11.4: The Peripheral Nervous System The Peripheral Nervous System (PNS) is in direct communication with the Central Nervous System (CNS). The PNS is responsible for the regulation both movement and the internal environment of the body. The PNS is divided into two sub-systems, the afferent systems which receives the stimulus and delivers it to the CNS and the response to this stimulus, follows along the efferent system to its effectors, muscles and glands, to maintain normal body function and homeostasis. Efferent System The efferent system is responsible for carrying the nerve impulse to the muscles and glands, acting as effectors and responding to the stimulus. The efferent system is divided into two systems, the somatic system which communicates to the skeletal muscles and the autonomic system, which communicates with the smooth muscles and glands. Somatic System Spinal nerves there are 31 pairs of nerves within the somatic system that transmit motor, sensory, and autonomic signals between the spinal cord and the rest of the body. The somatic system controls body s movements by carrying nerve signals from the CNS to the skeletal muscle groups through conscious and voluntary thought. The spinal nerves are identified by the area of the body in which they serve, there are 31 pairs of nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. The nerves emanate from the spinal column, between the vertebrate and reflect the segmental organization of the vertebrate body. Each spinal nerve pair serves a different part of the body. The cervical nerves serve the head and neck, the arms, and some back muscles. Thoracic nerves deal with the chest and abdomen. The lumbar nerves control the legs and sacral nerves deal with the bowel, bladder, and sex organs. The coccygeal nerve serves the rectum and anus.
Figure 1: the 31 pairs of spinal nerves are grouped according to the area of the body that they serve. Nerves can only carry impulses in one direction, sensory neurons delivers to the spinal cord and the motor neuron carries the impulses for a motor response. The dorsal roots carry the information to the CNS and the ventral root from the CNS. Both roots meet at the spinal cord (interneurons). Each of the roots branch repeatedly so that the nerves serve the entire body. The nerve cells of the efferent system, have their dendrites and cell bodies located in the spinal cord and only the axons radiate out from the CNS, to the skeletal muscles. Although the somatic system is voluntary and consciously controlled, there are some involuntary and non-conscious activity such as reflexes, shivering and constant muscle contractions that maintain body posture and balance.
Figure 2: from each side of the spinal cord, a dorsal and ventral root merge to form each of the 31 pairs of spinal nerves. Autonomic System and Homeostasis The autonomic nervous system is also an efferent system. It is responsible for maintain internal homeostasis as it response to both external and internal changes. This is an involuntary process, and controls such areas as; digestion, sweat glands, circulation, reproduction and excretion, by contracting the smooth muscles of the body. Sympathetic and Parasympathetic Divisions The autonomic system is divided into sub divisions, known as the sympathetic and parasympathetic systems. Each system has opposing action to the other, and is always active, making precise adjustments on organs. The sympathetic system will increase the heart rate, and the parasympathetic system will inhibit the response. In the autonomic system, two neurons are used. The first cell has its dendrites and cell body in the spinal cord, its axon then interacts with another neuron in the peripheral system. The axon meets an adjacent dendrite at a synapse, either a chemical or electrical synapse, and the impulse is passed on. The second nerve cell stimulates the effector and carries out the response.
Sympathetic division is associated with the nerves of the chest and abdomen. They exit via the thoracic and lumbar region of the spinal cord. Parasympathetic division is associated with the nerves that radiate from the cervical, dealing with the head, neck and body trunk, and sacral region, (lower digestive track and external genitalia), of the spinal cord. Figure 3: the autonomic system is divided in the sympathetic and parasympathetic divisions. These two divisions affect the functions of a variety of organs and glands. Only one side of each division is shown, since both divisions are duplicated on the left and right sides of the body. Pain and Painkillers Substantia Gelatinosa is a band of grey matter in the spinal cord, that when stimulated, produces a neurotransmitter that communicates with an injured region of the body; a simultaneous signal to the brain results in the perception of pain. Endorphins: natural painkillers belonging to a group of chemicals called neuro-peptides; contain 16 and 31 amino acids.
Enkephalins: natural painkillers belonging to a group of chemicals called neuro-peptides contain five amino acids and are produced by the splitting of larger endorphin chains.