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The Nervous System
Chapter 10
The nervous system consists of two principal division : A. Central nervous system (CNS)— includes
- Brain
- Spinal cord B. Peripheral nervous system (PNS)— includes
- Cranial nerves
- Spinal nerves
- Autonomic nervous system (ANS) (controls the automatic or involuntary functions (the heart rate, contraction of stomach and intestine, secretion from glands etc)
- Special senses (eye, ear, nose) and the sense organs found in the skin.
Cells of the Nervous System
The two types of cells found in the nervous system are called a. Neurons or nerve cells (Neurons conduct impulses), and b. Glia which are support cells (glia support neurons) Neurons Consist of three parts a. Cell body of neuron—main part b. Dendrites—highly branched projections of neuron that conduct impulses to cell body of neuron c. Axon—elongated projection that conducts impulses away from cell body of neuron to another neuron.
Neurons classified according to function, or direction of impulse they transmit. a. Sensory neurons : conduct impulses to the spinal cord and brain (CNS) from all parts of the body, also called afferent neurons b. Motor neurons : conduct impulses away from brain and spinal cord to muscles and glands; also called efferent neurons c. Interneurons : conduct impulses from sensory neurons to motor neurons; also called central or connecting neurons The axon is surrounded by a segmented wrapping of a material called myelin. Myelin is a white, fatty substance formed by Schwann cells in the peripheral nervous system. The same myelin sheath is produced by oligodendrocytes in the Central nervous system. The outer wrapped layer of a Schwann cell is called the neurilemma. It is clinically significant that axons in the brain and spinal cord have no neurilemma, because neurilemma plays an essential part in the regeneration of cut and injured axons. Therefore, the potential for regeneration in the brain and spinal cord is far less than it is in the peripheral nervous system. Glia (neuroglia) cell Glia or neuroglia do not specialize in transmitting impulses. Instead, they are special types of supporting cells. Types of glia cells: a. Astrocytes- Their threadlike branches attach to neurons and to small blood vessels, holding these structures close to each other. b. Microglia- They usually remain stationary, but in inflamed or degenerating brain tissue, they enlarge, move about, and act as microbe-eating scavengers. c. Oligodendrocytes- help hold nerve fibers together and also produce the myelin sheath that envelops nerve fibers located in the central nervous system.
Disorders of nervous tissue
Multiple sclerosis — The most common primary disease of the CNS is a myelin disorder called multiple sclerosis, or MS. It is characterized by myelin loss and destruction accompanied by varying degrees of oligodendrocyte injury and death. As the myelin around the axons is lost, nerve conduction is impaired, and weakness, in-coordination, visual impairment, and speech disturbances occur. The cause of multiple
Nerves- Bundles of axons in the Peripheral Nervous System bundled together like the strands of a cable. Tract - Bundles of axons in the Central Nervous System. White matter-Bundles of myelinated axons in the CNS form the white matter of the brain and cord. Gray matter- Brain and cord tissue composed of cell bodies and unmyelinated axons and dendrites is called gray matter because of its characteristic gray appearance. Each axon in a nerve is surrounded by a thin wrapping of fibrous connective tissue called the endoneurium. Groups of these wrapped axons are called fascicles. Each fascicle is surrounded by a thin, fibrous perineurium. A tough, fibrous sheath called the epineurium covers the whole nerve (outermost cover) Reflex arc- Reflex arc , a neurological and sensory mechanism that controls a reflex, an immediate response to a particular stimulus. The simplest kind of reflex arc (nervous system pathway) is a two-neuron pathway , so called because it consists of only two types of neurons: sensory neurons and motor neurons. Three-neuron arcs are the next simplest kind consists of all three kinds of neurons: sensory neurons, interneurons, and motor neurons. Examples of simple reflexes are the contraction of a muscle in response to stretching, the blink of the eye when the cornea is touched, and salivation at the sight of food. Ganglion -A ganglion is a group of nerve-cell bodies located in the PNS. This ganglion is located near the spinal cord Receptors - Receptors are the beginnings of dendrites of sensory neurons. Impulse conduction normally starts in receptors. They are often located far from the spinal cord (in tendons, skin, or mucous membranes,
for example).
