Spinal Cord, Brainstem, Cortex Control Of Motor Function I

Question 1

Name the characteristics of motor units and motor neuron pools?

Answer 1

Single motor neuron and the muscle fiber it innervates, composed of extrafusal fibers

Motor neuron pool - group of motor neurons that innervate fibers within the same muscle

Question 2

What is the funtions of interneurons?

Answer 2

Small and highly excitable

Capable of spontaneous activity

Responsible for most of spinal cord integrative function

Question 3

What is a renshaw cell?

Answer 3

Inhibitor cells in anterior horns of spinal cord - which recieve collateral branches from alpha motor neurons and transmitt inhibitory signals to surrounding motor neurons

Lateral inhibition

Purpose: To make movement more fluid

Inhibits the neuron that fires it.

Question 4

Small motor neurons Vs Large motor neurons

Answer 4

Smn - Innervate a few muscle fibers, have low thresholds and fire first with small force

Lmn - innervate many muscle fibers, have high thresholds and fire last with large force.

Question 5

Types of muscle sensors

Answer 5

Muscle spindle

• in parallel w/extrafusal fibers
• detects dynamic and static changes in muscle length

Golgi tendon organ

• arranged in series with extrafusal fibers
• detect muscle tension

Pacinian corpusles
-detect vibration

Free nerve endings
-detect noxious stimuli

Question 6

What composed a muscle spindle?

Answer 6

3-10 mm long and consists of 3-13 intrafusal fibers (muscle fibers)
-innervated by small γ motor neurons
-encapsulated
Parralel to extrafusal fibers

Central region noncontractile but for sensory

Question 7

Types of intrafusal fibers

Answer 7

Nuclear bag fibers

Nuclear chain fibers

Question 8

Nuclear bag fibers

Answer 8

Dectect rate in muscle length

Innervated by group 1α afferents and dynamic γ efferents

Multiple nuclei located in a central ‘bag-like’ config

Question 9

Nulear chain fibers

Answer 9

Detect static change in muscle length

Innervated by group II afferent and static γ efferents

More common

Nuclei arranged in row

Question 10

What types of gamma motor neurons are there and what do they do?

Answer 10

Aγ: Supply small intrafusal fibers in middle of muscle spindle

γ-dynamic: Excite nuclear bag intrafusal fibers

γ-static: Excite nuclear chain intrafusal fibers

Question 11

What activates a muscle spindle gamma motor neuron?

Answer 11

Legnthening of entire muscle

Stimulated with alpha-motor nerons

Innervate intrafusal fibers

Question 12

Functions of spindle function

Answer 12

Dynamic stretch reflex

Static reflex

Damping - prevents ferky movements

Question 13

Dynamic stretch relfex V.s Static reflex

Answer 13

Dynamic - signals transmitted from primary nerve endings, elicited by rapid stretch or unstretch

Static - transmitted by both primary and secondary endings , causes the degreee of muscle contraction to remain relatively constant

Question 14

Fuctions and sensory pathway of golgi tendon organ

Answer 14

It is an encapsulated sensory receptor that muscle tendon fibers pass

In sseries with extrafusal fibers

Stimulated by contracting or stretching of muscle - detects muscle tension

Opposite of the stretch reflex

Circuitry: type Ib afferent to inhibitory interneuron to anterior motor neuron

Question 15

Stretch reflex

Answer 15

Muscle stretch

• > group Ia afferent fibers stretch
• > synapse in α motor neuron of same muscle
• > muscle contracts and tecreases tension on muscle spindle

Synerggistic muscle activated and antagonistic muscles inhibited

Question 16

What is the structure and function of the golgi tendon organ?

Answer 16

Encapsulated sensory receptor that tendon fibers pass (in series with extrafusal fibers)

10-15 muscle fiber attach

Detect muscle tension

Circuitry: type Ib afferent +-> inhibitory interneuron -> anterior motor neuron

Question 17

What are the functions of the premotor, supplementary motor and motor cortex?

Answer 17

.premotor and supplementary areas transfer to primary motor cortex

Premotor - develops motor image

Supplementary - programs complex motor sequences

Question 18

Which areas of the motor homonculus have the greater area of control and which have the least?

Answer 18

Tongue/face and hands have most movement, legs have the least

Question 19

What are the outputs of the posterior motor cortex?

Answer 19

Sends signals to the primary motor cortes and the basal nuclei (which sends to the primary motor nucleus)

Question 20

Typical motor pathway, diff btwn upper and lower motor neurons

Answer 20

Upper motor neurons - entirely within the CNS and originate in the cerebral cortex, cerebellum and brainstem

Lower motor neurons - begin in the CNS and go into periphery, made of alpha motor neurons and make up spinal and cranial nerves