The functions of the cells of the nervous system are very diverse. One of the types is motor neuron (motoneuron). Its name, translated from Latin, means "setting in motion." It is with its mediation that muscle contraction occurs.
The peculiarity of motor nerve cells is that their cytoplasm does not evenly surround the nucleus, but forms two processes. One of them is shorter (dendrite) receives a nerve impulse, the second (axon) transmits it further.
Thus, a peripheral motor neuron conducts a nerve impulse from the central nervous system to the muscle. In muscle tissue, its long process forks and connects with dozens of muscle fibers.
Types of motor neurons
By localization, motor neurons are divided into central and peripheral. The central ones are located in the fabric brain. They are responsible for conscious, controlled muscle contractions.
Motor neurons that go directly to muscle fibers are called somatic.
The bodies of the motor neurons of the somatic nervous system are located in the region of the anterior horns of the dorsal brain and are located in groups, each of which is responsible for the contraction of a strictly defined musculature. For example, the motor neurons of the cervical spine control the muscles of the arms, while the lumbar spine is responsible for the innervation of the legs.
Peripheral nerve cells responsible for movement are classified as follows:
- large alpha motoneurons;
- small alpha motoneurons;
- gamma motor neurons;
- Renshaw cells.
Large alpha cells form large conductive stems. Small alpha and gamma neurons have thinner axons. Renshaw cells are part of large stems and are used for signal switching.
Gamma motor neuron loop
Motor neuron functions
Central and peripheral motor nerve cells work in concert. Together, they provide contraction of certain muscle groups and allow a person to perform any action.
For coordinated movements of the limbs, a simultaneous contraction of the flexors and extensors is necessary. When the flexors work, the initial excitation signal arises in the region of the precentral gyrus of the corresponding hemisphere.
Cells called pyramidal. Gathered together, their processes form the so-called pyramidal motor path. Further, the signal goes to the anterior horns of the spinal cord, from where it is transmitted directly to the myofibrils.
The activating effect on the motor neurons of the extensor muscles is exerted by special centers of the posterior parts of the cerebral hemispheres. They form the dorsal and ventral tract. Thus, two areas of the brain are involved in the formation of coordinated movement.
By the nature of the function, the nerve cells involved in the process of muscle contraction are subdivided into motor and intercalary neurons. The former are responsible for the executive function, while intercalary ones serve to coordinate nerve impulses. This particular species is smaller and more numerous.
For comparison, in the area of the anterior horns there are 30 times more of them than the motor ones. When the excitation is conducted along the axon of the motor nerve, it passes initially to the intercalary neuron. Depending on the nature of the signal, it can be amplified or attenuated, after which it is transmitted further.
The insertion type cells have more processes and are more sensitive. They have a large number of processes and are also called multipolar.
To optimize the signals outgoing along the axons and going to the muscle fibers, special Renshaw cells are used, which transmit excitation from one process to another. This mechanism serves to equalize the intensity of the nerve signal.
Along the process of the motor neuron, the impulse reaches the muscle fiber, which contracts. Each group of motor neurons and muscle fibers innervated by them are responsible for certain movements.
Nerve cells that provide motor function:
| Types of neurons | Localization | Function |
|---|---|---|
| central innervating flexors |
area of the precentral gyrus | contraction of skeletal flexor muscles by transmitting impulses to the anterior horns |
| central innervating extensors |
hindbrain area | contraction of skeletal extensor muscles by impulse transmission to the anterior horns |
| peripheral alpha | anterior horns of the spinal cord | direct skeletal muscle contraction |
| peripheral gamma | anterior horns of the spinal cord | regulation of tone |
| intercalary | all departments of the central nervous system | communication of signals within the central nervous system |
Large alpha neurons conducting a strong impulse cause myofibrils to contract. Small ones conduct weak signals and serve to maintain muscle tone.
In addition to the fibers responsible for contraction, muscle tissue also contains special spiral fibrils that regulate the strength of muscle tension.
These extrafusal muscle fibers are innervated by gamma neurons.
Excitation of the gamma motor neuron leads to an increase in the stretching of the myofibrils and facilitates the passage of the impulse of tendon reflexes. An example would be the passage of a nerve signal along the arch of the knee reflex.
The fine tuning of muscle tone is achieved by the well-coordinated work of peripheral motoneurons, which allows precise coordinated movements. When peripheral motor neurons are damaged, muscle tone disappears and movement is impossible.
How does a motor neuron work?
In order for a bioelectric impulse to arise, a potential difference is required on the nerve cell sheath. This occurs as a result of changes in the concentration of potassium and sodium ions from the outer and inner surfaces of the membrane.
In the future, the impulse passes to the end of the long appendix - axon and reaches the junction with another cell. The place of such contact is called synapse.
On the other side of the synapse, a short branching process is adjacent to the place of contact - dendrite. Signaling through the synapse is mediated by active chemicals called neurotransmitters.
Having arisen on the dendrite, the signal propagates along its shell and goes on to the axon. To contract skeletal muscle, the signal originates in the motor neuron of the cortex, passes along the pyramidal pathway, passes to the intercalary neuron, and then to the region of the anterior horns of the spinal cord. This chain ends in muscle tissue.
The result of the excitation of the motor center of the cortex will be a reduction in the group of muscle fibers.
Symptoms of the lesion of the central motor neuron
Damage to the central motor nerve cells occurs most often when stroke. At ischemia or hemorrhage into the substance of the cerebral hemispheres, a piece of tissue dies off. Such defeats are almost always one-sided.
As a consequence, at damage to central motor neurons dysfunctions of the muscles are observed on one side. The most noticeable symptom is unilateral paralysis, resulting in the inability to actively move the arm and leg.
On the same side, muscle tone in the trunk and facial muscles of the face decreases. The defeat of the central motor areas is accompanied by a number of changes in reflex activity.
Clinically, this is expressed in the appearance of a variety of pathological reflexes. Their combination, decreased muscle tone and sensitivity disorders allow the doctor to establish a diagnosis.
