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Neurons and neuroglia are the two main types of cells in the nervous system. The neuron is the actual nerve cell. The neuron is responsible for transmitting the impulses of the nervous system. It is made up of a cell body, an axon, and dendrites.
The cell body contains the nucleus and cytoplasm. The axon is a projection that extends from the cell body. Axons carry impulses away from the cell body. A myelin sheath, which covers some axons, protects the axon and increases the speed of transmission of the impulses. Axons covered with a myelin sheath make up the white matter of the nervous system. Those without the myelin sheath make up the gray matter of the nervous system. Dendrites are branching extensions of the cell body. These carry impulses toward the cell body. Neurons are not connected to one another. There is a space that separates the axon of one neuron and the dendrite of another. This space, known as a synapse, is where one neuron communicates with another. Neurotransmitters are released into the synapse. These activate or inhibit the transmission of nerve impulses across the synapses. Interneurons are connecting neurons that carry impulses from afferent nerves (nerves that transmit impulses toward the brain and spinal cord) to efferent nerves (nerves that transmit impulses away from the brain and spinal cord).
Neuroglia support neurons. They protect the nervous system through phagocytosis, a process of engulfing and digesting unwanted substances. The three types of neuroglia cells are astrocytes, microglia, and oligodendrocytes. The largest and most numerous of the neuroglial cells are astrocytes. Astrocytes have a star-shaped appearance due to their numerous radiating processes. These cells form a tight sheath by wrapping themselves around the brain's blood capillaries. The blood-brain barrier is formed by this sheath and the wall of the capillary. The blood-brain barrier prevents harmful substances from the bloodstream from entering the brain. Microglia cells are phagocytic in nature and increase in number during times of injury or infection. Oligodendrocytes are smaller than astrocytes. The processes of the oligodendrocytes form the myelin sheath that covers the axons of many neurons in the body. This sheath acts as an insulator and helps the transmission of nerve impulses.
The Structures and Functions of the Peripheral Nervous System
The peripheral nervous system (PNS) consists of nerves and ganglia. It contains afferent nerves and efferent nerves. Afferent nerves are sensory nerves which carry impulses from the body to the central nervous system (CNS). Efferent nerves are motor nerves which carry impulses from the CNS to the muscles and glands.
The PNS is further divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system (SNS) is responsible for the voluntary control of body movements through the skeletal muscles. The autonomic nervous system (ANS) is responsible for the involuntary control of the smooth muscles, cardiac muscles, and glands. The ANS consists of sympathetic and parasympathetic nerves. The sympathetic nerves control crucial involuntary body functions in response to stress such as increasing the heart rate, constricting blood vessels, raising blood pressure, and activating the "flight-or-flight" response. The parasympathetic nerves complement the somatic nervous system by returning the body to a more restful state (e.g., slowing down the heart rate, increasing glandular secretions, relaxing sphincters).
The Primary Components of the Central Nervous System
The two primary components of the central nervous system (CNS) are the brain and the spinal cord. The brain is protected by the skull; the spinal cord is protected by the vertebrae of the spinal column. The brain and spinal cord are also protected by meninges and cerebrospinal fluid. The meninges consist of three layers of protective membranes: the dura mater, arachnoid membrane, and the pia mater. The dura mater (a tough white connective tissue) is the outermost layer of the meninges. The arachnoid membrane is the middle layer of the meninges. Its threadlike strands give it a spider web appearance and are attached to the innermost layer of the meninges. The subarachnoid space beneath the arachnoid membrane contains cerebrospinal fluid which provides additional protection by serving as a shock absorber. Tightly bound to the surface of the brain and spinal cord is the pia mater. It is the innermost layer of the meninges.
Impulses are carried and processed through the CNS by way of afferent and efferent nerves. Afferent nerves are sensory nerves that carry impulses from the body to the CNS. Efferent nerves are motor nerves that carry impulses from the CNS to the muscles and glands.
The Structures and Functions of the Brain
The brain, one of the largest organs in adults, is a very complex structure. The four major divisions of the brain are the cerebrum, the cerebellum, the diencephalon, and the brain stem.
The largest part of the brain is the cerebrum. It is responsible for higher brain function such as thought and action. The cerebral cortex (the surface of the cerebrum) features convolutions called gyri (which give the brain its characteristic appearance). These are separated by grooves called sulci. The cerebrum is divided into two hemispheres: the right cerebral hemisphere and the left cerebral hemisphere.
The cerebellum is responsible for maintaining muscle tone and coordinating movement and balance. It is attached to the brain stem.
The diencephalon's main structures are the thalamus, hypothalamus, and the pineal gland. The thalamus is responsible for receiving and relaying all sensory stimuli (with the exception of smell) to the cerebral cortex. Located below the thalamus is the hypothalamus. This is the control center of all autonomic regulatory activities. It is also responsible for regulating the endocrine system processes and many sensory functions (i.e., body temperature, sleep, and appetite). The pineal gland, also known as the pineal body, is responsible for regulating the body's biological clock and producing melatonin, a hormone that regulates the body's circadian rhythm and controlling the female menstrual cycle.
The brain stem (located between the diencephalon and the spinal cord) consists of the midbrain, pons, and medulla oblongata. It serves as a pathway for the transmission of impulses between the brain and spinal cord. The brain stem also controls vital functions such as respiration, blood pressure, and heart rate.
10 Pathological Conditions That Affect the Nervous System
anencephaly: a congenital disorder in which the brain and spinal cord are absent at birth; a condition incompatible with life; detection can be done through an amnioscentesis or ultrasonography in early pregnancy.
Bell's palsy: a temporary or permanent unilateral weakness or paralysis of the muscles in the face resulting from trauma to the face, an unknown infection, or a tumor; treatment includes gentle massaging, application of warm moist heat, facial exercises, prednisone, and analgesics.
brain abscess: localized accumulation of pus located anywhere in the brain tissue due to an infectious process; treated with intravenous antibiotics and may include mannitol, in addition to steroids; may require surgical drainage if response to treatment is not good.
carpal tunnel syndrome: a pinching or compression of the median nerve within the carpal tunnel as a result of inflammation and swelling of the tendons; treatment includes anti-inflammatory medications, splints, physical therapy, discontinuing the repetitive overuse; surgery may be necessary if treatment fails to relieve the pain.
headache (cephalgia): involves mild to severe pain anywhere in the cranial cavity; may be chronic or acute; most headaches can be relieved by a mild analgesic.
intracranial tumors: occur in any structural region of the brain; causes the normal brain tissue to be displaced and compressed which leads to progressive neurological deficiencies; treatment is through surgical removal, when possible; may require use of radiation and/or chemotherapy based on location, classification, and type.
myasthenia gravis: chronic progressive neuromuscular disorder resulting in severe skeletal muscle weakness and fatigue; may require restricted activity, soft or liquid diet, and medications.
petit mal seizure: also known as absence seizures; small seizures resulting in a sudden temporary loss of consciousness lasting a few seconds; occur more frequently in children.
shingles (herpes zoster): an acute viral infection characterized by inflammation of the underlying spinal or cranial nerve pathway; treatment include antiviral medications, analgesics, and corticosteroids.
spina bifida: a congenital defect of the CNS; condition in which the back portion of one or more vertebrae is not closed; no treatment is recommended without symptoms.