HUF 2-44 Skeletal muscles and motor reflexes

Question 1

Motor unit

Answer 1

• each motoneuron innervates a few to thousands of ms. fibres
• each ms. is composed of 1k to 1m ms. fibres]
• Henneman’s Size Principle: smaller (fatigue-resistant) motor units are recruited before larger motor units

Question 2

Muscle fibres

Answer 2

Sarcomere:

• basic contractile unit of ms.
• wrapped by sarcoplasmic reticulum (Ca2+ for contraction; T tubules)
• overlapping thick (myosin) and thin (actin) filaments
• Ca2+ => troponin, tropomyosin
• Z disc: boundaries of sarcomere; midline of actin

Amount of force generated depends on neural activation level and ms. length

Question 3

Motor coordination in CNS

Answer 3

Motor cortical areas
- voluntary actions

Spinal cord and brainstem

• voluntary actions
• locomotion
• stereotypical motor patterns elicited by sensory stimuli (reflexes)

Question 4

Spinal reflexes

Answer 4

• Coordinated motor responses, mediated by spinal cord that can be elicited by sensory stimuli
• Monosynaptic vs. polysynaptic

Question 5

Functions of reflexes

Answer 5

1. Stereotypical motor responses
2. Limb stability
3. Prevent ms. damage from high ms. tension
4. Facilitate ms. activation in locomotion

Question 6

Stereotypical motor responses

Answer 6

e.g.
Withdrawal reflex
Righting reflex

Plantar reflex
- normal adult: downward motion of big toe
- adults with brain lesions / newborns: upward motion of big toe; fanning out of other toes
=> Babinski sign

Question 7

Limb stability

Answer 7

Monosynaptic stretch reflex
- ms. contraction elicited by stretching of ms.
- reflexive response that resists change in ms. length
- ms. spindle (intrafusal vs. extrafusal ms. fibres)
=> ms. length / change in ms. length / speed of length change

Monosynaptic: agonist activation
Polysynaptic: antagonist inhibition (Ia inhibitory interneuron)

Additional mechanisms for limb stability:

1. ms. elasticity (~spring)
2. co-contraction of agonist and antagonist (stiffness)

Question 8

Prevent ms. damage from high ms. tension

Answer 8

Autogenic inhibition reflex (polysynaptic)
- reflexive relaxation of ms. (inhibition of activation) when ms. tension is high
- Golgi tendon organ (GTO)
=> Ib aff. axons in collagen fibres
=> Ib inhibitory interneuron
=> extensor ms.

Question 9

Facilitate ms. activation in locomotion

Answer 9

State-dependent reflex reversal
- from inhibitory to excitatory reflex
- GTO
=> Ib aff.
=> excitatory interneuron
=> extensor ms.

Spinal reflexes under inhibitory modulation by cerebrum and brainstem in general
∵ UMN lesion => hyperreflexia