Chapter 6 1. uses millions of sensory receptors to monitor changes occurring inside and outside the body 2. processes and interprets the sensory input and makes decisions on what should be done (integration) 3.effects a response by activating muscles or glands (motor output) Has 2 subdivisions Central nervous system (CNS) Brain and spinal cord Occupy the dorsal body cavity Are the integrating and command centers Peripheral nervous system (PNS) Part that is outside the CNS Nerves outside the brain and spinal cord (spinal and cranial nerves) Communication lines from sensory receptors to CNS then to glands / muscles For the PNS only: there are two subdivisions Sensory (afferent) division Convey impulses TO the CNS from sensory receptors Somatic impulses come from skin, skeletal muscle, and joints Visceral impulses come from organs Motor (efferent) division Carries impulses from the CNS to effector organs (muscles and glands) They activate (get a response) from muscles and glands Motor Division has 2 subdivisions Somatic nervous system Allows conscious or voluntary control on skeletal muscles Reflexes are NOT voluntary but are carried by the same fibers Autonomic nervous system Regulates events that are involuntary (glands, organs) 2 parts (bring about opposite effects) Sympathetic parasympathetic Two types of cells in nervous tissue Supporting cells Grouped as “neuroglia” or “glia” Many cells that support, insulate, and protect neurons Neurons Or nerve cells, specialized to transmit messages All have common features Cell body (with nucleus and organelles) One or more slender processes extending from the cell Astrocytes Star-shaped Cling to neurons, anchoring them to nutrient supplies (capillaries) Help make exchanges between neurons and blood supply In the brain, pick up excess ions and recapture neurotransmitters Microglia Spiderlike phagocytes; dispose of debris (dead cells and bacteria) Ependymal cells Line the cavities of brain and spinal cord Cilia circulate cerebrospinal fluid Oligodendrocytes Wrap extensions around nerve fibers Produce fatty insulation (myelin sheath) Resemble neurons but do NOT transmit nerve impulses Never lose the ability to divide (mitosis) Most brain tumors are “gliomas” formed by glial cells In the PNS Schwann cells Form myelin sheaths Satellite cells Protective, cushioning cells Also called nerve cells Highly specialized to transmit messages from one area of the body to another Metabolic center of the neuron Contains the organelles (but no centrioles) Rough ER (Nissl substance) and neurofibrils are abundant Vary in length (microscopic to 3-4 feet) Dendrites – convey incoming messages to the cell body Axons – convey nerve impulses away from cell body Neurons may have a few to hundreds of dendrites, but will have only one axon. Arise from the axon hillock on the cell body Some give off collateral branches All branch profusely at the terminal end forming hundreds of axon terminals Terminals contain the vesicles full of neurotransmitters When nerve impulses reach the axon terminals, neurotransmitters are released to extracellular space Synaptic cleft separates the axon terminal from the next neuron (neurons do NOT touch each other) This functional junction is the synapse Long nerve fibers are covered with myelin (fatty material that protects and insulates) that increases the rate of impulse transmission Axons outside the CNS are myelinated with Schwann cells that wrap around the axon to produce the myelin sheath Neurilemma-cytoplasm of the Schwann cells just beneath the plasma membrane (external to the myelin) Schwann cells are individual so there are gaps between the cells called nodes of Ranvier In the CNS, myelinated fibers (myelin) is produced by the oligodendrocytes Schwann cells myelinate only a short segment of nerve fiber; oligodendrocytes can myelinate as many as 60 fibers at the same time CNS myelin sheaths do not have the neurilemma (so there is no regenerative properties for the CNS) Different names in CNS and PNS CNS – clusters are called nuclei are well protected by the skull and vertebral column; if the cell dies, it is NOT replaced PNS – clusters are called ganglia Bundles of nerve fibers (CNS – tracts; PNS – nerves) Gray matter and white matter refers to the presence or absence of myelin (white matter is highly myelinated) Functional classification Groups neurons according to the direction the nerve impulse is traveling relative to the CNS Structural classification Based on the number of processes extending from the cell body Sensory (afferent) Neurons carrying impulses from sensory receptors to the CNS Cell bodies located in ganglia outside CNS Give information about the outside and inside of body Dendrites have receptors activated by changes Cutaneous sense organs are in skin Pain receptors are bare dendrite endings Proprioceptors are in muscles and tendons Detect stretch or