Lecture 9

Question 1

Define and list characteristics of motor units

Answer 1

Composed of a single motor neuron and the muscle fibers it innervates; composed of extrafusal fibers

Question 2

Define and list characteristics of motor neuron pools

Answer 2

Group of motor neurons that innervate fibers within the same muscle

Question 3

List the functions of interneurons

Answer 3

Small and highly excitable, capable of spontaneous activity, responsible for most of spinal cord integrative function; most are inhibitor, where GABA is neurotransmitter; excitatory ones use glutamate as neurotransmitter

Question 4

Describe the anatomy and function of Renshaw cells

Answer 4

These are inhibitory cells in anterior horns of spinal cord that receive collateral branches from alpha motor neurons

Transmit inhibitory signals to surrounding motor neurons; this results in lateral inhibition and enhance fluidity of limb movement

Transmit inhibitory signals to same motor neuron, which results in recurrent inhibition (negative feedback)

Question 5

List and compare types of muscle sensors

Answer 5

Muscle Spindle (group Ia and II afferents): arranged in parallel with extrafusal fibers and detect both dynamic and static changes in muscle length

Golgi Tendon Organ (group Ib afferents): arranged in series with extrafusal fibers and detect muscle tension

Pacinian Corpuscles (group II afferents): detect vibration

Free Nerve Endings (groups III and IV afferents): detect noxious stimuli

Question 6

Describe the anatomy of a muscle spindle (including types of motor and sensory neurons associated with the spindle)

Answer 6

Consists of 3-12 intrafusal fibers; innervated by small gamma motor neurons (group II afferents), encapsulated within a sheath to form muscle spindle, and run parallel to extrafusal fibers; with finer movements, the number of muscle spindles required increases; central region of spindle has no contractile fibers and functions as a sensory receptor; sensory fibers originate from central region and stretching of central region of intrafusal fiber stimulates sensory fibers; detects changes in muscle length

Includes alpha motor neurons, gamma motor neurons, Ia sensory fibers, II sensory fibers,

Question 7

Describe and compare the three types of gamma motor neurons

Answer 7

A-gamma: 5 micrometers, supply small intrafusal fibers in middle of muscle spindle

Gamma-Dynamic: Excite nuclear bag intrafusal fibers

Gamma-Static: Excite nuclear chain intrafusal fibers

Can adjust sensitivity of muscle spindle

Question 8

Describe the functions of a muscle spindle

Answer 8

Dynamic stretch reflex, static reflex, and prevents jerkiness of body movements

Question 9

Describe how a myotactic reflex works

Answer 9

1. Stretching of muscle stretches group Ia afferent fibers
2. Group Ia afferents synapse directly on alpha-motor neurons of same muscle
3. Muscle contracts and decreases tension on muscle spindle
4. Synergistic muscles are activated and antagonistic muscles are inhibited

Question 10

Describe the structure, circuitry, and function of the golgi tendon organ

Answer 10

Encapsulated sensory receptor through which muscle tendon fibers pass; arranged in series with extrafusal fibers; 10-15 muscle fibers attached to each Golgi organ; organ stimulated by contracting or stretching of muscle; detects muscle tension; golgi tendon reflex is opposite of the stretch reflex

Circuitry: Type Ib afferent -> inhibitory interneuron -> anterior motor neuron

Question 11

List and compare the functions of the premotor, supplementary motor, and motor cortex

Answer 11

Premotor and supplementary motor cortices generate a plan for movement (transfer plan to primary motor cortex); signals generated here cause more complex patterns of movement than the more discrete pattern generated by the primary motor cortex, anterior part of the premotor cortex develops a “motor image” of the total muscle movement that is to be performed; supplementary motor cortex programs complex motor sequences and is responsible for mental rehearsal for a movement

Question 12

What are the outputs of the posterior motor cortex?

Answer 12

Posterior motor cortex sends signals to primary motor cortex -> basal nuclei and thalamus -> primary motor cortex

Question 13

What is the dynamic stretch reflex?

Answer 13

Signals transmitted from primary nerve endings, elicited by rapid stretch or unstretch, opposes sudden changes to muscle length

Question 14

What is the static stretch reflex?

Answer 14

Transmitted by both primary and secondary endings; causes degree of muscle contraction to remain relatively constant

Question 15

What are nuclear bag fibers?

Answer 15

Detect rate of change in muscle length, innervated by group Ia afferents and dynamic gamma efferents; multiple nuclei located in a central “bag-like” configuration in receptor area

Question 16

What are nuclear chain fibers?

Answer 16

Detect static change in muscle length, innervated by group II afferents and static gamma efferents; more numerous than nuclear bag fibers; multiple nuclei arranged in a single row; aligned in chain throughout receptor area

Question 17

What do alpha motor neurons give rise to?

Answer 17

A-alpha fibers

Question 18

Describe an upper motor neuron

Answer 18

Descends the spinal cord to level of the appropriate spinal nerve, synapses with lower motor neuron or interneuron, and the neurotransmitter from upper to lower motor neurons is glutamate, which binds to glutamatergic receptors

Originate in cerebral cortex, cerebellum, and brainstem

Question 19

Describe a lower motor neuron

Answer 19

Typically found in anterior gray portion of SC or cranial nerve nuclei of brainstem, terminate at effector with acetylcholine as neurotransmitter; cranial nerves are unique lower motor neurons

Begin in CNS from anterior horns of spinal cord and from brainstem cranial nerve nuclei

Question 20

What is recruitment?

Answer 20

Refers to the increase in tension of muscle contractions by the activation of additional motor units (size principle)

Question 21

Describe small motor neurons

Answer 21

Innervate a few muscle fibers, lowest thresholds, fire first, and generate smallest force

Question 22

Describe large motor neurons

Answer 22

Innervate many muscle fibers, highest thresholds, fire last, and generate largest force

Question 23

Describe Ia inhibitory interneurons

Answer 23

Inhibit antagonistic muscle, signal from muscle spindle, and avoid spasticity

Question 24

Describe Ib inhibitory interneurons

Answer 24

Autogenic inhibition reflex, golgi tendon activated with stretch, which connects with Ib to inhibit same muscle, and are responsible for smooth contraction and protects muscle from tears

Question 25

Describe excitatory interneurons

Answer 25

Involved in spinal reflex to remove extremity from damaging stimulus; place foot on a tack, sensory information must be translated into motor output, sensory group III afferents send signal to excitatory interneurons that send ipsilateral and contralateral signals

Question 26

Describe A fibers

Answer 26

Somatic and myelinated; include the subtypes alpha (largest, type I), beta, gamma, and delta (smallest, type IV)

Question 27

Describe B fibers

Answer 27

Lightly myelinated, preganglionic fibers of ANS

Question 28

Describe C fibers

Answer 28

Unmyelinated, found in somatic and autonomic systems; also referred to as type IV fibers

Question 29

What does the medial activation system innervate?

Answer 29

Innervates the postural and girdle muscles

Question 30

What is the lateral activation system associated with?

Answer 30

Associated with distally located muscles used for fine movements

Question 31

What does the nonspecific activating system facilitate?

Answer 31

Facilitates local reflex arcs

Question 32

What brain areas control gamma fibers?

Answer 32

Bulboreticular region of brain stem, cerebellum, basal nuclei, cerebral cortex

Question 33

What is after-discharge?

Answer 33

It is the result of persistent neural activity of polysynaptic circuits that prevents the muscle from relaxing for some time after the stimulus stops