Ch. 13 Nervous System - Lecture Notes

Ch. 13 Nervous System Anatomy and Physiology                  The nervous system is an intricate network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body o These messages may involve communication, higher mental function, and emotional expression, perception, and control of regulation of bodily functions Along with the endocrine system, the nervous system regulates many aspects of homeostasis, such as respiratory rate, blood pressure, body temperature, and the sleepwake cycle The nervous system can be described as having two interconnected divisions: the central nervous system (CNS) and the peripheral nervous system (PNS) The autonomic nervous systems (ANS) is a part of the peripheral nervous system and is divided into the sympathetic division The CNS includes the brain and spinal cord o It is enclosed by the bones of the skull and spinal columns The PNS consists of the network of nerves and neural tissues branching throughout the body from 12 pairs of cranial nerves and 31 pairs of spinal nerves Neurons (nerve cells)- structural and functional units of the nervous system act as specialized conductors of impulses that enable the body to interact with its internal and external environments Neuroglia- act as supporting tissue Nerve fibers and tracts- conduct impulses from one location to another Central nervous system- receives impulses from throughout the body, processes the information, and responds with an appropriate action Brain- governs sensory perception, emotions, consciousness, memory, and voluntary movements Spinal cord- conduct's sensory impulses to the brain and motor impulses from the brain to body parts; also serves as a reflex center for impulses entering and leaving the spinal cord without involvement of the brain (allows us to move and feel) Peripheral nervous system- links the central nervous system with other parts of the body Cranial nerves (12 pairs)- provide sensory input and motor control or a combination of these Spinal nerves (31 pairs)- carry impulses to the spinal cord and to muscles, organs, and glands Autonomic nervous system (sympathetic division and parasympathetic division)control involuntary bodily functions such as sweating, secretion of glands, arterial blood pressure, smooth muscle tissue, and the heart. Also stimulates the adrenal gland to release epinephrine (adrenaline), the hormone that causes the familiar adrenaline rush or the fight-or-flight response Tissues of the Nervous System The nervous system has 2 principle tissue types o These tissues are made up of neurons or nerve cells and their supporting tissue, collectively called neuroglia  Neurons are the structural and functional units of the nervous system o These cells are specialized conductors of impulses that enable the body to interact with its internal and external environments  There are several types of neurons, 3 of which are described in the following sections  Motor Neurons         Motor neurons cause contractions in muscles and secretions from glands and organs o They also act to inhibit the actions of glands and organs, thereby controlling most of the body's functions Motor neurons can be described as being efferent processes because they transmit impulses away from the neural cell body to the muscles or organs to be innervated Motor neurons consist of a nucleated cell body with protoplasmic processes extending away from it in several directions o These processes are known as axons and dendrites An axon is a single extension of a neuron that is defined as a process that can generate and conduct action potentials o The axons of certain neurons can carry a signal both toward and away from the cell body, although most axons carry information away from the cell body Depending on the type of neuron, the axon may range in length from short to very long Most axons are covered with a fatty substance, the myelin sheath, which acts as an insulator and increases the transmission velocity of the nerve fiber it surrounds Dendrites resemble the branches of a tree, are short, and transmit impulses to the cell body Neurons may have several dendrites with only a single axon Sensory Neurons Sensory neurons differ in structure from motor neurons because they do not have true dendrites  The processes transmitting sensory information to the cell bodies of these neurons are called peripheral processes, are sheathed, and resemble axons o They are attached to sensory receptors and transmit impulses to the CNS o After processing the information, the CNS can stimulate motor neurons in response to this sensory information   Sensory neurons are referred to as afferent nerves because they carry impulses from the sensory receptors to the synaptic endings in the central nervous system Interneurons  Interneurons are called central or associative neurons and are located entirely within the CNS o They function to mediate impulses between sensory and motor neurons Nerve Fibers, Nerves, and Tracts  The terms nerve fiber, nerve, and tract are used to describe neuronal processes conducting impulses from one location to another Nerve Fibers A nerve fiber is a single elongated process, the axon of a neuron Nerve fibers of the peripheral nervous system are wrapped by protective membranes called sheaths  The PNS has 2 types of sheaths: myelinated and unmyelinated, which are formed by accessory cells o Myelinated fibers have an inner sheath of myelin, a thick, fatty substance, and an outer sheath, or neurilemma, composes of Schwann cells o Unmyelinated fibers lack myelin and are sheathed only by the neurilemma  Nerve fibers of the central nervous