Nerve Impulses
Self-propagating wave of electrical disturbance that travels along the surface of a neuron membrane, sometimes called action potentials or neuron’s firing. A nerve impulse is initiated by a stimulus like pressure, temperature, and chemical changes. o The membrane of each resting neuron has a slight positive charge on the outside and a negative charge on the inside, a state called polarization. This occurs because there is normally an excess of sodium ions (Na+) on the outside of the membrane. o When a section of the membrane is stimulated, its Na+^ channels suddenly open, and Na+^ rushes inward. The inside of the membrane temporarily becomes positive, and the outside becomes negative—a process called depolarization. o This section of the membrane then immediately recovers—a process called repolarization. However, the depolarization has already stimulated Na+^ channels in the next section of the membrane to open. The impulse—or action potential — cannot go backward during the moment of repolarization and recovery of the previous section of membrane. Thus, a self-propagating wave of electrical disturbance —a nerve impulse—travels in one direction across the neuron's surface.
The CNS centrally located and its two major structures, the brain and spinal cord , are found along the midsagittal plane of the body. The brain is protected in the cranial cavity of the skull, and the spinal cord is surrounded in the spinal cavity by the vertebral column. Meninges : the brain and spinal cord are covered by the protective membranes called meninges.
A. THE BRAIN
The brain, one of our largest organs, consists of the following major divisions, named in ascending or- der beginning with the most inferior part:
I. Brainstem o Medulla oblongata o Pons o Midbrain II. Cerebellum III. Diencephalon o Hypothalamus o Thalamus o Pineal Gland IV. Cerebrum
(involuntary) of most internal organs like control the heartbeat, constriction and dilation of blood vessels, and contractions of the stomach and intestines o Some neurons in the hypothalamus make the posterior pituitary gland secretes antidiuretic hormone , or ADH into the blood that affect the volume of urine excreted, thus plays an essential role in maintaining the body's water balance. o Some of the neurons in the hypothalamus function as endocrine (ductless) glands. Their axons secrete chemicals called releasing hormones into the blood, which control the release of certain anterior pituitary hormones. These in turn influence the hormone secretion of other endocrine glands. Thus, the hypothalamus indirectly helps control the functioning of every cell in the body. o The hypothalamus is a crucial part of the mechanism for maintaining body temperature. In addition, this important center is involved in functions such as the regulation of water balance, sleep cycles, and the control of appetite and many emotions involved in pleasure, fear, anger, sexual arousal, and pain. Thalamus Just above the hypothalamus is a dumbbell-shaped section of gray matter called the thalamus. The thalamus performs the following functions: o Relays sensory information. Its neurons relay impulses to the cerebral cortex from the sense organs of the body. o Associates sensations with emotions. Almost all sensations are accompanied by a feeling of some degree of pleasantness or unpleasantness. o Regulates level of consciousness. It plays a part in the so-called arousal or alerting mechanism that keeps us awake. o Participates in motor reflexes. It plays a role in mechanisms that produce complex reflex movements. CEREBRUM o The cerebrum is the largest and uppermost part of the brain. o The outer surface of the cerebrum has many ridges and grooves. The ridges are called convolutions , or gyri/gyrus , and the grooves are called sulci. o The deepest sulci are called fissures ; the longitudinal fissure divides the cerebrum into right and left halves or hemispheres. These halves are almost separate structures except for their lower mid portions, which are connected by a structure called the corpus callosum. o Two deep sulci subdivide each cerebral hemisphere into four major lobes and each lobe into numerous convolutions. The lobes are named for the bones that lie over them: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. o Functions of the cerebrum —mental processes of all types, including consciousness, thinking, memory, sensa- tions, emotions, vision, hearing, and normal speech and voluntary control of movements. Specific areas of the cortex have very specific functions. For example, the temporal lobe's auditory areas interpret
