Lecture 14: Motor Reflexes Flashcards by Spandana Sama

What are examples of spinal reflexes?

Stretch (myotatic) - DTR
Golgi Tendon Reflex
Crossed Extensor

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What are examples of brainstem/midbrain reflexes?

Vestibular
Righting reflex: prevents falling down
Suckle
Yawn
Eye/head movements

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What are examples of cortical reflexes?

Placing reaction

Hopping reaction

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What is the purpose of a reflex?

Protection
Correct action w/o conscious though
Quick Reponse

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What are characteristics of reflexes?

Involuntary
Short-acting
Fast
Precise

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What is the biggest difference between a reflex and volitional movement?

Speed

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What does a myotatic (stretch) reflex do?

Contraction (shortening) of a stretched muscle that protects muscle from tearing due to stretch

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What initiates a myotatic reflex?

Muscle spindle

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What are muscle spindles embedded in?

Fusiform capsule

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How are intrafusal fibers organized?

Outer component: motor

Inner: sensory

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What is the sensory portion of the muscle spindle sensitive to?

Sensitive to length

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What are the two sensory parts of the muscle spindle?

Nuclear Bag and Chain Fiber

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What do primary Ia fibers innervate?

Both Nuclear Bag and Chain

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What are primary fibers sensitive to?

Length of muscle

2. How fast length of muscle is changing

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What happens when intrafusal fibers stretch?

Increase in AP

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What do secondary Group IIa fibers innervate?

Nuclear chain only

What are secondary Group IIa fibers sensitive to?

Length only

What innervates the motor portion of intrafusal fibers?

Gamma motor neuron

What does motor portion control?

Length of sensory portion

What happens when intrafusal fibers contract?

Sensory portion is stretched and increases sensitivity to Ia and II fibers

What is the difference between alpha and gamma motor neurons?

Alpha: activates extrafusal fibers
Gamma: controls intrafusal fibers

In the spinal cord, what does the Ia fiber synapse on?

Alpha motor neuron innervating stretched muscle

How do antagonist muscles relax in a reflex?

Ia fibers synapse on an interneuron which releases an inhibitory NT that relaxes antagonist muscle

What is a golgi tendon reflex?

Abrupt relaxation of a contracted muscle to protect it from damage

How many synapses are there in a golgi tendon reflex?

Polysynaptic (at least 2)

What fiber is used in a golgi tendon reflex

Ib afferent fiber to spinal cord

How does the golgi tendon reflex work?

1. Contracted muscle sends signal to spinal cord via 1b fiber 2. Interneuron releases GABA onto alpha motor neuron 3. Contracted muscle relaxes

What is a spinal shock?

With injury to the spinal cord, there is a combination of areflexia/hyporeflexia and autonomic dysfunction

What happens to alpha motor neurons in a spinal shock?

Hyperpolarization of spinal neurons due to loss of excitatory input from cortex

Why is there clonus and more expressive DTR when there is injury to the brain?

With cortical damage, there is less inhibition of gamma motor neurons, so there is unopposed excitation of the muscle fibers. With an increase gamma motor neuron firing rate, there is more contraction of intrafusal fibers, giving the more expressive DTR

What is decorticate posturing? How does it occur?

Damage in upper midbrain that causes rigidity | Stroke in internal capsule

What is decerebrate posturing? How does it occur?

Severe brain injury caudal to red nucleus and rostral to pons that causes contraction of all anti-gravity muscles (arms are in extension)

What is the difference between rigidity and spasticity?

Spasticity: Patient resists a passive stretch of muscles due to increased gamma motor neuron firing Rigidity: Patient contracts muscle in absence of stimuli due to continually active alpha motor neurons