WEEK 9 (Control of muscle movement)

Question 1

What is a motor unit?

Answer 1

A single motoneuron and the muscle fibers that it innervates

Question 2

What does Proper control of muscle function require?

Answer 2

• Excitation of the muscle by SPINAL CORD ANTERIOR MOTOR NEURONS
• CONTINUOUS FEEDBACK of sensory information from each muscle to the spinal cord indicating the functional status of each muscle at each instant

Question 3

What information does each muscle feed back to the spinal cord?

Answer 3

• Length of muscle
• Tension
• How rapidly the length or tension is changing

Question 4

What are the two types of Sensory receptors?

Answer 4

Muscle spindles & Golgi tendon organs

Question 5

What is the difference between Muscle spindles and Golgi tendon organs?

Answer 5

MUSCLE SPINDLES
- distributed throughout the belly of the muscle
- send info to the nervous system about MUSCLE LENGTH/RATE OF CHANGE OF LENGTH

GOLGI TENDON ORGANS
- located in the muscle tendons
- transmit info about TENDON TENSION/RATE OF CHANGE OF TENSION

Question 6

Signals from Muscle spindles and Golgi tendon organs are for the purpose of ____________ muscle control

Answer 6

Intrinsic

They operate almost completely at a subconscious level

Question 7

What are the two types of muscle fibers?

Answer 7

Extrafusal & Intrafusal fibers

Question 8

What are the properties of Extrafusal fibers?

Answer 8

• Make up the majority of skeletal muscle
• Innervated by alpha motoneurons
• Used to generate force

Question 9

What are the properties of Intrafusal fibers?

Answer 9

• Innervated by gamma motoneurons
• Too small to generate significant force
• Encapsulated in sheaths
• Form muscle spindles that run parallel to the extrafusal fibers

Question 10

What are Muscle spindles?

Answer 10

Sensory receptors (proprioreceptors) innervated by both sensory and motor nerve fibers

Question 11

What are the two types of intrafusal fibers present in muscle spindles?

Answer 11

Nuclear bag fibers & Nuclear chain fibers

Question 12

What is the difference between Nuclear bag fibers and Nuclear chain fibers?

Answer 12

NUCLEAR BAG FIBERS
- larger
- nuclei are accumulated in a central “bag” region

NUCLEAR CHAIN FIBERS
- smaller
- nuclei arranged in rows “chains”
- more plentiful

Question 13

What does Sensory innervation of the muscle spindle consist of?

Answer 13

• A single GROUP 1A AFFERENT NERVE which innervates the central region of the nuclear bag fibers and the nuclear chain fibers
• GROUP II AFFERENT NERVES which innervate nuclear chain fibers

Question 14

What does Motor innervation of the muscle spindle consist of?

Answer 14

Two types of gamma motoneurons: DYNAMIC and STATIC

Question 15

What is the difference between Dynamic γ motoneurons and Static γ motoneurons?

Answer 15

Dynamic γ motoneurons synapse on nuclear bag fibers in “plate endings”

Static γ motoneurons synapse on nuclear chain fibers in “trail endings” which spread out over longer distances

Question 16

How are γ Motoneurons different to α motoneurons?

Answer 16

γ Motoneurons are smaller and slower than the α motoneurons that innervate the extrafusal fibers

Question 17

What is the function of γ Motoneurons?

Answer 17

To regulate the sensitivity of the intrafusal muscle fibers they innervate

Question 18

What is the function of Muscle spindles?

Answer 18

Stretch receptors that correct changes in muscle length when extrafusal muscle fibers are either shortened (by contraction) or lengthened (by stretch). Muscle spindle reflexes therefore work to return muscle to its resting length after it has been shortened or lengthened.

Question 19

What are Spinal cord reflexes?

Answer 19

Stereotypical motor responses to specific kinds of stimuli

Question 20

What is the reflex arc?

Answer 20

The neuronal circuit that directs this motor response

Question 21

What does the reflex arc include?

Answer 21

• Sensory receptors
• Sensory afferent nerves which carry info to the spinal cord
• Interneurons in the spinal cord
• Motoneurons which direct the muscle to contract or relax

Question 22

What is the simplest of all the spinal cord reflexes?

