0519 spinal reflex and muscle control

Question 1

What is the function of alpha motor neurons? Define a motor unit and motor neuron pool

Answer 1

Alpha motor neurons generate force by signalling contraction of muscles (extrafusal). A motor unit = motor neuron + fibres it innervates. A motor neuron pool = all motor units of a single muscle

Question 2

What are the 2 different ways alpha motor nuerons control the grade/level of contraction of a muscle

Answer 2

The frequency of NT release (twitch summation) and the level of recruitment (number of contracting fibres)

Question 3

Define upper motor neurons and lower motor neurons

Answer 3

Upper motor neurons originate in cerebral cortex and command lower motor neurons. Lower motor neurons originate in ventral horn (spinal cord) and control somatic muscles

Question 4

List the 3 types of inputs alpha neurons receive and where they come from

Answer 4

Command inputs (from upper motor neurons), Sensory input (from afferent muscle spindles) and internueron input (from spinal cord)

Question 5

What is the function of proprioreceptors

Answer 5

Proprioreceptors provide information on location and position in space of muscles

Question 6

List the 3 types of proprioreceptors and brielfy outline their functions

Answer 6

Muscle spindles (info muscle stretch/length), Golgi tendon organs (joint position and forces on tendon) and mechanoreceptors (angle, velocity and movement of joints)

Question 7

What are the 2 types of muscle fibres that alpha motor neurons innervate? What is the size principle

Answer 7

Slow twitch (smaller, core postural) and fast twitch (larger, distal muscles). The size principle is recruitment of motor units based on size (smaller ones first)

Question 8

What type of fibres do alpha motor neurons and gamma motor neurons innervate/contract ? What is the function of each

Answer 8

Alpha motor neurons innervate extrafusal fibres which produce force. Gamma motor neurons innervate intrasual fibres which contain muscle spindles for proprioreceptive feedback)

Question 9

Describe the role of gamma motor neurons

Answer 9

Gamma motor neurons innervate and contract intrafusal muscle fibres which contain muscle spindles. It keeps the muscle spindles at the right length as the extrafusal fibres to ensure proprioreceptive feedback to CNS

Question 10

Describe the Gamma Loop?

Answer 10

The gamma loop is muscle contraction indirectly via gamma motor neurons. CNS sens info down and fires the Gamma motor neuron. Contraction of intrafusal musclefirest sensory neuron back to spinal cord which triggers an alpha motor neuron to fire (contraction, similar to monosynaptic reflex)

Question 11

What is the function of goli tendon organs? Do they act through excitatory or inhibitory synapses

Answer 11

Golgi tendon organs act as sensors of load/force on tendons and allow us regulate contraction via inhibitory interneurons

Question 12

Explain how a monosynaptic reflex occurs

Answer 12

A monosynaptic reflex is a single synapse between an sensory, afferent neuron which synapses at the spinal cord with a motor, efferent neuron

Question 13

Explain how a polysynaptic reflex occurs

Answer 13

A poly synaptic reflex has 2 or more synapses. A sensory, afferent neuron synapses with an interneuron which synapses with both an upper motor neuron and a motor, efferent neuron

Question 14

Explain how a muscle spindle can evoke a reflex

Answer 14

A stimulus (stretch in muscle) fires the sensory receptors in the intrafusal fibres. This relays directly with an alpha motor neuron in the spinal cord (monosynaptic) to contract (coutner-act) the muscle and inhibit antagonistic muscles)

Question 15

Explain a withdrawal reflex

Answer 15

A stimulus (sharp nail) fires a sensory neuron which synapses with an interneuron in spinal cord (polysynaptic). This interneuron 1) excites flexor muscle and inhibits extensor muscle of affected limb, and 2) excites extensor and inhibits flexor of other limb for balance