The nervous system is made up of two distinct parts: the central nervous system (brain and spinal cord) and the peripheral nervous system. In both systems, neurons communicate electrically and chemically with one another along pathways. These pathways allow the nervous system as a whole to conduct its incredibly broad array of functions, from motor control and sensory perception to complex thinking and emotions.
Neurons (or nerve cells) have several key anatomical features that contribute to their specialized functions. These cells typically contain an axon, a long projection from the cell that sends information over a distance. These cells also have dendrites, which are long, branching extensions of the cell that receive information from neighboring cells.
Neurons and nerve cells do not touch; instead, communication occurs across a specialized gap called a synapse. The chemicals that facilitate communication across synapses are known as neurotransmitters, and include serotonin and dopamine. Communication occurs when electrical signals cause the axon terminal to release neurotransmitters.
Nerve cells are accompanied by glia, or supporting cells, that surround the cell and provide support, protection, and nutrients. In the peripheral nervous system, the primary glial cell is a Schwann cell. Schwann cells secrete a fatty substance called myelin that wraps around the neuron and allows much faster transmission of the electrical signal the neuron is sending.
Generally, neurons can be divided into two types. Afferent (sensory) cells relay messages to the central nervous system, and efferent (motor) cells carry messages to the muscles.
The central nervous system, which includes the brain and spinal cord, is responsible for arguably the body’s most complex and abstract functions, including cognition, emotion, and behavioral regulation. The brain is divided into six general regions:
The cerebrum and cerebellum are further broken down into lobes that each carry out a broad common function. For example, in the cerebrum, the processing of visual information occurs in the occipital lobe, and the temporal lobe is involved in language comprehension and emotional associations.
Alzheimer’s disease, which causes dementia, is the result of damaged neurons in the cerebral cortex, the area of the brain responsible for higher order functions like information processing and language.
In addition to its organization by lobes and structures, regions of the brain are also designated by myelination status: white matter regions are myelinated and gray matter regions are unmyelinated. Brain structures in the cerebral cortex (the outermost brain layer) form a convoluted pattern of gyri (ridges) and sulci (valleys) that maximize the ratio of surface area to volume.
The peripheral nervous system, which includes all the nerve cells outside the brain and spinal cord, has one main function and that is to communicate between the central nervous system and the rest of the body.
The peripheral nervous system is further divided into two systems. The automatic nervous system is the part of the peripheral nervous system that controls involuntary bodily functions such as digestion, respiration, and heart rate. The autonomic nervous system is further broken down into the sympathetic nervous system and parasympathetic nervous system.
The sympathetic nervous system is responsible for the body’s reaction to stress and induces a “fight or flight” response to stimuli. For instance, if an individual is frightened, the sympathetic nervous system increases the person’s heart rate and blood pressure to prepare that person to either fight or flee.
The “fight or flight” reaction includes accelerated breathing and heart rate; dilation of blood vessels in muscles; release of energy molecules for use by muscles; relaxation of the bladder; and slowing or stopping movement in the upper digestive tract.
In contrast, the parasympathetic nervous system is stimulated by the body’s need for rest or recovery. The parasympathetic nervous system responds by decreasing heart rate, blood pressure, and muscular activation when a person is getting ready for activities such as sleeping or digesting food. For example, the body activates the parasympathetic nervous system after a person eats a large meal, which is why that individual may then feel sluggish.
The second part of the peripheral nervous system, called the somatic nervous system, controls sensory information and motor control. Signals from the brain travel down the spinal cord before exiting and communicating with motor neurons, which join with muscle fibers at neuromuscular junctions. Because individuals can control the movement of skeletal muscle, this part of the nervous system is considered voluntary.
Some reflexes, or automatic response to stimuli, are able to occur rapidly by bypassing the brain altogether. In a reflex arc, a signal is sent from the peripheral nervous system to the spinal cord, which then sends a signal directly to a motor cells, causing movement.
The nervous system can be affected by a number of degenerative diseases that result from the gradual breakdown of nervous tissue.
The nervous system is also susceptible to infections, some of which can be life threatening. Meningitis is inflammation of the meninges, the protective membrane that surrounds the brain and spinal cord, and encephalitis is inflammation of the brain. Both conditions can be caused by viral or bacterial pathogens.
Epileptic seizures are brief episodes caused by disturbed or overactive nerve cell activity in the brain. Seizures range widely in severity and may include confusion, convulsions, and loss of consciousness. They have many causes, including tumors, infections, head injuries, and medications.
In an ischemic stroke, blood flow in the brain is blocked by a thrombus, resulting in damage to brain cells. In a hemorrhagic stroke, uncontrolled bleeding in the brain prevents oxygen from reaching cells and increases the pressure inside the skull, damaging the brain.