system do not contain Schwann cells   Nerves   A nerve is a collection of nerve fibers, outside the central nervous system Nerves are usually described as being sensory or afferent (conducting to the CNS) or motor or efferent (conducting away from the CNS to muscles, organs, and glands) Tracts Groups of nerve fibers within the CNS are sometimes referred to as tracts when they have the same origin, function, and termination  The spinal cord contains afferent sensory tracts ascending to the brain and efferent motor tracts descending from the brain o The brain itself contains numerous tracts, the largest of which is the corpus callosum joining the left and right hemispheres  Transmission of Nerve Impulses  The transmission of an impulse by a nerve fiber is based on the all-or-none principle This means that no transmission occurs until the stimulus reaches a set minimum strength, which can vary for different receptors o Once the minimum stimulus or threshold is reached, a maximum impulse is produced o The impulse is then transmitted via a synapse, a specialized knoblike branch ending, with the help of certain chemical agents, across a space separating the axon's end knobs from the dendrites of the next neuron or from a motor end plate attached to a muscle  This space is called synaptic clef, and the chemical agents released are called neurotransmitters o Central Nervous System      Consisting of the brain and spinal cord, the central nervous system (CNS) receives the impulses from throughout the body, processes the information, and responds with an appropriate action o This activity can be at the conscious or unconscious level, depending on the source of the sensory stimulus Both the brain and spinal cord can be divided into gray matter and white matter o The gray matter consists of unsheathed cell bodies and true dendrites o The white matter is composed of myelinated nerve fibers In the spinal cord, the arrangement of white and gray matter results in an Hshaped core of gray cell bodies surrounded by tracts of nerve fibers interconnected to the brain The reverse is generally true of the brain where the surface layer of cortex is gray matter and most of the internal structures are white matter Neural tube development occurs about the 3rd to 4th week of embryonic life. This development becomes the central nervous system. At 6 weeks, a developing fetus's brain waves are measurable. At 28 weeks, the fetal nervous system begins some regulatory functions. By 32 weeks, the developing fetal nervous system is capable of sustaining rhythmic respirations and regulating body temperature. The growth rate of brain and nerve cells is at its most rapid pace up to about 4 years of age Brain      The nervous tissue of the brain consists of millions of nerve cells and fiber o It is the largest mass of nervous tissue in the body, weighing about 1380 g in the male and 1250 g in the female The brain is enclosed by 3 membranes known collectively as the meninges From the outside in, these are the dura mater, arachnoid, and pia mater The major structures of the brain are the cerebrum, cerebellum, diencephalon, and the brainstem, which is composed of the midbrain, pons, and medulla oblongata The Diencephalon includes the epithalamus, thalamus, and the hypothalamus The brainstem includes the midbrain, pons, and medulla oblongata Cerebrum- evaluates and controls all sensory and motor activity: sensory perception, emotions, consciousness, memory, and voluntary movements  Cerebellum- plays an important role in the integration of sensory perception and motor output. Its neural pathways link with the motor cortex, which sends information to the muscles causing them to move, and the spinocerebellar tract, which provides feedback on the position of the body in space (proprioception). The cerebellum integrates these pathways using the constant feedback on body position to fine-tune motor movements. Research shows that the cerebellum also has a broader role in a number of key cognitive functions, including attention and the processing of language, music, and other sensory temporal stimuli  Diencephalon o Epithalamus- involved in the maintenance of circadian rhythm, basically a 24-hour internal clock that cycles between sleepiness and alertness at regular intervals. Functions include the secretion of melatonin by the pineal gland and regulation of motor pathways and emotions o Thalamus- relay center for all sensory impulses (except olfactory) being transmitted to the sensory areas of the cortex, and relays motor impulses from the cerebellum and the basal ganglia to motor areas of the cortex, thought to be involved with emotions and arousal mechanisms o Hypothalamus- serves as the principal regulator of autonomic nervous activity that is associated with behavior and expression; also contains hormones that are important for the control of certain metabolic activities such as maintenance of water balance, sugar and fat metabolism, regulation of body temperature, sleep-cycle control, appetite, and sexual arousal  Brainstem o Midbrain- two-way conduction pathway that acts as a relay center for visual and auditory impulses; found in the midbrain are four small masses of gray cells known collectively as the corpora quadrigemina. The upper two, called the superior colliculi, are associated with visual reflexes. The lower two, or inferior colliculi, are involved with the sense of hearing o Pons- links the cerebellum and medulla to higher cortical areas; plays a role in somatic and visceral motor control; contains important centers for regulating breathing o Medulla oblongata- acts as the cardiac, respiratory, and