Answer 22

THE STRETCH REFLEX

It has only one synapse between the sensory afferent nerves and motor afferent nerves

Question 23

What is an example of a stretch reflex?

Answer 23

KNEE-JERK REFLEX

It is initiated by tapping the patellar tendon, causing the quadriceps muscle to stretch

Question 24

Describe the knee-jerk reflex

Answer 24

1) When the muscle is stretched, group Ia afferent fibers (sensory afferent nerves) in the muscle spindle are activated and their firing rate increases. They enter the spinal cord and synapse directly on and activate alpha motoneurons. This pool of alpha motoneurons innervates the homonymous muscle.
2) When alpha motoneurons are activated, they cause CONTRACTION of the muscle that was originally stretched (HOMONYMOUS MUSCLE). When the muscle contracts, it shortens, decreasing stretch on the muscle spindle. The muscle spindle returns to its original length and the firing rate of group Ia afferents returns to baseline
3) Simultaneously, info is sent from the spinal cord to cause contraction of SYNERGISTIC MUSCLES and relaxation of ANTAGONISTIC MUSCLES

Question 25

Describe the Golgi tendon reflex

Answer 25

1) When the muscle contracts, the extrafusal muscle fibers shorten which activates the Golgi tendon organs attached to them. The group Ib afferent fibers that synapse on INHIBITORY NEURONS in the spinal cord are activated and these inhibitory interneurons synapse on the alpha motoneurons
2) When the inhibitory interneurons are activated, they inhibit firing of the alpha motoneurons producing relaxation of the homonymous muscle
3) As HOMONYMOUS muscle relaxes, the reflex causes SYNERGISTIC MUSCLES to relax and ANTAGONISTIC MUSCLES to contract

Question 26

What happens when Golgi tendon organs of a muscle tendon are stimulated by increased tension?

Answer 26

Signals are transmitted to the spinal cord to cause INHIBITORY REFLEX EFFECTS in the respective muscle
(provides a -ve feedback mechanism that prevents the development of too much tension on the muscle)

Question 27

What happens when tension on the muscle/tendon becomes extreme?

Answer 27

Significant inhibitory effect leads to a sudden reaction in the spinal cord that causes INSTANTANEOUS relaxation of the entire muscle (LENGTHENING REACTION)

it is a protective mechanism to prevent tearing of the muscle or avulsion of the tendon from its attachments to the bone

Question 28

What is the flexor-withdrawal reflex?

Answer 28

A polysynaptic reflex that occurs in response to a tactile, painful or noxious stimulus

Question 29

Describe the Flexor-withdrawal reflex

Answer 29

1) When a limb touches a painful stimulus, flexor reflex afferent fibers are activated which synapse on multiple interneurons in the spinal cord
2) On the ipsilateral side of the pain stimulus, reflexes are activated that cause flexor muscles to contract and extensor muscles to relax. This produces flexion on the IPSILATERAL SIDE.
3) On the contralateral side of the pain stimulus, reflexes are activated that cause extensor muscles to contract and flexor muscles to relax. This produces extension on the contralateral side (CROSSDEXTENSION REFLEX).
4) A persistent neural discharge (AFTERDISCHARGE) occurs in the polysynaptic reflex circuits which causes contracted muscles to remain contracted for a period of time after the reflex is activated.

Question 30

What are the differences between the different types of Reflexes?

Answer 30

STRETCH REFLEX
NO. OF SYNAPSES: one
STIMULUS FOR REFLEX: stretch (lengthening) of the muscle
SENSORY AFFERENT FIBERS: Ia
RESPONSES: contraction of the muscle

GOLGI TENDON REFLEX
NO. OF SYNAPSES: two
STIMULUS FOR REFLEX: contraction (shortening) of the muscle
SENSORY AFFERENT FIBERS: Ib
RESPONSES: relaxation of the muscle

FLEXOR-WITHDRAWAL REFLEX
NO. OF SYNAPSES: many
STIMULUS FOR REFLEX: pain, temp etc
SENSORY AFFERENT FIBERS: II, III and IV
RESPONSES: flexion on ipsilateral side & extension on contralateral side