Lecture 14: Motor Reflexes Flashcards
What are examples of spinal reflexes?
Stretch (myotatic) - DTR Golgi Tendon Reflex Crossed Extensor
What are examples of brainstem/midbrain reflexes?
Vestibular Righting reflex: prevents falling down Suckle Yawn Eye/head movements
What are examples of cortical reflexes?
Placing reaction Hopping reaction
What is the purpose of a reflex?
Protection Correct action w/o conscious though Quick Reponse
What are characteristics of reflexes?
Involuntary Short-acting Fast Precise
What is the biggest difference between a reflex and volitional movement?
Speed
What does a myotatic (stretch) reflex do?
Contraction (shortening) of a stretched muscle that protects muscle from tearing due to stretch
What initiates a myotatic reflex?
Muscle spindle
What are muscle spindles embedded in?
Fusiform capsule
How are intrafusal fibers organized?
Outer component: motor Inner: sensory
What is the sensory portion of the muscle spindle sensitive to?
Sensitive to length
What are the two sensory parts of the muscle spindle?
Nuclear Bag and Chain Fiber
What do primary Ia fibers innervate?
Both Nuclear Bag and Chain
What are primary fibers sensitive to?
- Length of muscle 2. How fast length of muscle is changing
What happens when intrafusal fibers stretch?
Increase in AP
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?
- 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