tension and send messages to maintain balance and posture Motor (efferent) Carry impulses from CNS to the viscera (muscles and glands) Cell bodies inside CNS Association neurons (interneurons) Connect motor and sensory neurons in neural pathways Cell bodies inside CNS Multipolar neurons Bipolar neurons Several processes coming off cell body Most common Includes motor and association neurons Have one axon and one dendrite Rare in adults Found only in special sense organs (eye, ear) for sensory reception Unipolar neurons A single process extending from the cell body Divides into proximal and distal fibers Axon conducts impulses toward and away from cell body Found in sensory organs of PNS Two major functions of neurons Irritability Ability to respond to a stimulus and convert it to a nerve impulse Conductivity Ability to transmit the impulse to other neurons, muscles, glands Plasma membrane of a resting (inactive) neuron is polarized Fewer positive ions inside the cell than outside K+ is inside; Na+ is outside Inside must be more negative than the outside (so there are less K+ inside, but more Na+ outside) Neuron must be stimulated which causes the sodium gates to open Na+ ions rush into the cell (making inside more +) If strong enough, depolarization activates the neuron to initiate and transmit an action potential or nerve impulse This is “all or none”; impulse either happens completely or it doesn’t Immediately after Na+ rushes in, membrane permeability changes (so now it’s impermeable to Na+) Na+ cannot go into the cell but K+ rushes out to restore the electrical conditions of the cell (repolarization) Neurons cannot conduct another impulse until repolarization occurs Na+-K+ pump restores initial concentrations of Na+ (outside) and K+ (inside) using ATP This is polarization, depolarization, and repolarization Slower on unmyelinated axons Those neurons with myelinated axons carry impulses much faster since the impulse can jump from node to node (between the myelin sheaths) Carrying the impulse from one neuron to the next When the impulse reaches the axon terminals, neurotransmitters are released Neurotransmitters diffuse across the synapse to bind to receptors on the membrane of the next neuron If enough neurotransmitter is released, irritability of the next neuron occurs This is an electrochemical event Transmission down the axon of one neuron is electrical Stimulating the next neuron is chemical (sending neurotransmitters across the synapse) Reflexes – rapid, predictable, and involuntary responses to stimuli Once it begins, it goes in the same direction Reflexes occur over reflex arcs Autonomic reflexes Regulate activity of smooth muscles, heart, glands Regulate body functions like digestion, elimination, blood pressure, sweating Somatic reflexes Reflexes that stimulate skeletal muscles All reflexes have 5 elements Sensory receptor – reacts to a stimulus Effector organ – muscle or gland stimulated Afferent and efferent neurons to connect the sensory and effector CNS integration center - synapse between the afferent and efferent neurons Knee-jerk reflex two-neuron reflex Quadriceps muscle attached to the hit tendon is stretched Tested to determine general health of motor portion of nervous system Withdrawal reflex Three-neuron reflex arc Has 5 elements Receptor Afferent neuron Association neuron Efferent neuron effector Always a delay at the synapses (takes time for the neurotransmitters to diffuse across) The more synapses involved, the longer it takes for the reflex to occur Involve only spinal cord neurons (no brain involvement) Some need brain involvement to “interpret” the information so the appropriate reflex occurs, such as contracting the pupils in bright light Embryonic development First appears as a neural tube extending down the dorsal median plane By 4th week, anterior expansion (brain development) begins; the rest becomes spinal cord Central canal of the neural tube enlarges into 4 regions /chambers called ventricles “two fist-fulls” of gray tissue all wrinkled Texture of cold oatmeal Weighs a little over 3 pounds Largest and most complex mass of nervous tissue Discussed in terms of the 4 major regions Cerebral hemispheres Diencephalon Brain stem Cerebellum Most superior part of the brain Much larger than other 3 brain regions Enclose and obscure most of brain stem Surface has gyri (ridges) separated by sulci (grooves) Has less numerous fissures that separate larger regions Fissures and gyri serve as “landmarks” 2 hemispheres separated by the longitudinal fissure Other fissures/gyri separate the hemispheres into smaller lobes (named for the cranial bones over them) Control Speech Memory Logical and emotional response Consciousness Interpretation of sensation Voluntary movement Has somatic sensory area (posterior to the central sulcus) Interprets impulses from sensory receptors Recognize Pain Coldness Light