Motor System Physiology and Hypothalamus Flashcards
What is a motor unit?
The smallest units of motor control. Comprises a single alpha motor neurone and the collection of muscles fibres it innervates.
The number and type of fibres within a motor unit determines the amount of force that unit can generate.
Describe alpha motor neurones.
Each controls the contraction of a number of muscle fibres of the same type. Because the force depends on the total cross sectional area, alpha motor neurones that control a larger number of muscle fibres generate larger force than those that control fewer fibres.
What are the 3 main types of skeletal muscle fibre innervated by alpha motor neurones?
- Type 1
- Type 2a
- Type 2b
Describe type 1 skeletal muscle fibres.
Slow twitch, oxidative phosphorylation, fatigue resistant, do not generate much force but are for sustained contraction, like anti-gravity muscles for sustained posture.
Describe type 2a skeletal muscle fibres.
Fast twitch, oxidative phosphorylation, relatively fatigue resistant, generate more force than slow twitch.
Describe type 2b skeletal muscle fibres.
Fast twitch, glycolytic metabolism, highly fatigable, high tension and a lot of force.
How does muscle tension increase with motor neurones firing?
- Action potential firing rate also determines the amount of force produced by a muscle.
- As motor neurone firing rate increases, there is not enough time for action potential to recover, so the force increases until fused tetanic contraction occurs, the maximum force that can be generated.
What is motor units recruitment?
- Graded increases in force are obtained by the orderly recruitment of motor neurones.
- Ordered recruitment by fibre type.
- Force generated by a muscle is controlled by recruitment of alpha motor neurones.
- Recruitment is the addition of an active motor neurone to an existing pool of active motor neurones.
How are motor units recruited?
- Smaller motor neurones controlling fewer muscle fibres fire first. Standing conditions, for example.
- Larger motor neurones controlling a greater number of fibres recruited as more force is required. Jumping and running, for example.
- So total force form a muscle increases.
Distinguish extrafusal and intrafusal muscle fibres.
Alpha motor neurone gives rise to end plates to muscle fibres in a muscle.
- Intrafusal fibres generate this force and are spindle shaped.
- Extrafusal muscle fibres are not within spindles but are controlled by alpha motor neurones.
- Intrafusal are contractile and ends but central region is enclosed in a capsule/muscle spindle. Isolates it mechanically from surrounding muscle but is in parallel with extrafusal fibres.
- If length of muscle changes lengths of intrafusal and extrafusal fibres changes in the same way.
What are Golgi tendon organs?
Gogi tendon organs have nerve endings between the collagen interwoven fibres. As tension on muscles increases, collagen fibres pull more and more, closing up gaps between the collagen fibres, squeezing the nerve endings and distort the membrane, increasing action potential fibres. So Golgi tendon organs are sensory to the tension and force, not length.
What are type Ia and II intrafusal fibres?
Type Ia fibres signal rate of change of stretch and so peaks and then decreases again when increasing and then decreasing length. But type II signals stretch directly so increases and decreases directly with length change.
What is the role of proprioceptive afferent fibres?
Information about length and tension needs to be transmitted quickly, because it provides feedback that is used to control movement.
What is the fibre group, modality and conductive velocity of primary muscle spindle receptor?
A-alpha (Ia)
Change of muscle length
75-120 m/s
What is the fibre group, modality and conductive velocity of secondary muscle spindle receptors?
A-beta (II)
Muscle length
35-75 m/s
What is the fibre group, modality and conductive velocity of Golgi tendon organ receptors?
A-alpha (Ib)
Active tension
75-120
What is the fibre group, modality and conductive velocity of joint capsule machnoreceptors?
A-beta
Joint angle
35-75
What is the fibre group, modality and conductive velocity of stretch sensitive free endings?
Aδ
Excessive stretch or force
5-35
Summarise the direction of information between muscle fibres and the spinal cord.
Extrafusal alpha motor neurones and gamma muscle spindles to motor nerve fibres from the spinal cord. Muscle spindles and Golgi tendon organs to sensory nerve fibres and on to spinal cord.
Describe the tendon jerk reflex.
Key part of the neurological exam:
- Provides vital diagnostic information about the integrity of the neural pathway.
- The monosynaptic tendon jerk reflex (stretch reflex) opposes changes in muscle length and helps to maintain the basal level of moto neurone activity that produces resting muscle tone.
What does the stretch reflex generate and how?
Generates muscle tone:
- By opposing changes in muscle length, the stretch reflex helps to maintain limb position and muscle tone.
- Changes in sensitivity of the stretch reflex change resting muscle tone.
What is the role of gamma motor neurones?
Gamma motor neurone activity changes the sensitivity of muscle spindles to match desired muscle length.
What signals are sent upon muscle contraction?
Signal sent to alpha motor neurones for contraction and similar signal must be sent to gamma motor fibres for increased tension to in muscle contraction so that length can still be altered.
What is the role of muscle spindles in motor control?
- Movements are produced by alpha-gamma coactivation.
- Alpha motor neurone activity signals force of contraction.
- Gamma motor neurones activity signals desired muscle length, such as joint position.
- Muscle spindles compare actual muscle length with desired muscle length.
- The stretch reflex then corrects actual muscle length to desired muscle length.