Central nervous system(CNS)

 To comprehend how even the most basic stimulus can cause muscle activity, we must next consider the complexity of the CNS.

Brain

The brain is composed of numerous parts. For our purposes, we subdivide it into the four major regions: the cerebrum, diencephalons, cerebellum, and brain stem.

Cerebrum

The cerebrum is composed of the right and left cerebral hemispheres. These are connected to each other by fiber bundles(tracts) reffered to as the corpus callosum, which allows the two hemispheres to communicate with each other. The cerebral cortex forms the outer portion of the cerebral hemispheres and has been referred to as the site of the mind and intellect. It is also called the gray matter, which simply reflects its distinctive color resulting from lack of myelin on the cell bodies located in this area. The cerebral cortex is the conscious brain.

It allows people to think, to be aware of sensory stimuli, and to voluntarily control their movements.

The cerebrum consists of five lobes – four outer lobes and the central insula. Its four outer lobes have the following general functions:

• Frontal lobe: general intellect and motor control ;
• Temporal lobe: auditory input and its interpretation ;
• Parietal lobe: general sensory input and its interpretation ;
• Occipital lobe: visual input and its interpretation.

The three areas in the cerebrum that are of primary concern to our discussion and that we discuss later in this chapter are the primary motor cortex, in the frontal lobe; the basal ganglia, in the white matter below the cerebral cortex; and the primary sensory cortex, in the parietal lobe.

Diencephalon

The region of the brain known as the diencephalons is composed mostly of the thalamus and the hypothalamus. The thalamus is an important sensory integration center. All sensory input(except smell) enters the thalamus and is relayed to appropriate area of the cortex. The thalamus regulates what sensory input reaches the conscious brain and thus is very important for motor control.

The hypothalamus, directly below the thalamus, is responsible for maintaining homeostasis by regulating almost all processes that affect the body’s internal environment. Neural centers here assist in the regulation of:

• Blood pressure, heart rate and contractility, respiration, and digestion;
• Body temperature;
• Fluid balance;
• Neuroendocrine control;
• Emotions;
• Thirst;
• Food intake;
• Sleep-wake cycles.

Cerebellum

The cerebellum is located behind the brain stem. It is connected to numerous parts of the brain and has a crucial role in coordinating movement.

Brain stem

The brain stem, composed of the midbrain, the pons, and the medulla oblongata; is the stalk of the brain, connecting the brain and the spinal cord. Sensory and motor neurons pass through the brain stem as they relay information between the brain and the spinal cord. This is the site of origin for 10 of the 12 pairs of cranial nerves. The brain stem also contains the major autonomic regulatory centers that control the respiratory and cardiovascular systems.

A specialized collection of neurons in the brain stem known as the reticular formation, is influenced by, and has an influence on, nearly all areas of the CNS. These neurons help:

• Coordinate skeletal muscle function;
• Maintain muscle tone;
• Control cardiovascular and respiratory functions;
• Determine our state of consciousness(both arousal and sleep).

The brain has a pain control system, called an analgesia system. The enkephalins and beta-endorphin are important opiate substances that act on the opiate receptors in the analgesia system to help reduce pain. Research has demonstrated that exercise of long duration increases the natural levels of these opiate substances.

Spinal cord

The lowest part of the brain stem, the medulla oblongata, is continuous with the spinal cord below. The spinal cord is composed of tracts of nerve fibers that allow two-way conduction of nerve impulses. The sensory(afferent) fibers carry neural signals from sensory receptors, such as those in the skin, muscles, and joints, to the upper levels of the CNS. Motor(efferent) fibers from the brain and upper spinal cord transmit action potentials to end organs.