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Ch. 7 The Nervous System NOTES Functions of the nervous system 1. Sensory input – gathering information o To monitor changes occurring inside and outside the body o Changes = stimuli 2. Integration o To process and interpret sensory input and decide if action is needed 3. Motor output o A response to integrated stimuli o The response activates muscles or glands Structural Classification of the Nervous CNS: Central nervous system o o Brain Spinal cord PNS: Peripheral nervous system – nerves outside the brain and spinal cord Functional Classification of the PNS To go toward Sensory (afferent) division o nerve fibers that carry information toward the central nervous system Motor (efferent) division Nerve fibers that carry impulses away from the central nervous system o Two subdivisions Somatic nervous system = voluntary Autonomic nervous system = involuntary Organization of the nervous system o Nervous Tissue: Support Cells (Neuroglia) – cannot transmit nerve impulses CNS glia: a) Astrocytes o Abundant, star-shaped cells o Brace neurons o Form barrier between capillaries and neurons o Control the chemical environment of the brain b) Microglia o Spider-like phagocytes o Dispose of debris c) Ependymal cells o Line cavities of the brain and spinal cord o Circulate cerebrospinal fluid with cilia d) Oligodendrocytes o Produce myelin sheath around nerve fibers in the central nervous system PNS glia: e) Satellite cells o f) Protect neuron cell bodies Schwann cells o Form myelin sheath in the peripheral nervous system Neurons = nerve cells (do not routinely undergo cell division after birth) Cells specialized to transmit messages Structure: o Cell body– nucleus (& large nucleolus) and metabolic center of the cell Nissl substance – specialized rough endoplasmic reticulum Neurofibrils – intermediate cytoskeleton that maintains cell shape Neuron Cell Body Location: Most are found in the CNS o Gray matter – cell bodies and unmylenated fibers Page | 2 o Nuclei – clusters of cell bodies within the white matter of the central nervous system Ganglia – collections of cell bodies outside the CNS o Processes – fiber extensions outside the cell body Dendrites – conduct impulses toward the cell body Axons – conduct impulses away from the cell body Axons & Nerve Impulses: Axons end in axonal terminals Axonal terminals contain vesicles with chemicals called neurotransmitters Axonal terminals are separated from the next neuron by a gap o Synaptic cleft – gap between adjacent neurons o Synapse – junction between nerves Nerve Fiber Coverings o Schwann cells – produce myelin sheaths in jelly-roll like fashion o Nodes of Ranvier – gaps in myelin sheath along the axon A lack of or, deterioration of myelin sheaths causes the nervous current to short-circuit. This results in a loss of the ability to control muscles. It’s commonly referred to as multiple sclerosis (MS). Functional Classification of Neurons o Sensory (afferent) neurons Carry impulses from the sensory receptors Cutaneous sense organs Proprioceptors (in muscles & tendons) – detect stretch or tension o Motor (efferent) neurons o Carry impulses from the CNS Interneurons (association neurons) Found in neural pathways in the CNS Connect sensory and motor neurons Structural Classification of Neurons o Multipolar neurons – o Bipolar neurons – one o Unipolar neurons – have a many extensions from axon and one dendrite short single process leaving the cell body the cell body Page | 3 Functional Properties of Neurons o Irritability – ability to respond to stimuli o Conductivity – ability to transmit an impulse o The plasma membrane at rest is polarized Fewer positive ions are inside the cell than outside the cell Nerve Impulses Starting a Nerve Impulse o Depolarization – a stimulus depolarizes the neuron’s membrane o A deploarized membrane allows sodium (Na+) to flow inside the membrane o The exchange of ions initiates an action potential in the neuron The Action Potential o If the action potential (nerve impulse) starts, it is propagated over the entire axon o Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane o The sodium-potassium pump restores the original configuration This action requires ATP Nerve Impulse Propagation o The impulse continues to move toward the cell body o Impulses travel faster when fibers have a myelin sheath o Impulses are able to cross the synapse to another nerve Neurotransmitter is released from a nerve’s axon terminal The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter An action potential is started in the dendrite Alcohol, sedatives, & anesthetics can block nerve impulses by reducing membrane permeability to Na+. Cold, & continuous pressure can hinder circulation (and hence delivery of nutrients) to neurons. Page | 4 Reflex Arc Reflex – rapid, predictable, and involuntary responses to stimuli (always in the same direction) o Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector Simple reflex arc o Types of Reflexes & Regulation o o Autonomic reflexes: (involuntary) ex.: salivary reflex, pupillary reflex, etc… Smooth muscle regulation Heart and blood pressure regulation Regulation of glands Digestive system regulation Somatic reflexes: (voluntary) ex.: pulling your hand away from a hot object Activation of skeletal muscles All reflex arcs have a minimum of five elements o sensory receptor – reacts to a stimulus o effector organ – the muscle or gland eventually stimulated o afferent & efferent neurons – connect the previous two o integration center – the synapse between the afferent & efferent neurons two-neuron reflex arc (knee-jerk reflex) three-neuron reflex arc (withdrawal reflex) Central Nervous System CNS develops from the embryonic neural tube o The neural tube becomes the brain and spinal cord o The opening of the neural tube becomes the ventricles Four chambers within the brain Filled with cerebrospinal fluid Regions of the Brain o Cerebral hemispheres o Diencephalon o Brain stem o Cerebellum Page | 5 Cerebral Hemispheres (Cerebrum) Paired (left and right) superior parts of the brain Include more than half of the brain mass The surface is made of ridges (gyri), shallow grooves (sulci), and deeper grooves (fissures) Lobes of the Cerebrum o Fissures divide the cerebrum into lobes o Surface lobes of the cerebrum Frontal lobe Parietal lobe Occipital lobe Temporal lobe Sensory and Motor Areas of the Cerebral Cortex Specialized Areas of the Cerebrum o Somatic sensory area – receives impulses from the body’s sensory receptors pain, coldness, light touch o Primary motor area – sends impulses to skeletal muscles face, mouth, hands o Broca’s area – involved in our ability to speak Cerebral areas involved in special senses o Gustatory area (taste) o Visual area o Auditory area o Olfactory area Interpretation areas of the cerebrum o Speech/language region o Language comprehension region o General interpretation area Layers of the Cerebrum o Gray matter o White matter Outer layer Fiber tracts inside the gray matter Composed mostly of neuron cell bodies Example: corpus callosum connects Basal nuclei – internal islands of gray matter hemispheres Page | 6 Diencephalon Sits on top of the brain stem Enclosed by the cerebral hemispheres Made of three parts o Thalamus o Hypothalamus o Epithalamus Thalamus o Surrounds the third ventricle o The relay station for sensory impulses o Transfers impulses to the correct part of the cortex for localization and interpretation Hypothalamus o Under the thalamus o Important autonomic nervous system center o Helps regulate body temperature Controls water balance Regulates metabolism An important part of the limbic system (emotions) o thirst, appetite, sex, pain, & pleasure The pituitary gland is attached to the hypothalamus Epithalamus o Forms the roof of the third ventricle o Houses the pineal body (an endocrine gland) o Includes the choroid plexus – forms cerebrospinal fluid Brain Stem Attaches to the spinal cord Parts of the brain stem o Midbrain o Pons o Medulla oblongata Midbrain: o Mostly composed of tracts of nerve fibers Has two bulging fiber tracts – cerebral peduncles Has four rounded protrusions – corpora quadrigemina Reflex centers for vision and hearing Page | 7 o o Pons The bulging center part of the brain stem Mostly composed of fiber tracts Includes nuclei involved in the control of breathing Medulla Oblongata The lowest part of the brain stem Merges into the spinal cord Includes important fiber tracts Contains important control centers Heart rate control Blood pressure regulation Breathing Swallowing Vomiting Reticular Formation o Diffuse mass of gray matter along the brain stem o Involved in motor control of visceral organs o Reticular activating system plays a role in awake/sleep cycles and consciousness Cerebellum Two hemispheres with convoluted surfaces Provides involuntary coordination of body movements o like an automatic pilot Page | 8 Protection of Central Nervous System Scalp and skin Skull and vertebral column Meninges Cerebrospinal fluid Blood brain barrier Meninges Dura mater – “tough or hard mother” o o Double-layered external covering Periosteum – attached to surface of the skull Meningeal layer – outer covering of the brain Folds inward in several areas Arachnoid layer – “spider/cobweb” o Middle layer o Web-like Pia mater – “gentle mother” o Internal layer o Clings to the surface of the brain Cerebrospinal fluid (CSF) Similar to blood plasma composition Formed by the choroid plexus Forms a watery cushion to protect the brain Circulated in arachnoid space, ventricles, and central canal of the spinal cord Blood Brain Barrier Includes the least permeable capillaries of the body Excludes many potentially harmful substances Useless against some substances o Fats and fat soluble molecules o Respiratory gases o Alcohol o Nicotine o Anesthesia Traumatic Brain Injuries Concussion