Neuromuscular 2

Question 1

Walking without corticospinal input

Answer 1

• Cutting corticospinal neurons does not severely disrupt walking on a flat surface.
• However, it disrupts more difficult tasks, such as walking on a pipe or horizontal ladder (Liddell & Phillips, 1944, Brain 67(1) 1-9).
• These early findings suggested that the spinal cord controls repetitive movement patterns…
• …but other neural input (cortical and sensory) is required for more complex tasks or adapting to a changing environment.

Question 2

Spinal control of rapid movements

Answer 2

• Rapid, repeated patterns of movement in nature involve rapid action and relaxation by muscles.
• Spinal circuits form the basic neural circuitry to enable this to happen.
• This spinal control allows the brain to attend to other tasks.
  
  • The brain involvement is directed towards changing muscles forces and actions

Question 3

Central pattern generators and repetitive

HINT: Rhythmic motor patterns…

Answer 3

• Rhythmic motor patterns in the absence of voluntary effort (e.g., breathing, walking) can be sustained by central pattern generators (CPGs).
• CPGs are neuronal circuits which, when connected to a-motoneurons, can generate intermittent bursts of muscle activity.
• Locomotor CPGs in the spinal cord generate alternating bursts of activity in opposing flexor and extensor muscles.

Question 4

How central pattern generators create rhythmic movement

Answer 4

• CPGs provide alternating excitatory input to motor neurons in opposing muscles.
• Results in alternating bursts of activity in opposing muscles.
• Stimulus for activating CPGs can be supraspinal input (brain) or sensory input (moving limbs)

Question 5

Sensory input to spinal control

Answer 5

• Changing a basic motor pattern generated by spinal CPGs requires a change in supraspinal or sensory input (activation of sensory neurons).
• Sensory neurons important to movement originate within muscle, tendon and joints.
• They can be activated by several types of stimuli – mechanical, chemical, thermal.
• These stimuli can change the activity of sensory neurons and alter the recruitment and firing of motor units.
• Sensory input is also transmitted to the brain via ascending tracts in the spinal cord to create sensations of movement.
• Damage to sensory input can have devastating consequences:

Question 6

Simple examples of spinal control of muscle

HINT: Monosynaptic and disynaptic

Answer 6

• Stretch reflex.
• Stretching muscle creates a sensory stimulus that evokes more than one motor response at the same time.
• A monosynaptic pathway mediates one response - contraction of the stretched muscles
• A disynaptic pathway, created by adding an interneuron, mediates the opposite response – relaxation of antagonists.
• Increasing the complexity of neural connections within the spinal cord ONLY increases the control over muscle action.
• The brain is not involved, and we see a simple example of increasing spinal control (by adding an interneuron and connections with other a-motoneurons) over muscle actions.

Question 7

Sensory receptors in muscle

Answer 7

• Muscle mechanoreceptors and chemoreceptors provide continuous sensory information to spinal cord.
• Muscle spindles sense length (2nd pic).
• Golgi tendon organs sense force (1st pic).
• Other mechanoreceptors in joints sense joint position.
• Sensory input about chemical state by type III and IV afferent nerve endings.
• The spinal cord and brain continually monitor this sensory input to help control complex repetitive tasks.

Question 8

Strength tasks

Answer 8

• Rock climbing

- Sumo wrestling

Question 9

Motor pathway and strength

Answer 9

• Strength is the highest force generated.
• During voluntary effort strength involves the entire motor pathway.
• We will focus on mechanisms inside the dashed box.

Question 10

Mechanisms of skeletal muscle contraction

REFER TO LECTURE OR EXERCISE NUTRITION FOR EXACT STEPS

Answer 10

1. The propagation of impulses / action potential along the axon terminal
2. Action potential will stimulate the release of a neurotransmitter called acetycholine
3. The acetycholine binds to receptors on the surface of the membrane

Question 11

Muscle shortening and movement

Answer 11

• Muscle fibres apply force to the tendons by shortening towards their middle (concentric, isometric), or attempting to shorten (eccentric).
• This stabilises or moves bones about joints.

Question 12

How muscle fibres shorten and develop force

• Sarcomeres
• Force directions

Answer 12

• Sarcomeres are basic contractile units of skeletal muscle fibres.
• Sarcomeres shorten towards their centre by attachment of actin filaments to a thicker myosin filaments followed by cross-bridge cycling. (the attachment of the projection of the myosin head to the thin filament)
• Force is applied by sarcomeres to surrounding structures within muscle in both a longitudinal and radial directions.

Question 13

Mechanisms of muscle relaxation

Answer 13

1. No action potential repolarise membrane
2. Stop acetylcholine release & receptor binding
3. No sarcolemma action potential
4. Repolarise sarcolemma ion pumping
5. Ca reuptake by sarcoplasmic reticulum
6. Myosin binding of ATP
7. Relaxation and lengthening

Question 14

Strength (highest force)

- Sarcomeres

Answer 14

• Muscle strength is the highest force developed during a maximum voluntary effort.
• Increasing force depends on increasing the number of contracting sarcomeres (basic contractile unit) in parallel, rather than in series.
• Therefore, a muscle’s strength is a function of the number of sarcomeres in parallel, the cross-sectional areas of muscle fibres, and the number of muscle fibres in parallel

Question 15

How to we enlarge muscle

Answer 15

Hypertrophy - width of the muscle cell get larger

Question 16

Strength and muscle length

- Inverted U

Answer 16

• Strength is a function of muscle length
• Inverted ‘U’ relationship between sarcomere length and maximum force
• Function of number of cross-bridge attachment

Question 17

Strength and speed

Answer 17

• Strength is a function of speed
• Maximum amount of force generated by muscle depends on how quickly it shortens: the quicker it shortens, the less force it generates.
• The figure relates to concentric and isometric actions.
• “Load” is equivalent to force.
• Maximum force is inversely related to the muscle shortening velocity.
• Maximum force at zero velocity (‘P0 ’ ; isometric).
• Zero force at maximum velocity (Vmax).

Question 18

Strength is highest during lengthening action

Answer 18

• Muscle is able to apply higher force when it lengthens than when it shortens or acts isometrically.
• Higher loads can be tolerated during eccentric versus isometric or concentric contractions.

Question 19

Motor unit recruitment and strength

HINT: MU pool

Answer 19

• Maximum strength depends on maximising MU recruitment.
• Training-induced increases in strength occur through increases in MU recruitment and firing frequency – ‘Neural adaptation – and muscle fibre hypertrophy.
• The extent to which the entire MU pool (for a given muscle) is recruited might depend on the task and varies between people.
• Mental imagery can improve muscle strength.