurones innervating homonymous muscle containing that spindle and synergist muscles
Excite 1a inhibitory interneurones which inhibit alpha motor neuron innervating antagonist muscle


Stretch reflex across several motor pools to ensure...

locally coordinated response i.e. synergistic muscles across joint


Describe experiment by Llloyd 1946 and Eccles 1950

Took recordings after stimulating extensor and flexor afferents for antagonistic muscles --> for extensor: 0.7 msec after stimulation there was EPSP in alpha motor neurone --> single synapse. Flezor: 1.6 msec delay before IPSP in alpha motor neurone --> suggests inhibitory interneuron in pathway


What is short latency (M1) component of stretch reflex involved in

Axial and proximal muscle control


What is long latency (M2)component of stretch reflex involved in

Finger proprioceptors to dorsal column --> VPL --> S1 ---> Motor cortex --> CST --> hononymous motor neurone


What does long latency component of stretch reflex do

Mediates fine voluntary distal limb movements


What does lesion of sensory/alpha motor neurone do

Impairs reflex arc so flaccid paralysis


What does lesion of suppressive descending pathway to do reflex arc

Hyperactive reflex and spastic paralysis


What is the role of gamma efferent control

Motor commands coactivate alpha and gamma neurones so extrafusal and intrafusal fibres contract simultaneously --> prevent muscle spindle going slack and muscle so remains sensitive to muscle stretch


What do gamma motor neurones innervate

Contractile ends of intrafusal fibres


How can stretch reflexes be modified

CST: signals voluntary movement to inhibit stretch reflex
Vestibulospinal tract: engages extensor antigravity muscles
Reticulospinal tract: modulates reflex intensity via gamma motor neurones


Describe location of golgi tendon organs

In muscle tendon junctions in series with extrafusal fibres so signal tension produced by muscle contraction


Describe innervation of golgi tendon organs

Innervated by type 1b afferents, entwinted with collagen fibres


What is the golgi tendon reflex

Muscle contractions --> tension on collagen fibres icrease --> 1b afferent firing increases --> enters via dorsal horn ---> branches forming excitatory synpse onto 1b inhibitory interneurone in ventral horn --> inhibits alpha motor neurone of homonymus muscle --> agonist muscle relaxes


What is the main role of golgi tendon

Regulate muscle tension (sensitive to small changes) within optimal range to facilitate fine movements e..g handling objects with steady grip


How can golgi tendon reflex protect muscle in extreme cases

From overstretch if excessive load placed on muscle


How do 1b inhibitory interneurones show state dependent reflex reversal

Through receiving inputs from other primary sense axons and descending pathways...1b afferents inhibit extensor motor neurones at rest but excite them during walking by recruiting alternative excitatory interneurone pathway


How do joint mechanoreceptors contribute to proprioception

Proprioceptive axons found in connective tissue of joints, esp ligaments and capsules
Rapidly adapting encoding changes in angle/velocity of moving joint
Joint afferents respond at extreme angles of joint flexion not midrange


What is crossed extensor reflex

At end of swing phase, interrupt motor neurone and engage swing phase due to painful stimulus. Inhibit ipsilateral extensor and engage ipsilateral flexor --> contralateral leg flexors inhibited so extensors maintain posture --> contralateral leg begins swing phase


What is Babinski response

Normally, Flexor withdrawal effect to assess if higher control intact --> usually flexion via CST, of toes as you stroke sole of foot. But extension of toe shows CST lesion = Babinski sign (upper motor neurone lesion)


Why don't kids show Babinsky response

Immaturity of CST tract