o Slight brain injury o No permanent brain damage Page | 9 Contusion o Nervous tissue destruction occurs o Nervous tissue does not regenerate Cerebral edema o Swelling from the inflammatory response o May compress and kill brain tissue Cerebrovascular Accident (CVA) Commonly called a stroke The result of a ruptured blood vessel supplying a region of the brain Brain tissue supplied with oxygen from that blood source dies Loss of some functions or death may result Alzheimer’s Disease Progressive degenerative brain disease Mostly seen in the elderly, but may begin in middle age Structural changes in the brain include abnormal protein deposits and twisted fibers within neurons Victims experience memory loss, irritability, confusion and ultimately, hallucinations and death Spinal Cord Extends from the medulla oblongata to the region of T12 Below T12 is the cauda equina (a collection of spinal nerves) Enlargements occur in the cervical and lumbar regions Anatomy: o Exterior white mater – conduction tracts o Internal gray matter - mostly cell bodies o Dorsal (posterior) horns Anterior (ventral) horns Central canal filled with cerebrospinal fluid Meninges cover the spinal cord Nerves leave at the level of each vertebrae o Dorsal root Associated with the dorsal root ganglia – collections of cell bodies outside the central nervous system o Ventral root Page | 10 Peripheral Nervous System Nerves and ganglia outside the central nervous system Nerve = bundle of neuron fibers Neuron fibers are bundled by connective tissue Structure of a Nerve Endoneurium surrounds each fiber Groups of fibers are bound into fascicles by perineurium Fascicles are bound together by epineurium Classifications of Nerves Mixed nerves – both sensory and motor fibers Afferent (sensory) nerves – carry impulses toward the CNS Efferent (motor) nerves – carry impulses away from the CNS Cranial Nerves 12 pairs of nerves that mostly serve the head and neck Numbered in order, front to back Most are mixed nerves, but three are sensory only “Oh, Oh, Oh, to touch and feel very good velvet ah.” o I Olfactory nerve – sensory for smell o II Optic nerve – sensory for vision o III Oculomotor nerve – motor fibers to eye muscles o IV Trochlear – motor fiber to eye muscles o V Trigeminal nerve – sensory for the face; motor fibers to chewing muscles o VI Abducens nerve – motor fibers to eye muscles o VII Facial nerve – sensory for taste; motor fibers to the face o VIII Vestibulocochlear nerve – sensory for balance and hearing o IX Glossopharyngeal nerve – sensory for taste; motor fibers to the pharynx o X Vagus nerves – sensory and motor fibers for pharynx, larynx, and viscera o XI Accessory nerve – motor fibers to neck and upper back o XII Hypoglossal nerve – motor fibers to tongue Page | 11 Spinal Nerves There is a pair of spinal nerves at the level of each vertebrae for a total of 31 pairs Spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord Spinal nerves are named for the region from which they arise Anatomy: o Spinal nerves divide soon after leaving the spinal cord Dorsal rami – serve the skin and muscles of the posterior trunk Ventral rami – forms a complex of networks (plexus) for the anterior Page | 12 Autonomic Nervous System The involuntary branch of the nervous system Consists of only motor nerves Divided into two divisions o Sympathetic division o Parasympathetic division Differences Between Somatic and Autonomic Nervous Systems Somatic Autonomic Nerves One motor neuron preganglionic and postganglionic Effector organs skeletal muscle smooth muscle, cardiac muscle,and glands Nerurotransmitters always use acetylcholine use acetylcholine, epinephrine, or norepinephrine Anatomy of the Sympathetic Division Originates from T1 through L2 Ganglia are at the sympathetic trunk (near the spinal cord) Short pre-ganglionic neuron and long postganglionic neuron transmit impulse from CNS to the effector Norepinephrine and epinephrine are neurotransmitters to the effector organs Page | 13 Anatomy of the Parasympathetic Division Originates from the brain stem and S1 through S4 Terminal ganglia are at the effector organs Always uses acetylcholine as a neurotransmitter Autonomic Functioning Sympathetic – “fight-or-flight” o Response to unusual stimulus o Takes over to increase activities o Remember as the “E” division = exercise, excitement, emergency, and embarrassment Parasympathetic – housekeeping activities o Conserves energy o Maintains daily necessary body functions o Remember as the “D” division - digestion, defecation, and diuresis Development Aspects of the Nervous System The nervous system is formed during the first month of embryonic development Any maternal infection can have extremely harmful effects The hypothalamus is one of the last areas of the brain to develop No more neurons are formed after birth, but growth and maturation continues for several years The brain reaches maximum weight as a young adult Page | 14