Sonoma State University
Department of Biology - Hanes
Neurophysiology

Chapter 13: Somatic Motor System

Terms

Extensors, flexors, antagonists, motor unit, reciprocal inhibition, axial, proximal, distal muscles, pattern generator

Outflow

Mixed spinal nerves. Arms from C3-T-1 and legs from L1-S3 have swollen ventral horns. Contractile fibers are enervated by A-alpha motor neurons. All of the muscle fibers controlled by one alpha motor neuron are called a motor unit. They all act together. There is a one-to-one correlation between nerve action potentials and muscle action potentials. A continual contraction requires a train of input action potentials. Graded responses are controlled with frequency of stimulation and recruitment of more or fewer motor units. A motor unit can be from 3-1000 muscle fibers depending on the strength or accuracy of control needed. Most muscles have varied motor unit size and they recruit from smallest to largest. Smaller units have smaller alpha motor neurons and they are more easily excited.

Inputs

Sensory input from muscle spindles - muscle stretch and position
Input from the cerebral cortex and brain stem - voluntary control and trained activities, smoothing contraction
Excitatory and inhibitory inputs from spinal interneurons - automatic control from load, balance, and effects of other muscles

Skeletal Muscle Cell Types

	Fast Glycolytic	Fast Aerobic	Intermediate
Contraction speed	Fast	Not as fast
Strength	Very Strong	Strong
Fatigue rate	Fast	Slow
Mitochondria	Few	Many
Nerve firing frequency	30-60 Hz	20-30 Hz
Blood supply	Little	Lots
Waste Products	Lactic Acid	H2O & CO2
Glycogen stores	Lots	Little
O2 requirement	Little	Lots
Exercise effect	Increase diam. and strength	Increase blood supply

The nerve determines the type of muscle and motor unit. p 444.

Molecular basis of contraction

Anatomy

Fiber or Myofiber, Myofibrile, Thick filament, thin filament, Sarcoplasmic reticulum, T-tubule, Sarcolemma, Actin, Myosin, Myosin heads, Troponin, Tropomyosin, Dihydropyridine receptor, (on T-tubule) Ryanodine receptor (on Sarcoplasmic reticulum)

Know the cycle of the myosin head attachment and detachment to actin and the influence of ATP.

Spinal control of motor units

Proprioception

Muscle spindles p 452 detect stretch in muscle. All of the alpha motor neurons that serve muscle fibers monitored by one muscle spindle reciprocally stimulate each other.

A muscle spindle is an encapsulated group of modified skeletal muscle cells called Intafusal fibers. Normal muscle cells are then called extrafusal fibers. The intrafusal fibers are attached to the same tendons at the extrafusal fibers, have contractile ends and a stretch receptor in the middle that is monitored by a Ia sensory neuron.

When the frontal cortex wants to contract a muscle, it sends a message to both alpha motor neurons and gamma efferent neurons. The alpha motor will get the process started quickly and the gamma efferent will help to monitor the speed and strength of the contraction.

Events:

1. gamma efferent stimulated

2. ends of intrafusal fiber contract

3. stretch placed on center of intrafusal fiber

4. Sensory IA excited

5. Monosynaptic input to alpha motor

6. Contraction of extrafusal fibers

7. relieves stretch on intrafusal

8. inhibits or excites sensory IA and back to 5.

Golgi tendon organs p 456

Stretch receptors located in the junction between muscle cells and tendons. These are arranged in series with the contraction process. Ib sensory fibers synapse in the ventral horn with numerous interneurons that are inhibitory to alpha motor neurons that supply the muscle. Used to adjust pressures on the tendon.

It is these inhibitory cells that are blocked by strychnine.

Proprioception from joints.

Receptors in joints are rapidly adapting and thus give information about speed of angle change between bones. They are located in joint capsules and are connected to inhibitory interneurons. p. 457

Interneuron inhibition is necessary for precise control of motion. The spinal chord is also hard wired for reciprocal inhibition. p. 458. The alpha motor neurons of Flexors and antagonistic Extensors inhibit each other via inhibitory interneurons. The flexor of one limb is also hard wired to inhibit the flexor of the opposite limb in a similar fashion.

In the same manner the alpha motor neurons of flexor of one limb excite the extensor of the opposite limb which brings us to central pattern generators.

Walking and generators

Rhythmic activities abound in the nervous system. They include heart beats, breathing, walking, running, chewing, peristaltic waves, and many more. The following can all happen in one cell as a heart muscle cell.

1. membrane depolarizes

2. Na+ and Ca++ flow into cell via NMDA receptors

3. Ca++ activates K+ channels

4. K+ flows out of cell

5. Membrane hyperpolarizes

6. K+ channels close

7. Back to 1.

One can accomplish a similar effect with circuitry. See Figure 13.27 p 462