vasomotor control center; regulates and controls breathing, swallowing, coughing, sneezing, and vomiting as well as heartbeat and arterial blood pressure, thereby exerting control over the circulation of blood   Cerebrum          Representing seven-eighths of the brain's total weight, the cerebrum contains nerve centers that evaluate and control all sensory and motor activity, including sensory perception, emotions, consciousness, memory, and voluntary movements The cerebrum is divided by the longitudinal fissure into 2 cerebral hemispheres, the right and left, that are joined by large fiber tracts (corpus callosum) that allow information to pass from one hemisphere to the other. The surface or cortex of each hemisphere is arranged in folds creating bulges and shallow furrows. o Each bulge is called a gyrus or convolution o A furrow is known as a sulcus  This surface is composed of gray, unmyelinated cell bodies and is known as the cerebral cortex  The cortex has been divided into lobes as a means of identifying certain location  These lobes correspond to the overlying bones of the skull and are the frontal, parietal, temporal, and occipital lobes The hippocampus is an extension of the cerebral cortex, situated deep in the temporal lobe o It is part of the limbic system, which is considered to be like a "primitive brain" that is involved with basic emotions and reactions, such as fear, anger, hunger, pleasure, sex, motivation, mood, pain, appetite, and memory The hippocampus is currently one of the most studied areas of the brain o Studies have found that patients with conditions such as Alzheimer disease (AD), depression, and schizophrenia show signs of hippocampal atrophy (shrinkage or volume reduction_ o In fact, all people with AD show some amount of hippocampal atrophy, and in individuals experiencing depression, the duration of the depressed state has been correlated with the severity of hippocampal atrophy o Evidence suggests that atrophy thus produced may be permanent even through though the depression has undergone remission Hippocampal atrophy is one of the most consistent findings found in MRIs of schizophrenic patients along with functional and biochemical abnormalities Electrical stimulation of the various areas of the cortex during neurosurgery has identified specialized cell activity within the different lobes The frontal lobe has been identified as the brain's major motor area and the site for personality and speech The parietal lobe contains centers for sensory input from all parts of the body and is known as the somesthetic area and the site for the interpretation of language o Temperature, pressure, touch, and an awareness of muscle control are some of the sensory activities localized in this area The temporal lobe contains centers for hearing, smell, and language input   The occipital lobe is the primary interpretive processing area for vision The occipital lobe is directly posterior to the temporal lobe Cerebellum The cerebellum is the second largest part of the brain o It occupies a space in the back of the skull, inferior to the cerebrum and dorsal to the pons and medulla oblongata  The cerebellum is oval shape and is divided into lobes by deep fissures o It has a cortex of gray cell bodies, and its interior contains nerve fibers and white matter connecting it to every part of the CNS  The cerebellum plays an important part in the coordination of voluntary and involuntary complex patterns of movement and adjusts to maintain posture  Diencephalon  The word diencephalon means second portion of the brain and refers to the epithalamus (a part of the dorsal forebrain including the pineal gland and a region in the roof of the third ventricle of the brain), thalamus, and hypothalamus Epithalamus The dorsal posterior segment of the diencephalon, the epithalamus is involved in the maintenance of circadian rhythms and serves as a connection between the limbic system and other parts of the brain  Some functions of its components include the secretion of melatonin by the pineal gland and regulation of motor pathways and emotions  Thalamus The thalamus is the largest structure derived from the embryonic diencephalon; it is the larger of two divisions of the diencephalon and is actually two large masses of gray cell bodies joined by a third or intermediate mass  The thalamus serves as a relay center for all sensory impulses (except olfactory) being transmitted to the sensory areas of the cortex  Besides it sensory function, the thalamus also relays motor impulses from the cerebellum and the basal ganglia to motor areas of the cortex  Some impulses related to emotional behavior are also passed from the hypothalamus through the thalamus, to the cerebral cortex  Hypothalamus  The hypothalamus lies beneath the thalamus and is a principal regulator of autonomic nervous activity that is associated with behavior and emotional expression It also produced neurosecretions for the control of water balance, sugar and fat metabolism, regulation of body temperature, and other metabolic activities  The pituitary gland is attached to the hypothalamus by a narrow stalk, the infundibulum  Brainstem The Brainstem is the lower part of the brain, adjoining and structurally continuous with the spinal cord  The brainstem provides the main motor and sensory innervation to the face and neck via the cranial nerves o It consists of 3 structures: the mesencephalon or midbrain, the pons, and the medulla oblongata  The brainstem processes visual, auditory, and sensory information and plays an important role in the regulation of cardiac and respiratory function o It