skeletal muscle: whole function and reflex control

Question 1

Twitch- what is it and phases

Answer 1

= Mechanical Response to A Single Action Potential
Reproducible for a given fiber but varies between fibers

Latent Period: excitation/contraction coupling prior to cross-bridge formation
Contraction: cytosolic [Ca2+] increases and the number of active cross-bridges increases
Relaxation: cytosolic [Ca2+] decreases and the number of active cross-bridges decreases

Question 2

velocity of contraction

Answer 2

• Depends upon the myosin heavy chain isoform
• the speed of cross-bridge cycling depends upon ATPase activity
• Fast muscle myosin has higher ATPase activity

Question 3

force generation

Answer 3

Fibre Diameter
- The number of myofibrils per unit of cross-sectional area is constant: the greater the cross-sectional area, the more sarcomeres in parallel, the more cross-bridges
Fibre Length
A relaxed fiber is at its optimal length for force generation
A fibre cannot generate tension if:
- Stretched to >175%of the optimal length
- Compressed to <60%of the optimal length

Question 4

length-tension relationship

Answer 4

When stretched to >175% of resting length, the thin and thick filaments do not overlap
- No cross bridges, no tension
Optimal overlap occurs during resting state
- Increasing overlap increases the number of cross bridges and tension
When compressed to <60% of resting length, the thin filaments overlap each other and disrupt cross bridge formation
- The thick filaments collide with the Z discs

Question 5

summation of muscle force.- activation of muscle repeatedly

Answer 5

Arrival of 2nd stimulus before the muscle has fully relaxed
->Increased tension is developed -summation

Several stimuli applied over a short period- tetanus

High-frequency stimulation (smooth development of tension) – fused tetanus

Question 6

spinal cord & sending infromation

Answer 6

The dorsal (ascending) and ventral (descending) roots carry information in opposite directions

Each myelinated motor axon innervates one muscle fiber
- The axon splits or branches to make several terminal contacts with the muscle-neuromuscular junction
- Release of ACh from axon terminals activates nicotinic ACh receptors at the motor endplate

Question 7

cascade of muscle contraction

Answer 7

nerve action potential
nerve ending secretes ACh
endplate potential
muscle AP
T-tubules depolarize & open Ca2+ channels of SR
Sarcoplasmic Ca2+ increases
muscle fibre contracts
Ca2+ is pumped back to SR
Muscle fibre relaxes

Question 8

whole muscle contraction

Answer 8

• Whole muscle function involves the activation and contraction of a motor units
• A single motor neurone innervates a variable number of muscle fibres through axon branching
• A motor neurone and the fibres it innervates are a motor unit
  At any time, the tension generated by a whole muscle depends on:
  1. The tension generated by each contracting fibre
• The larger the fibre diameter, the greater the tension
  2. The number of fibres contracting
  A. The number of fibres per motor unit (small or large)
  e.g. Delicate movements of the eye involve many small motor units (approximately 15 fibres per motor unit)
  e.g. Gross movements of the leg involve a few large motor units (100’s-1000’s fibres per motor unit)

B. The number of active motor units

Question 9

Motor unit recruitment

Answer 9

=Regulate the number of active motor units
- It is controlled by motor centers in the brain and involves the sequential activation of motor units
- Order of recruitment depends upon motor neuron cell body size
- Presynaptic cells elicit graded potentials in the motor neurone cell body – the smaller the cell body, the less input is required to fire an action potential

Question 10

Order of motor unit recruitment

Answer 10

1. Smallest motor neurons recruit 1st
   - Innervate slow oxidative fibres
2. Intermediate motor neurons recruit 2nd
   - Innervate fast oxidative fibres
3. Largest motor neurons recruit 3rd
   - Innervate fast glycolytic fibres
   Few fast glycolytic fibres (metabolically expensive) are recruited for moderate strength contractions (i.e. <40% maximal tension)

Question 11

simple neuronal circuit

Answer 11

draw and describe

Question 12

muscle spindles (stretch receptors) provide sensory information

Answer 12

Spinal reflexes convert afferent input to efferent output without the involvement of brain centres
- Achieved within the spinal
cord
- Requires a “sensor”
Muscle spindle: Capsule, intrafusal fibres, stretch receptors
- Specialized group of intrafusal muscle fibres that run parallel to muscle fibres and “sense” muscle length
- Stretching increases firing in afferent nerve
- Contraction reduces firing in afferent nerve

Question 13

Monosynaptic reflex

Answer 13

Muscle spindle -> afferent neurone -> synapse -> efferent neurone -> NMJ -> skeletal muscle

Question 14

Polysynaptic reflex

Answer 14

=multiple muscle groups controlling limb movement

• Muscles work as antagonistic pairs (e.g. flexors & extensors)
• Movement requires contraction of one whilst the other relaxes
• This is achieved through reciprocal inhibition
• Polysynaptic pathway involving one (or many) interneurons
• The motor neurone innervating the antagonistic muscle is inhibited via an inhibitory interneuron
• Interneurons prevent hamstring muscle contraction during the knee jerk response

Question 15

Regulation and adjustment of muscle contraction- movements

Answer 15

Movements are adjusted through ascending pathways
- The afferent neurone enters the spinal cord and continues to the brainstem
-> Communicates via interneurons with motor control areas
-> Provides conscious perception allowing adjustment of slow controlled movements of body position

Question 16

flexor reflex

Answer 16

= sensory info modulates reflex-withdrawl
Interneurons receive sensory input and transmit that information to the brain and to the motor neurone to initiate limb withdrawal from averse stimuli
- Involves excitatory interneurons

Question 17

crossed-extensor reflex

Answer 17

stops you from falling over
- Another set of interneurons receive information from the sensory inputs and inhibit motor neurons that control flexion and activate those that control (extensor) muscles in the other leg.

Question 18

Golgi tendon

Answer 18

Stretch receptors located in tendons (acts as a strain gauge)
- Fire in response to muscle tension
->Sends afferent information to the spinal cord via group Ib afferent fibres

Question 19

Golgi-tendon reflex

Answer 19

= maintaining posture
When the rectus femoris muscle starts to fatigue, signals from the Golgi tendon organ decline
->reduces interneuron inhibition of the motor neurons controlling the rectus femoris muscle in the spinal cord
-> Increases contraction of the rectus femoris muscle to maintain posture
=inverse myotactic reflex

Question 20

excitatory and inhibatory inputs

Answer 20

• Excitatory inputs can increase neuronal excitability (glutamate)
• Inhibitory inputs can reduce neuronal excitability (GABA and glycine)

->allow us to generate responsive reflexes and complex movements (e.g. walking)
- Interneurons allow integration of signals between the brain and the sensory and motor pathways

Question 21

synaptic integration can modulate neuronal responses

Answer 21

• Multiple synaptic inputs (from interneurons) allow the integration of spatial and temporal signals between the brain and the sensory and motor pathways

Question 22

spinal cord injury

Answer 22

Motor neurone axons are unable to regenerate in the spinal cord if severed
- The degree of paralysis depends at which level the spinal cord injury occurs and how much of the cord is damaged (the higher level of injury, the greater the paralysis)