BIPN 100 Lecture #8 Motor Pathways, Muscle Types, and Metabolism

Creatine Kinase (CK)

enzyme, catalyzes transfer of phosphate from ATP to creatine

• E stored by PCr in high-E phosphate bonds

• at rest for phosphocreatine

Phosphocreatine during contraction

• creatine kinase catalyzes transfer of phosphate from PCr to ADP

• regenerates ATP

Fatigue

condition wherein muscle cannot generate or sustain expected power output

Determinants of force of contraction

1. Type of skeletal muscle

2. length/strength of the muscle at rest

3. frequency of MAP

4. motor units recruited

Slow-twitch muscle fibers (ST or Type I fibers)

• Function: produce long-lasting movements; i.e posture, standing, walking

• Metabolism: oxidative phosphorylation —> lots of ATP

• Mechanism: low force, slow-twitch

  • fatigue resistant due to low ATP use

• Structure: red muscle: high myoglobin, more mitochondria, smaller diameter —> O2 diffuses easily across cell and into mitochondria

Fast-twitch Glycolytic Muscle Fibers (FG or type IIX)

• Function: fine, fast movement

• Metabolism: anaerobic glycosis —> ATP + H+

• Mechanism: high-force, fast twitch, fast-fatiguing due to higher ATP use

• Structure: white muscle, lower myoglobin levels, fewer mitochondria, large diameter

Fast-twitch oxidative/glycolytic fibers (FOG or type IIA)

• metabolism: combination of oxidative and glycolytic metabolism

  • fatigue-resistant

  • medium twitch force and speed

Length-Tension Relationship

• tension determined by the length of resting fibers

  • tension generated by at overlapping myofilaments

• Mechanism

  • long sarcomere —> low overlap, fewer crossbridges, less tension

  • short sarcomere —> excess overlap, filaments run into M-line and Z disks, thin filaments overlap, less tension

Summation-Contraction relationship

force generated by contraction of 1 muscle fiber

• APs temporally spaced —> relaxation between contraction

• tetanus: maximal contraction produced by high frequency APs

Recruitment of motor units

• small stimulus: lower-threshold motor neurons recruited

  • controls slow twitch fibers

• medium stimulus: medium threshold motor neurons recruited

  • controls FOG fibers

• large stimulus: highest-threshold motor neurons recruited

  • controls FG fibers

• ST + FOG + FG fibers —> greatest contraction force

Lateral Corticospinal Tract

pyramidal tract

• Decussate in medulla

• Descend ipsilaterally down lateral

  parts of spinal cord

• Synapse in spinal cord

• Function: Control of fine muscle

  movement

Anterior Corticospinal Tract

Descend contralaterally down

anterior parts of spinal cord

○ Decussate at spinal level

○ Synapse in spinal cord

● Function: Control bilateral

“posture” muscles

Proprioceptors

sensory receptors that detect the position of the body in space, movement, exertion, and changes to skeleton and muscles

Synapses

• Monosynaptic reflex: afferent neuron synapses directly on efferent neuron in ventral horn

• Polysynaptic reflex: interneurons converge and diverge to control motor neurons

Efferent neurons and effectors

• alpha motor neurons (a-mn) innervate extrafusal muscle fibers and generate force of contraction in skeletal muscle

• gamma (y) motor neurons innervate intrafusal muscle fibers

Intrafusal muscle fibers

serves as proprioceptors AND effectors —> produce compensatory contractions to retain proprioceptive info about muscle fiber stretch

• Structure: Non-contractile center region of intrafusal muscle fibers is

  • passively stretched when extrafusal muscle fiber elongates

Muscle spindle afferents

proprioceptors that innervate intrafusal fiber

• Structure: nerve endings surrounded by a sheath/connective tissue that encloses noncontractile regions

• Function: Detect elongation of muscle

Muscle Tone

monosynaptic reflex that maintains muscle tension @ rest

Sequencing Muscle Tone Reflex

1. Muscle @ resting length

2. Muscle spindle afferents are tonically active

3. Synapse on and excite a-mn

4. a-mn tonically active

5. Tonic extrafusal muscle contraction

Stretch reflex: (myotatic reflex)

Monosynaptic reflex that prevents damage from over-stretching

Sequencing Stretch Reflex

1. Movement or placement of load

2. Increased muscle length

3. Noncontractile center of intrafusal

   fibers stretch

4. Spindle sensory fiber firing increases

5. Synapse on & excite α-mn

6. Increased firing of α-mn

7. Contraction of extrafusal muscle fibers