The human nervous system is a communication network of neurons (nerve cells) which serve to conduct a variety of messages to and from various parts of the body. There are two distinct types of nerve tissues: neurons which are the basic structural units of the nervous system, and neuroglia which act as supporting nerve tissue for the neurons. The nervous system is divided into two basic components: the central nervous system (CNS), and the peripheral nervous system (PNS). the following are the basic components of the human nervous system.
The central nervous system:
The CNS consists of the brain and the spinal cord. The brain functions as the central processing unit of the CNS. The spinal cord functions as a conductor of nerve impulses which travel to the brain on ascending fibers (afferent sensory fibers), and away from the brain and out to the rest of the body on descending fibers (efferent motor fibers). There are 31 pairs of spinal nerve fibers. The CNS is covered by skeletal and fibrous tissue and surrounded by cerebrospinal fluid (CSF) which serves as protection and circulatory vehicle. The brain and spinal column are protected by three different types of membranes (meninges) known as dura matter, arachnoid matter and pia matter.
Peripheral nervous system:
The PNS consists of sensory neurons and motor neurons. Sensory neurons have sensory receptors which recognize external and internal information that is received by the dendrites. This information travels along the nerve cell body of the neuron and out through the axon into the CNS. This information is processed and sent out via the motor pathway. Within this pathway, the information can go either through the somatic (voluntary) or autonomic (involuntary) nervous systems.
The somatic nervous system controls conscious activities, such as heat, cold, light, etc. The autonomic nervous system controls unconscious activities, including temperature, pain, pressure, etc. the autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system´s main function is to respond to stressful (flight or fight) situations. The parasympathetic nervous system´s main function is to bring the body back to a calm state.
Neurons can be categorized into three distinct types: sensory, motor, and inter neurons. Sensory neurons transmit information detected by the senses (touch, smell, sight, taste, and hearing) and send this information to the CNS. Motor neurons transmit information from the CNS to various organs in the body. Inter neurons, which represent the majority of neurons, integrate sensory and motor transmissions in the brain. They´re related to perception, thought, and sight.
Nerve tissue receives, transmits, and stores information throughout the nervous system. Information is transported as electrical impulses along the neurons. The majority of neurons in the CNS are multipolar neurons containing short fibers known as dendrites and one long fiber known as an axon. Bipolar neurons have one dendrite and one axon. Unipolar neurons only have one fiber which functions as both a dendrite and an axon.
The nerve cell body of the neuron contains the nucleus, mitochondria, Golgi bodies, and lysosomes. Dendrites receive nerve impulses and conduct them to the cell body. These nerve impulses are carried away from the nerve cell body by axons; axons, though, microscopic in diameter can extend up to three feet long, for instance, the ones extending from the spine to the foot. The myelin sheath, which wraps around the axons, is produced by Schwann cells and functions as an electrical insulator during impulse transmission.
Neurons do not actually make contact. A space between the terminal bulb of one neuron and the dendrites of the second neuron, known as the synaptic cleft, prevents the transmission transfer from one neuron to the next. Information is transferred from one neuron to another by chemicals known as neurotransmitters. Neurotransmitters are made and stored on one side of the synaptic cleft allowing the information to flow in only one direction. The presynaptic cleft is the membrane through which the neurotransmitters enter the synaptic cleft. Within the postsynaptic membrane of a second neuron lies a chemical which inactivates the used neurotransmitter.
The nervous system main function is to generate a response to a stimulus. To respond to a stimulus, the nervous system first has to receive a signal; after the signal is received, the sensory neurons must transmit that signal to the CNS; the CNS must integrate the electrical impulse into a suitable reaction and send it to the appropriate receptors which will bring about a response. All this process occurs continuously throughout the day.