also regulates the CNS and is pivotal in maintaining consciousness and regulating the sleep cycle  Midbrain The midbrain is located below the cerebrum and above the pons The midbrain has 4 small masses of gray cells known collectively as the corporal quadrigemina  The upper two of these masses, called the superior colliculi, are associated with visual reflexes such as the tracking movements of the eyes  The lower two, or inferior colliculi, are involved with the sense of hearing   Pons The pons is a broad band of white matter located anterior to the cerebellum and between the midbrain and the medulla oblongata  The pons is composed of fiber tracts linking the cerebellum and medulla oblongata to higher cortical areas 'it also plays a role in somatic and visceral motor control and contains important centers for regulating breathing  Medulla Oblongata The medulla oblongata connects the pons and the rest of the brain to the spinal cord  All afferent and efferent tracts from the spinal cord either pass through or terminate in the medulla oblongata o It contains nerve centers for regulation and control of breathing, swallowing, coughing, sneezing, vomiting, heartbeat, and blood pressure  Spinal Cord        The spinal cord has an H-shaped gray area of cell bodies encircles by an outer region of white matter The white matter consists of nerve tracts and fibers providing sensory input to the brain and conducting motor impulses from the brain to spinal neurons The adult spinal cord is about 44 cm long and extends down the vertebral canal from the medulla oblongata to terminate near the junction of the first (L1) and second (L2) lumbar vertebrae Between the 12th thoracic vertebra (T12) and L1 is a region known as the conus medullaris, where the spial cord becomes conically tapered The filum terminale of terminal thread of fibrous tissue extends from the conus medullaris to the second sacral vertebra The cauda equina (known as the horse's tail) is the terminal portion of the spinal cord that forms the nerve fibers that are the lumbar, sacral, and coccygeal spinal nerves The functions of the spinal cord are to conduct sensory impulses from the brain, and to serve as a reflex center for impulses entering and leaving the spinal cord without direct involvement of the brain Cerebrospinal Fluid      The brain and spinal cord are surrounded by cerebrospinal fluid (CSF) o This colorless fluid is produced as a filtrate or blood by the choroid plexuses within the ventricles of the brain Cerebrospinal fluid circulates through the ventricles, the central canal, and the subarachnoid space The normal adult will have between 120 and 150 mL of cerebrospinal fluid in circulation The cerebrospinal fluid serves to cushion the brain and spinal cord from shocks that could cause injury o It also helps to support the brain by allowing it two float within the supporting liquid It also contains neurotransmitters such as monoamines, acetylcholine (ACh), and neuropeptides Peripheral Nervous System The network of nerves branching throughout the body from the brain and spinal cord is known as the peripheral nervous system (PNS)  There are 12 pairs of cranial nerves that attach to the brain and 31 pairs of spinal nerves connected to the spinal cord  Cranial Nerves  The nerves described in the following sections attach to the brain and provide sensory input, motor control, or a combination of these functions              They are arranged symmetrically, 12 to each side of the brain, and are generally named for the area or function they serve Olfactory (1)- detects and provides the sense of small Optic (II)- provides vision Oculomotor (III)- conducts motor impulses to 4 of the 6 external muscles of the eye and to the muscle that raises the eyelid Trochlear (IV)- conducts motor impulses to control the superior oblique muscle of the eyeball (controls the movement of the eyeball) Trigeminal (V)- provides sensory input from the face, nose, mouth, forehead, and top of the head; motor fibers to the muscles of the jaw (chewing) Abducens (VI)- conducts motor impulses to the lateral rectus muscle of the eyeball Facial (VII)- controls the muscles of the face and scalp; the lacrimal glands of the eye and the sub-mandibular and sublingual salivary glands; input from the tongue for the sense of taste Vestibulocochlear (Acoustic) (VIII)- provides input for hearing and equilibrium Glossopharyngeal (IX)- provides general sense of taste; regulates swallowing; controls secretion of saliva Vagus (X)- controls muscles of the pharynx, larynx, thoracic, and abdominal organs; responsible for swallowing, voice production, slowing of heartbeat, acceleration of peristalsis Accessory (XI)- controls the trapezius and sternocleidomastoid muscles, permitting movement of the head and shoulders Hypoglossal (XII)- controls the tongue; tongue movements. Connects to the brain. Spinal Nerves       There are 31 pairs of spinal nerves distributed along the length of the spinal cord and emerging from the vertebral canal on either side through the intervertebral foramina At the point of attachment, each nerve is divided into 2 roots The dorsal or sensory root is composed of afferent fibers carrying impulses to the cord The ventral root contains motor fibers carrying efferent impulses to muscles and organs Names for the region of the vertebral column from which they exit, there are 8 pairs of cervical spinal nerves, 12 pairs of thoracic spinal nerves, 5 pairs of lumbar spinal nerves, 5 pairs of sacral spinal nerves , and 1 pair of coccygeal spinal nerves A short distance from the cord, the fibers of the two roots unite to form a spinal nerve