incoming nervous signals from the ear as very specific sounds. The visual area of the cortex in the occipital lobe helps you identify and understand specific images. Brain disorders Physical Injury (Injury or disease can destroy neurons in the brain). Concussion -a common type of traumatic brain injury (TBI) resulting from a jolt to the head that bends the brainstem and causes temporary chemical changes in the brain. A concussion can be characterized by changes in thinking or concentration; physical symptoms like headache, nausea, or light sensitivity; a change in mood; and/or changes in sleep. Cerebrovascular accident (CVA)-. A CVA, or stroke , is a hemorrhage from or cessation of blood flow through cerebral blood vessels. When this happens, the oxygen supply to portions of the brain is disrupted, and neurons cease functioning. If the lack of oxygen is prolonged, the neurons die. If the damage occurs in a motor control area of the brain, the victim can no longer voluntarily move the parts of the body controlled by the affected areas. The term hemiplegia refers to paralysis (loss of voluntary muscle control) of one whole side of the body. Cerebral palsy (CP)— One of the most common crippling diseases that can appear during childhood, cerebral palsy (CP), also results from damage to brain tissue. Cerebral palsy involves permanent, non-progressive damage to motor control areas of the brain, which in turn causes abnormal muscle tension (spasticity) that hinders movement. Such damage is present at birth or occurs shortly after birth and remains throughout life. Possible causes of brain damage include prenatal infections or diseases of the mother; mechanical trauma to the head before, during, or after birth; nerve- damaging poisons; and reduced oxygen supply to the brain. Many people with cerebral palsy exhibit spastic paralysis, a type of paralysis characterized by involuntary contractions of affected muscles. In cerebral palsy, spastic paralysis may affect one entire side of the body (hemiplegia), both legs (paraplegia), both legs and one arm (triplegia), or all four extremities (quadriplegia). Degenerative disease Dementia -A variety of degenerative diseases can result in destruction of neurons in the brain. This degeneration can progress to adversely affect memory, attention span, intellectual capacity, personality, and motor control. The general term for this syndrome is dementia. Dementia is characteristic of Alzheimer disease (AD). Its characteristic lesions develop in the cortex during the middle to late adult years. Huntington disease (HD ) is an inherited disease characterized by chorea (involuntary, purposeless movements) that progresses to severe dementia and death. The human immunodeficiency virus (HIV ) that causes acquired immunodeficiency syndrome (AIDS) also can cause dementia. SEIZURE DISORDERS Seizures —sudden bursts of abnormal neuron activity that result in temporary changes in brain function. Seizures may be very mild, causing subtle changes in the level of consciousness, motor control, or sensory perception. On the other hand, seizures may be quite severe, resulting in jerky, involuntary muscle contractions called convulsions or even unconsciousness. Epilepsy —Recurring or chronic seizure episodes constitute a condition called epilepsy. Although some cases of epilepsy can be traced to specific causes such as tumors, trauma, or chemical imbalances, most epilepsy is idiopathic (of unknown cause). Electroencephalogram (EEG) is graphic representation of brain’s electrical activity. Diagnosis and evaluation of epilepsy or any seizure disorder often rely on EEG.
B. SPINAL CORD
is formed continually from fluid filtering out of the blood in a network of brain capillaries known as the choroid plexus and into the ventricles. CSF also fills spaces in the brain called cerebral ventricles. LUMBAR PUNCTURE/SPINAL TAP Spinal tap is the withdrawal of some CSF from the Subarachnoid space of the meninges in the lumbar region of the spinal cord. It is done to reduce pressure from swelling after injury or disease or for analysis to diagnose some diseases. PERIPHERAL NERVOUS SYSTEM The nerves connecting the brain and spinal cord to other parts of the body constitute the peripheral nervous system (PNS). This system includes cranial and spinal nerves that connect the brain and spinal cord, respectively, to peripheral structures such as the skin surface and the skeletal muscles. In addition, The Autonomic Nervous System (ANS) is considered a subdivision of the PNS. A. Cranial Nerves Twelve pairs of cranial nerves are attached to the undersurface of the brain, extending mostly from the brainstem. Their fibers conduct impulses between the brain and structures in the head and neck and in the thoracic and abdominal cavities. The names of each cranial nerve and a brief description of their functions are listed in
B. Spinal Nerves Thirty-one pairs (31 pairs) of spinal nerves are attached to the spinal cord in the following order: o 8 pairs are attached to the cervical segments, o 12 pairs are attached to the thoracic segments, o 5 pairs are attached to the lumbar segments,
