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Module 1: Módulo 17: Introdução à mecânica e controle nervoso dos músculos esqueléticos

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Motor Commands into the CNS from Skeletal Muscles

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Motor Homunculus Tab.
The various parts of the body are represented in the substance of the brain. If one plots the sequence and the amount of tissue devoted to each part of the body, one comes up with a caricature of the human being. This caricature (distorted image) is referred to as the motor homunculus.

Pyramidal/Extrapyramidal Motor Systems Tab.
Various collections of neurons and their processes carry commands for actions to the individual skeletal muscles.
Originating in the brain, these neurons and processes pass through the brain stem into the spinal cord. In general, they are grouped into the pyramidal motor system and extra pyramidal motor system.

Since the pyramidal motor system is subject to volitional control, it can be used for testing during medical examinations.

The extra pyramidal motor system is more automatic. For the most part, control in this system is non-volitional.

Modulation of Commands Tab.
Several areas of the brain act as coordinators and modulators of the muscle activity of the body. These areas include the cerebellum and the basal ganglia. The sequential patterns of action to produce an overall motion appear to be programmed in the brain, particularly the cerebellum.

Motor Neurons.
The individual motor neuron has its cell body in the brainstem or spinal cord. The axon of the motor neuron passes out of the CNS to become a part of the nerves going to the individual skeletal muscles.

Motor Units.
In the skeletal muscle, the individual motor neuron (axon) has a terminal branching so that it contacts several striated muscle fibers. The actual number of striated muscle fibers contacted (innervated) by a single motor neuron are together known as a motor unit.

When its motor units are small, a muscle can produce very fine actions. The extra ocular muscles are an example of this.

With larger motor units, the muscle action is coarse.

A variable number of motor units may be called into action at a given moment. The number recruited is the number needed for the required action.

Neuromuscular Junctions Tab.
At the end of each branch of the terminal branching of the motor neuron, is an enlargement known as the bouton. The bouton has a specific relationship with the sarcolemma of the striated muscle fiber. There is no actual physical contact. Instead, there is a little space known as the synaptic cleft.

Across this space to the striated muscle fiber, the command message is carried in the form of a special chemical acetylcholine (ACh). Once the message has been transferred, the ACh is degraded to no longer function.

Nerve gases and organic phosphate insecticides produce their effects by interfering with the transmission or reception of messages across the synaptic cleft.

Axial Versus Appendicular Muscular Control Tab.
The musculature of the body can be thought of in two categories: axial and appendicular.

The axial musculature includes the skeletal muscles of the trunk and the upper and lower girdle regions.

The appendicular musculature includes the skeletal muscles of the upper and lower limbs beyond the girdles.

These two categories are important because they are controlled by the nervous system in different ways. Also, they react quite differently to various physiological and nervous situations.

The muscles that operate the hands tend to be very specifically and highly controlled by the nervous system.