Exam 3 Lower Motor System

Question 1

General organization of the motor system—e.g., lower motor neurons, upper motor
neurons,

Answer 1

Lower motor neurons: innervate skeletal muscles of head and body
Local circuit neurons: major source of synaptic input to lower motor neurons
Upper Motor Neurons: Cell bodies in the cortex, projections to local circuit neurons and lower motor neurons
Basal Ganglia: Initiated movement and prevent unwanted movement
Cerebellum: Detects and corrects motor errors

Question 2

What is meant by the “final common pathway”?

Answer 2

Motor neurons in the spinal cord are the final common pathway for all motor action

Question 3

Alpha Motor Neurons

Answer 3

large motor neurons in the ventral horn; innervate striate muscle fibers

Question 4

Motor Unit

Answer 4

Combination of muscle fiber & ⍺motor neuron

Combination of muscle fiber & ⍺motor neuron

Question 5

Motor Neuron Pool

Answer 5

⍺motor neurons that innervate a particular muscle form a column in the spinal cord

Question 6

End-Plate potential (EPP)

Answer 6

depolarization of the postsynaptic muscle fiber

Question 7

Miniature EPP’s (MEPP)

Answer 7

occur spontaneously

Question 8

Describe the somatotopic organization of lower motor neuron pools in the spinal cord:

Answer 8

Medial motor neurons project to more proximal muscles & Lateral motor neurons project to more distal muscles.

Question 9

Describe the anatomy and function of medial local circuit neurons:

Answer 9

Medial: extend over several spinal cord segments, terminate bilaterally, synchronous bilateral coordination of muscles (posture, breathing, locomotion)

Question 10

Describe the anatomy and function of lateral local circuit neurons

Answer 10

Lateral: extend over a few spinal cord segments, terminate ipsilaterally, fine control of distal extremities (independent movement of fingers)

Question 11

What neurotransmitter is released at the neuromuscular junction?

Answer 11

Acetylcholine (Ach) released into the synaptic cleft at the neuromuscular junction

Question 12

What are the 3 types of motor units and their properties? How are they recruited during
a movement?

Answer 12

Slow (S) motor units: Small, innervate weaker muscle fibers, sustained motor activity (posture)
Fast fatigable (FF) motor units: Large, innervate stronger fibers, brief exertions of large force (jumping).
Fast fatigue-resistant (FR) motor units: intermediate.
Recruited from weakest to strongest•Deactivation follows reversed order

Question 13

What is the function and the components of muscle spindles?

Answer 13

Consist of intrafusal muscle fibers:
Are surrounded by parallel extrafusal fibers(force producing)
Detects stretch of the muscle
Intrafusal fibers controlled by gamma motor neurons

Question 14

Group Ia afferent axon

Answer 14

Group Ia afferent axon: Rapidly adapting responses, velocity & direction of movement

Question 15

Group II afferent axons:

Answer 15

Group II afferent axons: Sustained responses to constant muscle lengths, static position of limbs

Question 16

Gamma (y) motor neurons

Answer 16

Gamma (y) motor neurons impact the sensitivity of the spindle afferents to changes in muscle length

Question 17

What are the main purposes of reflexes? Give examples:

Answer 17

Spinal Reflexes: Simplest sensorimotor circuits, circuitry is completely contained within the spinal cord. Sensory stimuli are provided by receptors in muscles, joints, and skin
Then: reflexes are automatic and stereotyped
Now: Reflexes can be modified to adapt to the task, Reflexes are not all-or-nothing responses
This allows reflexes to be smoothly incorporated into complex movements initiated by central commands.

Question 18

What is reciprocal innervation?

Answer 18

Reciprocal innervation is a general principle in the motor system.

Question 19

How do reflexes differ in upper vs. lower extremities? Why?

Answer 19

Contraction of stretched muscle and simultaneous relaxation of the antagonist muscle
Upper extremities: withdrawal reflex only affects one limb.
Lower extremities: reflex affects both limbs.

Question 20

Can descending pathways modulate reflexes? How?

Answer 20

Descending pathways can block the reciprocal inhibition when a co-contraction is desired (e.g., for stiffening of joints).

Question 21

Describe the anatomy and function of Golgi tendon organs:

Answer 21

Afferent nerve endings located at the junction of muscle and tendon
Innervated by a group Ib sensory axon
Arranged in series with extrafusal muscle fibers

Question 22

What’s the function of Ib afferents and Ib inhibitory neurons?

Answer 22

Contraction of muscle>force acts on tendon & tendon organ>activates Ib sensory receptors
Ib afferent synapse: Ib inhibitory neuron>inhibits flexor Excitatory interneuron>excites extensor
The Golgi tendon organ signals tension in a muscle: Ib inhibitory interneurons receive their principal input from Golgi tendon organs, but also a variety of other inputs.
Function: Protective: maintains a steady level of tension and stable joint angle (steady tone during fatigue). Also provides precise information about the state of contraction of a muscle

Question 23

What is a central pattern generator?

Answer 23

The spinal cord also contains circuits capable of generating basic motor patterns.
Ex: Cat (or dog) on a treadmill with a transected spinal cord can walk and even bring a paw around an obstacle after hitting it

Question 24

Describe what happens to the ability to walk in a cat before and after transection of the spinal cord:

Answer 24

Before: Stancephase: limb is extended and placed in contact with the ground to propel animal forward. Swing phase: limb is flexed to leave the ground and then brought forward to begin the next stance phase.
After: the cat’s hindlimbs will still make the coordinated movement.

Question 25

Draw and explain what a half center oscillator is:

Answer 25

Two “half centers” inhibit each other: Some “neural fatigue” mechanism is required to obtain rhythmically alternating activity.

Question 26

Do humans have the same ability that’s been shown in the cat?

Answer 26

Humans don’t walk spontaneously after spinal lesions, but there are observations that rhythmic movements can be triggered.
Immediately after birth babies can produce stepping movements when held upright and moved over a horizontal surface.