Chapter 14: Spinal Cord Control of Movement

Question 1

What are the categories of movement?

Answer 1

Reflexive, rhythmic, voluntary

Question 2

What is reflexive movement?

Answer 2

Involuntary coordinated patterns of muscle contraction and relaxation elicited by peripheral stimuli, e.g. knee jerk reflex

Question 3

What is rhythmic movement?

Answer 3

Repetitive motor patterns e.g. breathing, chewing, swallowing, scratching, walking, swimming
Circuits lie in brainstem and spinal cord

Question 4

What is voluntary movement?

Answer 4

Goal-directed
Improve with practice

Question 5

How are motor systems are organized?

Answer 5

Hierarchically
Forebrain
Brainstem
Spinal cord

Question 6

What does the spinal cord do?

Answer 6

Carries motor information from the brain to the periphery
Carries sensory input from the periphery to the brain
Mediates reflexes for body

Question 7

What is the spinal cord anatomy?

Answer 7

Dorsal root are afferents (sensory)
Dorsal root ganglion contains somas of sensory afferents entering the cord
Ventral root are efferents (motor)
Spinal nerves mix of sensory and motor neurons

Question 8

What is gray matter?

Answer 8

Nerve cell bodies, dendrites and axons

Question 9

What is white matter?

Answer 9

Bundles of myelinated axons organized into tracts

Question 10

How is the spinal gray matter divided?

Answer 10

Dorsal horn
Intermediate gray
Ventral horn

Question 11

What types of neurons are in the dorsal horn?

Answer 11

Mainly neurons responding to sensory input

Question 12

What types of neurons are in the ventral horn?

Answer 12

Mainly motor neurons whose axons exit the spinal cord

Question 13

What types of neurons are in the intermediate gray?

Answer 13

Some sensory neurons, some motor neurons, interneurons

Question 14

What is a motor unit?

Answer 14

One alpha motor neuron and all the muscle fiber it innervates

Question 15

What is a motor neuron pool?

Answer 15

The collection of alpha motor neurons that innervate a single muscle

Question 16

What are the two types of lower motor neurons?

Answer 16

Alpha motor neurons
Gamma motor neurons

Question 17

What are alpha motor neurons?

Answer 17

They innervate extrafusal muscle
Trigger the generation of force in the muscle

Question 18

What are gamma motor neurons?

Answer 18

Innervate intrafusal muscle fibers of the muscle spindle

Question 19

What is the organization of motor neuronal pools in the ventral horn?

Answer 19

It is divided like an arm
Axial muscles close to the middle
Extensors at the middle towards the ventral end
Flexors are above extensors
Distal muscles most far away from middle

Question 20

What type of interaction of neurons is needed for voluntary contraction of skeletal muscle?

Answer 20

Upper motor neuron goes down the tract synapses to lower motor neuron on spinal nerve to skeletal muscle

Question 21

What are the mechanisms done by lower motor neurons to control the force of a muscle contraction?

Answer 21

By varying firing rare of motor units
Recruitment of synergistic motor units

Question 22

What means by varying firing rate of motor units?

Answer 22

One AP in the alpha motor neuron causes an endplate potential large enough to generate one postsynaptic AP in the muscle
AP in the muscle causes a muscle twitch
Increasing AP frequency leads to twitch summation and eventually sustained contraction

Question 23

What means by recruitment of synergistic motor unit?

Answer 23

Size principle: small motor units are recruited first (slow) then larger motor units (fast)

Question 24

What are large motor units (fast twitch)?

Answer 24

Generate more force
Tire easily

Question 25

What are small motor units (slow twitch)?

Answer 25

Generate less force For endurance

Question 26

What are fast-fatigable motor units?

Answer 26

Generates the most force but fatigues easily

Question 27

What are fast fatigue-resistant motor units?

Answer 27

Generates force but takes less time to fatigue

Question 28

What are slow motor units?

Answer 28

Generate low force and lasts a long time

Question 29

How much input does an alpha motor neuron receives?

Answer 29

Three sources- interneurons, muscle spindles, upper motor neurons

Question 30

What is a muscle spindle?

Answer 30

Proprioceptor (body position)- stretch receptor detect changes in muscle length intrafusal muscle in a fibrous capsule inside the extrafusal muscle fiber Situated in parallel with extrafusal muscle fibers Ia sensory afferents wrap around intrafusal muscle fibers and inform CNS about muscle length

Question 31

What is the monosynaptic myotatic reflex arc (stretch reflex)?

Answer 31

When weight is added to muscle, it will stretch, but then it will contract due to the Ia axons and alpha motor neuron

Question 32

What is the knee-jerk reflex?

Answer 32

When you hit the tendon of quadriceps that has a muscle spindle that is wrap around Ia sensory afferent and activates alpha motor neuron causing the muscle to contract

Question 33

What happens when you stretch a muscle?

Answer 33

Stretching the muscle stretches the Ia nerve endings and activates alpha motor neuron Extrafusal muscle contracts and activity in Ia axon stops Spindle no longer sensitive to stretch Gamma motor neuron activity resets the length of the spindle by contracting intrafusal muscle fiber. Ia can continue to inform CNS of changes in length

Question 34

What do gamma motor neurons do for the muscle spindle?

Answer 34

Gamma motor neuron activity resets the length of the spindle by contracting intrafusal muscle fiber. Ia can continue to inform CNS of changes in length Thus, alpha and gamma co-activation normally

Question 35

What are golgi tendon organs?

Answer 35

Proprioceptors Monitors muscle tension- the force of contraction Situated in series with muscle fiber Ib afferents encode muscle tension information

Question 36

What is the reverse myotatic reflex?

Answer 36

Ib axon synapse with inhibitory interneuron inhibiting the alpha motor Reflex protects muscle from being overloaded

Question 37

What is reciprocal inhibition?

Answer 37

The contraction of one set of muscles is accompanied by the relaxation of the antagonist muscles.

Question 38

What is the flexor crossed-extensor reflex?

Answer 38

Ipsilateral: excite flexors, inhibit extensors Contralateral: excite extensors, inhibit flexors

Question 39

What are central pattern generators?

Answer 39

Circuits that give rise to rhythmic motor activity