Motor Units

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In vivo, each muscle fiber receives a single axon terminal from a somatic motor neuron. The motor neuron stimulates the muscle fiber to contract by liberating acetylcholine at the neuromuscu-lar junction (described in chapter 7). The specialized region of the sarcolemma of the muscle fiber at the neuromuscular junction is known as a motor end plate (fig. 12.3).

The acetylcholine (ACh) released by the axon terminals diffuses across the synaptic cleft and binds to ACh receptors in the plasma membrane of the end plate, thereby stimulating the muscle fiber. Prior to its release, the ACh is contained in synaptic vesicles that dock and fuse with the plasma membrane of the axon terminal and undergo exo-cytosis (see chapter 7, fig. 7.21). The potentially deadly botulinum toxin, produced by the bacteria Clostridium botulinum, is selectively taken into cholinergic nerve endings and cleaves the proteins needed for the exocytosis of the synaptic vesicles. This blocks nerve stimulation of the muscles, producing a flaccid paralysis. Interestingly, botulinum toxin is now used medically in certain cases to relieve muscle spasms due to excessive nerve stimulation. For example, it is injected into an affected extraocular muscle in order to help correct strabismus (deviation of the eye). Intramuscular injections of Botox (a brand name for botulinum toxin) are also given for the temporary cosmetic treatment of skin wrinkles.

Somatic motor neuron axon - Muscle fiber nucleus

Nerve fiber branches

Motor end plate Myofibril

Somatic motor neuron axon - Muscle fiber nucleus

Axon Muscle FiberMotor Unit Axonal Terminal

Neuromuscular junction Somatic motor axons

Motor end plate of muscle fiber

Neuromuscular junction Somatic motor axons

Motor end plate of muscle fiber

Muscle fibers

■ Figure 12.3 Motor end plates at the neuromuscular junction. The neuromuscular junction is the synapse between the nerve fiber and muscle fiber. The motor end plate is the specialized portion of the sarcolemma of a muscle fiber surrounding the terminal end of the axon. (a) An illustration of the neuromuscular junction. Notice the slight gap between the membrane of the axon and that of the muscle fiber. (b) A photomicrograph of muscle fibers and neuromuscular junctions.

Fox: Human Physiology, I 12. Muscle: Mechanisms of I Text I © The McGraw-Hill

Eighth Edition Contraction and Neural Companies, 2003

Control

Muscle 329

The cell body of a somatic motor neuron is located in the ventral horn of the gray matter of the spinal cord and gives rise to a single axon that emerges in the ventral root of a spinal nerve (chapter 8). Each axon, however, can produce a number of collateral branches to innervate an equal number of muscle fibers. Each somatic motor neuron, together with all of the muscle fibers that it innervates, is known as a motor unit (fig. 12.4).

Whenever a somatic motor neuron is activated, all of the muscle fibers that it innervates are stimulated to contract. In vivo, graded contractions of whole muscles are produced by variations in the number of motor units that are activated. In order for these graded contractions to be smooth and sustained, different motor units must be activated by rapid, asynchronous stimulation.

Fine neural control over the strength of muscle contraction is optimal when there are many small motor units involved. In the extraocular muscles that position the eyes, for example, the innervation ratio (motor neuron:muscle fibers) of an average motor unit is one neuron per twenty-three muscle fibers. This affords a fine degree of control. The innervation ratio of the gas-trocnemius, by contrast, averages one neuron per thousand muscle fibers. Stimulation of these motor units results in more powerful contractions at the expense of finer gradations in contraction strength.

All of the motor units controlling the gastrocnemius, however, are not the same size. Innervation ratios vary from 1:100 to 1:2,000. A neuron that innervates fewer muscle fibers has a smaller cell body and is stimulated by lower levels of excitatory input than a larger neuron that innervates a greater number of muscle fibers. The smaller motor units, as a result, are the ones that are used most often. When contractions of greater strength are required, larger and larger motor units are activated in a process known as recruitment of motor units.

Test Yourself Before You Continue

1. Describe the actions of muscles when they contract, and define the terms agonist and antagonist in muscle action.

2. Describe the different levels of muscle structure, explaining how the muscle and its substructures are packaged in connective tissues.

3. Define the terms motor unit and innervation ratio as they relate to muscle function, and draw a simple diagram of a motor unit with a 1:5 innervation ratio.

4. Using the concept of recruitment, explain how muscle contraction can be graded in its strength.

Somatic motor neuron

Motor unit r

Motor unit

Somatic motor neuron

Motor unit

Spinal cord

■ Figure 12.4 Motor units. A motor unit consists of a somatic motor neuron and the muscle fibers it innervates. (a) Illustration of a muscle containing two motor units. In reality, a muscle would contain many hundreds of motor units, and each motor unit would contain many more muscle fibers than are shown here. (b) A single motor unit consisting of a branched motor axon and the three muscle fibers it innervates (the fibers that are highlighted) is depicted. The other muscle fibers would be part of different motor units and would be innervated by different neurons (not shown).

Motor unit

Skeletal muscle fibers

Neuromuscular Somatic junctions motor axon

Skeletal muscle fibers

Neuromuscular Somatic junctions motor axon

Skeletal Muscle Motor Unit

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Responses

  • fabrizia pirozzi
    How many motor units in a human?
    8 years ago
  • REIJA TAIPALE
    What is muscle tone motor unit activation in?
    8 years ago
  • Milla
    What contains a somatic motor neuron and all the skeletal muscle fibers it stimulates?
    8 years ago
  • Fre-Swera
    What are the steps for the neuron to stimulate a muscle fiber to contract in a sequence?
    8 years ago
  • saradas
    How a motor unit depicts 3 different contractions?
    8 years ago
  • EDOARDO FERRARI
    Is a motor unit one nerve fiber?
    8 years ago
  • ashleigh
    How many neurons innervate a muscle fiber?
    8 years ago
  • hildigrim
    How many neurons control each muscle fibre?
    8 years ago
  • virginio
    What single axon controls all the muscle fibers?
    7 years ago
  • otto
    What are the motor units in the human body?
    7 years ago
  • Kinfe
    Which motor unit has a neuron fiber ratio of 1:20?
    5 years ago

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