Peripheral Nerve Disorders Neuritis —general term referring to nerve inflammation a. Sciatica is inflammation of the sciatic nerve (largest nerve in the body) that innervates the legs. b. Neuralgia, or muscle pain, often accompanies neuritis Trigeminal neuralgia —Compression or degeneration of the fifth cranial nerve , the trigeminal nerve, may result in a condition called trigeminal neuralgia or tic douloureux. This condition is characterized by recurring episodes of stabbing pain radiating from the angle of the jaw along a branch of the trigeminal nerve Bell palsy — Compression, degeneration, or infection of the seventh cranial nerve , the facial nerve, may result in Bell palsy. Bell palsy is characterized by paralysis of some or all of the facial features innervated by the facial nerve, including the eyelids and mouth. Herpes zoster, or shingles- Herpes zoster, or shingles, is a unique viral infection that almost always affects the skin of
a single dermatome. In most cases the disease results from reactivation of the varicella virus (chicken pox). The virus probably travels through a cutaneous nerve and remains dormant in a dorsal root ganglion for years after an episode of the chickenpox. If the body's immunological protective mechanism becomes diminished in the elderly after stress, or in individuals undergoing radiation therapy or taking immunosuppressive drugs, the virus may reactivate. C. Autonomic Nervous System The autonomic nervous system (ANS) consists of certain motor neurons that conduct impulses from the spinal cord or brainstem to the following kinds of tissues that control their involuntary functions: Cardiac muscle tissue, Smooth muscle tissue and Glandular epithelial tissue. For example, the heartbeat, contractions of the stomach and intestines, and secretions by glands of the internal organs and also acts as reflex center of the body. somatic nervous system : Motor nerves that control the voluntary actions of skeletal muscles are sometimes called the somatic nervous system. They carry impulses form the brain and spinal cord to the skeletal muscle. Somatic motor neurons connect the spinal cord to the somatic effector organs with only one neuron with no intervening synapses The autonomic nervous system consists of two divisions : a. The sympathetic nervous system and b. The parasympathetic nervous system Functional Anatomy Autonomic neurons are motor neuron also called preganglionic neurons conduct from spinal cord or brainstem to an autonomic ganglion; postganglionic neurons conduct from autonomic ganglion to cardiac muscle, smooth muscle, and glandular epithelial tissue. Ganglion -A ganglion is a group of nerve-cell bodies located in the PNS. This ganglion is located near the spinal cord Sympathetic Nervous System o Dendrites and cell bodies of sympathetic preganglionic neurons are located in the gray matter of the thoracic and upper lumbar segments of the spinal cord. The sympathetic system also has been referred to as the thoracolumbar system. o Axons of sympathetic postganglionic neurons travel in spinal nerves to blood vessels, sweat glands, and arrector hair muscles all over the body Functions of the sympathetic nervous system Sympathetic nervous system functions to produce localized adjustments (such as sweating as a response to an increase in temperature) and reflex adjustments of the cardiovascular system. Under conditions of stress, the entire sympathetic nervous system is activated, producing an immediate widespread response called the fight-or-flight response. This response is characterized by the release of large quantities of epinephrine from the adrenal gland, an increase in heart rate, an increase in cardiac output, skeletal muscle vasodilation, cutaneous and gastrointestinal vasoconstriction, pupillary dilation, slow peristalsis, bronchial dilation, and piloerection (goose bump). The overall effect is to prepare the individual for imminent danger. Parasympathetic Nervous System Structure- The dendrites and cell bodies of parasympathetic preganglionic neurons are located in the gray mat- ter of the brainstem and the sacral segments of the spinal cord. The parasympathetic system also has been referred to as the craniosacral system. Functions of the parasympathetic nervous system The parasympathetic nervous system is responsible for the body's rest and digestion response when the body is relaxed, resting, or feeding. It basically undoes the work of sympathetic division after a stressful situation. The parasympathetic nervous system decreases respiration and heart rate and increases digestion, increase peristalsis, constriction of pupils and bronchial muscles, relax sphincter muscles in GIT, increase production of saliva and mucus.