touch Lips/fingertips send impulses to neurons that occupy most of sensory cortex Sensory pathways are crossed Left lobe receives info from the right side of the body; right lobe receives info from left side of the body Interprets impulses from the eyes Visual area is in the posterior portion Interprets impulses from the ears Auditory area borders the lateral sulcus Olfactory area (smell) is deep inside this lobe Also involved in complex memories Primary motor area; anterior to central sulcus Conscious movement of the skeletal muscles Axons form major voluntary motor tract – pyramidal or corticospinal tract Pathways are crossed Most neurons control fine motor areas: face, mouth, hands Anterior portion controls higher intellectual reasoning Also involved in complex memories and language comprehension Controls ability to speak Base of the precentral gyrus (anterior to central sulcus) Usually located in left hemisphere Damage prevents ability to say words properly Junction of the temporal, parietal, and occipital lobes Allows one to sound out words Usually only in one hemisphere Gray matter Contains the cell bodies of neurons involved in speech, motor skills, responding to the senses Highly ridged and convoluted (more surface area=more neurons) Deeper tissue (below gray matter) Composed of fiber tracts carrying impulses to and from the cortex Corpus callosum – large fiber tract that connects the two hemispheres and allows them to communicate Islands of gray matter deep within the white matter Regulate voluntary motor activities by modifying instructions sent to the skeletal muscles by primary motor cortex Also called “interbrain” Located on top of the brain stem and enclosed by the hemispheres Contains Thalamus Hypothalamus epithalamus Encloses the third ventricle of the brain (a relay station for sensory impulses going to the sensory cortex) Gives a crude recognition of sensation (a forewarning of liking or disliking the response) Localization and interpretation of the sensation is done in the sensory cortex Floor of the diencephalon Autonomic nervous system center Regulates body temperature, water balance, metabolism Center for emotions Part of the limbic system (emotional-visceral brain) Thirst, appetite, sex, pain, and pleasure Regulates the pituitary gland Pituitary gland Endocrine organ Releases hormones Hangs from the floor of the hypothalamus Mammillary bodies Reflex centers for olfaction Bulge from floor of hypothalamus Posterior to the pituitary Roof of the third ventricle Contain the Pineal body (part of the endocrine system) Choroid plexus (knots of capillaries that form cerebral spinal fluid) Thumb size in diameter and about 3 inches long Contains Midbrain pons Medulla oblongata Provides a pathway for ascending and descending tracts Has small gray matter areas (which controls activities such as breathing, blood pressure) Small part of brain stem Extends from mammillary bodies to pons Contains the cerebral aqueduct (canal through the midbrain from third ventricle to fourth ventricle) Cerebral peduncles (anterior portion) convey ascending and descending impulses Corpora quadrigemina (dorsal) are reflex centers for vision and hearing Most inferior of the midbrain Merges into spinal cord Regulates vital visceral activities (heart rate, blood pressure, breathing, swallowing, vomiting, etc.) Fourth ventricle is posterior to pons and medulla and anterior to cerebellum Extends entire length of brain stem Large mass of gray matter Involved in motor control of visceral organs Reticular activating system (RAS) has a role in consciousness and awake/sleep cycles Damage can result in coma Dorsal under the occipital lobe 2 hemispheres and convoluted surface Has outer cortex of gray matter; inner region of white matter Provides precise timing for skeletal muscle activity Controls balance and equilibrium Smooth body movements Receives fibers from inner ear, eye, proprioceptors of skeletal muscles and tendons Monitors body position and tension Bone enclosures (skull and vertebrae) Membranes (meninges) Watery cushion (cerebrospinal fluid) Blood-brain barrier 3 connective tissue membranes Outer most layer is dura mater – double layered membrane around brain Outer layer is the periosteum connected to the skull Inner layer is the meningeal layer forming the outermost layer of the brain and continues as dura mater of spinal cord Two layers are fused except at dural sinuses where venous blood is collected Two folds of the meningeal layer Falx cerebri extends inward to attach the brain to the cranial cavity Tentorium cerebelli separates the cerebellum from the cerebrum Arachnoid mater – middle meningeal layer Pia mater Clings to the surfaces of brain and spinal cord Subarachnoid space Threadlike extensions attach to pia mater Filled with cerebrospinal fluid Arachnoid villi Part of arachnoid mater Protrude through dura mater Absorb