W7 - Physiology of Motor Control_2

Question 1

What mechanisms control how much force a muscle generates?

Answer 1

Number of and frequency of motor units

Question 2

What biomechanical factors affect the amount of force generated to the same neural stimulus?

Answer 2

Angle of the joint when the contraction occurs as this influences the line of muscle pull and △s the mechanical advantage. The maximum occurs when the lever arm is perpendicular to the line of pull. Greater than 90° there will be some contraction force < 90° distraction force. This can be represented by free body diagrams.

Question 3

Why does the weight seem heaviest when it’s pull is perpendicular to the limb/ lever arm?

Answer 3

1. Max gravitational pull in this position 2. Maximum force production occurs during an isometric movement, as soon as you start to contract the muscle the incresaed velocity of the limb causes ↓ force production. 3. Length tension relationship - actin and mysoin crossover and therefore cross bridge production is at it’s maximum in this position, generating max force.

Question 4

Define a reflex

Answer 4

Pre-programmed protective response to rapid muscle stretch i.e the speed of the muscle stretch not the force is important eg patellar tap. No cortical input. They are all monosynaptic and ipsilateral.

Question 5

What are the three common characteristics of reflexes?

Answer 5

They are:
1. stimulus-specific - muscle stretch not heat
2. graded - greater the stimulus the greater the reflex
3. There is a degree of delay involved between the stimulus and the response.

Question 6

How does a reflex occur?

Answer 6

When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles.

Question 7

What’s the difference between Group 1a and Group 2 afferent nerve endings?

Answer 7

1a - conducting information about rapid stretch movement to the spinal cord; Group 2, slower length changes

Question 8

What two sources does the proprioceptor input come from to ensure smooth skeletal muscle activity?

Answer 8

◦ Muscle spindles inform the nervous system of the length of the muscle
◦ Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons

Question 9

What do Golgie Tendon reflexes do and and what afferent nerve axons do they transmit their signals by?

Answer 9

They act as a protective mechanism which inhibits muscles contracting to a force level which may potentially damage the tissue (dampening the force of the agonist muscle and causing some contraction of the antagonist). 1b

Question 10

What are the similarities and differences b/w flexor and crossed extnesor reflexes?

Answer 10

The flexor is ◦ Initiated by a painful stimulus
◦ Causes automatic withdrawal of the threatened body part ◦ Ipsilateral and polysynaptic. The Crossed extensor reflex
• Occurs with flexor reflexes in weight-bearing limbs to maintain
balance
•Consists of an ipsilateral flexor reflex and a contra lateral extensor reflex eg limb stands on painful stimulus withdrawls, other limb takes the whole bodyweight

Question 11

Where does the spinal cord end?

Answer 11

L1/ L2 where it becomes caudal equina

Question 12

What information is carried by the ventral (anterior) root of each spinal nerve, and dorsal (posterior)? What does the white matter do?

Answer 12

Ventral → efferent motor info Dorsal → afferent sensory. White matter transmits signals within the brain, up, down or across. Note: the spinal nerve roots connect the peripheral nervous system to the spinal cord.

Question 13

What are ascednding and descneding tracts?

Answer 13

they are pathways that carry information up and down the spinal cord between brain and body

Question 14

What similarities are there with most spinal tracts?

Answer 14

• Generally, the names of the spinal tracts reveal both the origin and the destination
• Most pathways:
• Most pathways cross over from one side of the CNS to the other
(decussate) at some point
• Consist of a chain of 2 or 3 neurons (1st, 2nd & 3rd order)that contribute to the
successive tracts in the pathway
• Exhibit somatotopy - a precise spatial relationship among the
tract fibersthatreflectstheorderlymappingofthebody
• All pathways and tracts are paired

Question 15

What is the difference between the Dorsal column–medial pathway lemniscal pathway and Corticospinal tract?

Answer 15

Dorsal afferent; Cortic efferent

Question 16

What differentiates the Brainstem,
Cerebellum,
Diencephalon,
Cerebrum?

Answer 16

Brainstem – the pons, medulla and reticular formation receive, integrate and relay information for involuntary movement
Cerebellum – regulation of muscle tone, co- ordination of movement, timing and learning
Diencephalon – the thalamus receives information from the cerebellum and the basal ganglia
Cerebrum – contains the motor cortex and basal ganglia

Question 17

What do the basal ganglia do?

Answer 17

The basal ganglia are classified as part of the cerebral hemispheres.
They receive inputs from multiple areas of the cerebral cortex (largely pre-frontal and pre-motor) - and ultimately influence motor cortex (efferent signals) output via the thalamus.

Question 18

How does movement occur?

Answer 18

Information from the frontal lobe areas concerned with movement planning and sequencing travels to
the Basal Ganglia which modifies their output to the pre motor cortex by exciting or inhibiting the thalamus

The (Cerebellum) CBM has obtained sensory info regarding limb position from
a) Directly from sensory receptors
b) Via the somatosensory cortex
And has obtained the motor command from the frontal lobe (not shown here)
The CBM influences motor cortex output also via the thalamus

Question 19

How is the motor cortex related to muscular movement?

Answer 19

•the specific area that controls each muscle may be topographically located on the surface of the motor cortex (hormunculus - sensory and motor) • the relative size of each area of the brain is proportional to the degree of movement precision required by the muscle eg hand has greater than foot

Question 20

Is there a link b/w the PA & the brain?

Answer 20

Regular exercise can improve brain function by up to 15%. Weekly moderate physical activity increased people’s cognitive function on average by 5% for men and 14% for women.

Question 21

What is the role of intrafusal vs extrafusalfibres?

Answer 21

Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs (proprioceptors). They detect the amount and rate of change in length of a muscle. They constitute the muscle spindle, and are innervated by both sensory (afferent) and motor (efferent) fibers.

Intrafusal muscle fibers are not to be confused with extrafusal muscle fibers, which contract, generating skeletal movement and are innervated by alpha motor neurons.

Question 22

What does reciprocal inhibition mean in relation to reflexes?

Answer 22

branches of the afferent fibers also synapse with interneurons that inhibit the motor neurons of antagonistic muscles

Example: In the patellar reflex, the stretched muscle (quadriceps) contracts and the antagonists (hamstrings) relax