1
Q
Walking without corticospinal input
A
- 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.
2
Q
Spinal control of rapid movements
A
- 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
3
Q
Central pattern generators and repetitive
HINT: Rhythmic motor patterns…
A
- 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.
4
Q
How central pattern generators create rhythmic movement
A
- 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)
5
Q
Sensory input to spinal control
A
- 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:
6
Q
Simple examples of spinal control of muscle
HINT: Monosynaptic and disynaptic
A
- 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.
7
Q
Sensory receptors in muscle
A
- 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.
8
Q
Strength tasks
A
- Rock climbing
- Sumo wrestling
9
Q
Motor pathway and strength
A
- Strength is the highest force generated.
- During voluntary effort strength involves the entire motor pathway.
- We will focus on mechanisms inside the dashed box.
10
Q
Mechanisms of skeletal muscle contraction
REFER TO LECTURE OR EXERCISE NUTRITION FOR EXACT STEPS
A
- The propagation of impulses / action potential along the axon terminal
- Action potential will stimulate the release of a neurotransmitter called acetycholine
- The acetycholine binds to receptors on the surface of the membrane
11
Q
Muscle shortening and movement
A
- 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.
12
Q
How muscle fibres shorten and develop force
- Sarcomeres
- Force directions
A
- 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.
13
Q
Mechanisms of muscle relaxation
A
- No action potential repolarise membrane
- Stop acetylcholine release & receptor binding
- No sarcolemma action potential
- Repolarise sarcolemma ion pumping
- Ca reuptake by sarcoplasmic reticulum
- Myosin binding of ATP
- Relaxation and lengthening
14
Q
Strength (highest force)
- Sarcomeres
A
- 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
15
Q
How to we enlarge muscle
A
Hypertrophy - width of the muscle cell get larger
16
Q
Strength and muscle length
- Inverted U
A
- Strength is a function of muscle length
- Inverted ‘U’ relationship between sarcomere length and maximum force
- Function of number of cross-bridge attachment
17
Q
Strength and speed
A
- 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).
18
Q
Strength is highest during lengthening action
A
- 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.
19
Q
Motor unit recruitment and strength
HINT: MU pool
A
- 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.