cerebrospinal fluid into venous blood of the dural sinuses Formed from blood plasma (has less protein, more vitamin C, different ion composition) Continually formed from blood by choroid plexus (capillaries in brain ventricles) Continually circulates Forms and drains at a constant rate to retain normal pressure and volume Changes may indicate meningitis or other brain pathologies Brain must have a constant internal environment (some ions (Na+ and K+) are needed for impulses) Blood-brain barrier separates the brain from bloodborne substances Made of the least permeable capillaries only water, glucose, and essential amino acids pass through (all are water soluble) Metabolic wastes, toxins, proteins, and most drugs cannot pass into brain tissue Nonessential amino acids and K+ are pumped out of brain tissue into the blood Cannot stop the passage fats, respiratory gases and fatsoluble molecules So blood-borne alcohol, nicotine, anesthetics can pass easily Concussion Brain injury is slight Dizziness, “see stars”, lose consciousness briefly No permanent damage Contusion Marked tissue destruction Damage to cerebral cortex may not cause unconsciousness Damage to brain stem results in coma (lasting hours to lifetime) Cerebral edema Intercranial hemorrhage (bleeding) Swelling due to inflammatory response Both compress vital brain tissue Commonly called strokes Third leading cause of death in US Occur when blood circulation to the brain is blocked (clot or ruptured vessel) Vital brain tissue dies Can result in paralysis Can result in aphasia (loss of or impaired speech) About 17 inches long; extends from foramen magnum (of skull) to the first or second lumbar vertebra (it ends just below ribs) Continuation of the brain stem Provides two-way conduction pathway to and from brain Major reflex center Enclosed inside the vertebral column Cushioned / protected by meninges (which extend past the end of the spinal cord inside the vertebral canal) Humans have 31 pairs of spinal nerves arising from the cord that exit the vertebral column Cord is the diameter of a thumb in most areas except in cervical and lumbar areas (larger here since nerves that serve the upper / lower body areas arise here and exit the cord) Cauda equina – spinal nerves that extend past the lower part of the cord and travel through the vertebral canal before exiting Resembles a butterfly in cross section Has two posterior (dorsal) horns and two anterior (ventral) horns Dorsal horns have interneurons Sensory neurons enter the cord by the dorsal root Ventral horns have neurons of somatic nervous system Send axons by the ventral root Dorsal and ventral roots fuse to form spinal nerves Surrounds the central canal of the cord and contains cerebrospinal fluid Composed of myelinated fiber tracts Some run to higher centers Some travel from brain to cord Some send impulses from one side of cord to the other Divided into 3 regions Posterior column-ascending tracts carrying sensory impulses to brain Lateral column-ascending and descending motor tracts Anterior column-ascending and descending motor tracts Nerves and groups of neuronal cell bodies (ganglia) outside the CNS Nerve-bundle of neuron fibers outside the CNS Neuron fibers are wrapped in delicate connective tissue (endoneurium) Groups of fibers are wrapped in course connective tissue (perineurium) to make bundles (fascicles) Fascicles are bound by tough sheaths (epineurium) to form cord-like nerves Nerves classified according to direction in which they transmit impulses Mixed nerves-carry both sensory and motor fibers Afferent nerves-carry impulses toward the CNS Efferent nerves-carry only motor fibers 12 pairs that serve the head and neck One pair extends to the thoracic and abdominal cavities (vagus nerves) Most are mixed nerves 3 pairs (optic, olfactory, and vestibularcochlear) are purely sensory I. olfactory – purely sensory; carries impulses for the sense of smell II. Optic – purely sensory; carries impulses for vision III. Oculomotor – supplies motor fibers to four of 6 muscles that direct the eyeball; eyelid; internal eye muscles controlling lens shape and pupil size IV. Trochlear – supplies motor fibers for external eye muscle V. trigeminal – conducts sensory impulses from skin of face and mucosa of nose and mouth; contains motor fibers that activate the chewing muscles VI. Abducens – supplies motor fibers to lateral rectus muscle which rolls the eye laterally VII. Facial – activates the muscles of facial expression and lacrimal and salivary glands; carries sensory impulses from taste buds of anterior tongue VIII. Vestibulocochloear – purely sensory; transmits impulses for sense of balance, and hearing IX. Glossopharyngeal – supplies motor fibers to the pharynx that promote swallowing and saliva production; carries sensory impulses from taste buds of posterior tongue and from pressure receptors of the carotid artery X. Vagus – fibers carry sensory impulses from and motor impulses to the pharynx, larynx, and abdominal and thoracic viscera; most are parasympathetic that promote digestive activity and regulate heart activity XI. Accessory – mostly motor fibers that activate the sternocleidomastoid and trapezius muscles XII. Hypoglossal – motor fibers control tongue movements; sensory fibers carry impulses from the tongue 31 pairs Formed by combination of the ventral and dorsal roots of the spinal cord Named for the region of the cord from which they arise (cervical, thoracic, lumbar, sacral) Spinal nerves divide into dorsal and ventral rami Rami contain both sensory and motor fibers Damage to the spinal nerve or its rami can result in loss of sensation and flaccid paralysis Dorsal rami serve skin and muscles of posterior body trunk Ventral rami (T1-T12) form intercostal nerves that supply muscles between ribs and skin and muscles of the anterior and lateral trunk Ventral rami of other spinal nerves for plexuses that serve motor and sensory needs of limbs Motor subdivision of the PNS that controls body activities automatically Special neurons that regulate cardiac muscle, smooth muscle, and glands Also called the involuntary nervous system Somatic system Cell bodies of motor neurons are inside the CNS Axons extend all the way to the skeletal muscles they serve Autonomic system Uses 2 motor neurons The first motor neuron is inside the CNS Its preganglionic axon leaves the CNS to synapse with the second motor neuron (outside the CNS) The postganglionic axon extends to the organ it serves 2 parts Sympathetic Mobilizes the body during extreme stress Parasympathetic Returns the body to normal First neurons in the cranium (III, VII, IX, X) vagus is most important; S2-S4 of spinal nerves Axons of neurons serve the head and neck organs Synapse with a second motor neuron in a terminal ganglion; postganglionic axon extends a short distance to the organ it serves Sacral region Preganglionic axons leave spinal cord and form the pelvic nerves These travel to the pelvic cavity to synapse with the second motor neurons in terminal ganglia on the organs they serve Thoracolumbar division First neurons are in the gray matter of the spinal cord from T1 through L2 Preganglionic axons leave the cord in the ventral root, enter the spinal nerve, and pass through a ramus communicans to enter the sympathetic chain ganglion to synapse with a second neuron Sympathetic chain lies alongside the vertebral column on each side Postganglionic axon reenters the spinal nerve to travel to the skin If the first axon does not synapse in the sympathetic chain ganglion, it will form part of the splanchnic nerves Travel to viscera to synapse with second neuron in the collateral ganglion Major collateral ganglia (celiac, superior and inferior mesenteric ganglia) supply abdominal and pelvic organs Postganglionic axon leaves the collateral ganglion and travels to serve a nearby visceral organ Body organs receive fibers from both the sympathetic and parasympathetic divisions Except blood vessels, skin, some glands, adrenal medulla Causes antagonistic effects (due to different neurotransmitters) Parasympathetic (cholinergic fibers release acetylcholine) Sympathetic (adrenergic fibers release norepinephrine) “fight or flight” system Activates systems in times of stress (emotional or physical) Increases Heart rate, blood pressure, blood glucose levels Dilation of bronchioles in lungs Dilation of blood vessels in skeletal muscles Decreases Blood flow to digestive system Most active when the body is at rest “resting and digesting” system Promotes normal digestion, elimination of wastes, and conserving body energy Formed during 1st month of embryonic development (maternal infections can be devastating during pregnancy) Nervous tissue has a high metabolic rate and requires large amounts of oxygen (maternal use of cigarettes, drugs, and alcohol decrease oxygen levels and can cause brain damage) One last part to develop is hypothalamus (regulates body temperature); premature babies have difficulty regulating body temperature No new neurons are produced after birth but growth and maturation continue due to myelination that continues to occur during early childhood Brain reaches maximum weight in the young adult Over the next 60 years, neurons are damaged and die Unlimited number of neural pathways are available and ready to develop With age, sympathetic division becomes less effecient Nervous system deterioration caused by poor circulation Can be caused by heart disease Can be caused by hypertension Decreases oxygen levels Can lead to senility (forgetfulness, irritability, difficulty concentrating or thinking clearly) Reversible senility can be caused by Some drugs Constipation Poor nutrition or dehydration Hormone imbalances While reversible, this type of senility, most of the time, is undiagnosed