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8 The Nervous System The Nervous System Two Organ Systems Control All the Other Organ Systems: • Nervous System characteristics • Rapid response • Brief duration • Endocrine System characteristics • Slower response • Long duration The Nervous System Two Anatomical Divisions: • Central Nervous System (CNS) • • • Brain Spinal cord Peripheral Nervous System (PNS) • • • All the neural tissue outside CNS Afferent division (sensory input) Efferent division (motor output) • Somatic nervous system (controls skeletal muscle) • Autonomic nervous system (controls smooth & cardiac muscle) CENTRAL NERVOUS SYSTEM Information Processing PERIPHERAL Sensory information NERVOUS within SYSTEM afferent division Motor commands within efferent division includes Somatic nervous system Autonomic nervous system Parasympathetic division Receptors Somatic sensory receptors (monitor the outside world and our position in it) Sympathetic division Effectors Visceral sensory receptors (monitor internal conditions and the status of other organ systems) Smooth muscle Skeletal muscle Cardiac muscle Glands Adipose tissue Neural Tissue Organization Two Classes of Neural Cells: • Neurons • For information transfer, processing, and storage • Neuroglia • Supporting framework for neurons • Contain imbedded phagocytes Neural Tissue Organization Three Classes of Neurons: • Sensory neurons • Deliver information to CNS • Motor neurons • Stimulate or inhibit peripheral tissues • Interneurons (association neurons) • Located between sensory and motor neurons • Analyze inputs, coordinate outputs Neural Tissue Organization Neuron Anatomy: • Cell body • Nucleus • Mitochondria, RoughER, other organelles • Dendrites • Several branches • Signal receptors (inward) • Axon • Signal propagation (outward) Anatomy of a Typical Neuron Neural Tissue Organization Structural Classes of Neurons: • Unipolar • Dendrite, axon continuous, cell body off to one side • Sensory neurons • Multipolar • Many dendrites, one axon • Most common class of neuron • Motor Neurons • Bipolar • One dendrite, one axon • Very rare, found in sense organs Structural Classification of Neurons Neural Tissue Organization Types of Neuroglia (glia): • Astrocytes • Part of blood-brain barrier • Oligodendrocytes • Responsible for myelination(protective covering) around axons • Microglia • Phagocytic defense cells • Ependymal cells • Lining of brain, spinal cord cavities • Source of cerebrospinal fluid Neuroglia in the CNS Neural Tissue Organization Two Types of Neuroglia in the PNS: • Satellite cells • Surround cell bodies • Schwann cells • Surround all peripheral axons • Form myelin sheath on myelinated axons Schwann Cells & Peripheral Axons Neural Tissue Organization Key Note: Neurons perform all of the communication, information processing, and control functions of the nervous system. Neuroglia outnumber neurons and have functions essential to preserving the physical and biochemical structure of neural tissue and the survival of neurons. Neural Tissue Organization Anatomic Organization of CNS Neurons: • Center—Collection of neurons with a shared function • Nucleus—A center with a discrete anatomical boundary • Neural cortex—Gray matter covering of brain portions • White matter—Bundles of axons (tracts) that share origins, destinations, and functions Neural Tissue Organization Anatomic Organization of PNS Neurons: • Ganglia—Groupings of neuron cell bodies • Nerve—Bundle of axons supported by connective tissue • Spinal nerves • To/from spinal cord • Cranial nerves • To/from brain Neural Tissue Organization Neural Tissue Organization Pathways in the CNS: • Ascending pathways - Carry information from sensory receptors to processing centers in the brain • Descending pathways - Carry commands from specialized CNS centers to skeletal muscles Neuron Function The Membrane Potential • Resting potential • Excess negative charge inside the neuron • Created and maintained by Na-K ion pump • Negative voltage (potential) inside • -70 mV (0.07 Volts) Cell Membrane at the Resting Potential Neuron Function A membrane potential exists across the cell membrane because: (1) the cytosol and the extracellular fluid differ in their ionic composition, and (2) the cell membrane is selectively permeable to these ions. The membrane potential can quickly change, as the ionic permeability of the cell membrane changes, in response to chemical or physical stimuli. Depolarization to threshold Activation of voltageregulated sodium channels and rapid depolarization Sodium ions Local current Potassium ions Inactivation of sodium channels and activation of voltage-regulated potassium channels Transmembrane potential (mV) +30 DEPOLARIZATION 3 REPOLARIZATION 0 2 _ 60 _ 70 The return to normal permeability and resting state Threshold 1 4 Resting potential REFRACTORY PERIOD 0 1 2 Time (msec) 3 Neuron Function Key Note: “Information” travels within the nervous system primarily in the form of propagated electrical signals known as action potentials. The most important information (e.g., vision, balance, movement), is carried by myelinated axons. Neural Communication Synapse Basics • Intercellular communication • Axon terminal receives electrical signal from cell body • Input to next cell • Chemical signaling to cross synapse • Neurotransmitter release Neural Communication Structure of a Synapse: • Presynaptic components • Axon terminal • Synaptic knob • Synaptic vesicles • Synaptic cleft • Postsynaptic components • Neurotransmitter receptors Neural Communication The Structure of a Typical Synapse Neural Communication Synaptic Function and Neurotransmitters • Cholinergic synapses • Release neurotransmitter acetylcholine • Enzyme in synaptic cleft (acetylcholinesterase) breaks it down • Adrenergic synapses • Release neurotransmitter norepinephrine • Dopaminergic synapses • Release neurotransmitter dopamine Neural Communication Extracellular Ca2+ enters the synaptic cleft triggering the exocytosis of ACh An action potential arrives and depolarizes the synaptic knob PRESYNAPTIC NEURON Synaptic vesicles Action potential EXTRACELLULAR FLUID ACh ER Synaptic knob Ca2+ Synaptic cleft Ca2+ AChE CYTOSOL Chemically regulated sodium channels POSTSYNAPTIC NEURON ACh is removed by AChE (acetylcholinesterase) ACh binds to receptors and depolarizes the postsynaptic membrane Initiation of action potential if threshold is reached Propagation of action potential (if generated) Na2+ Na2+ Na2+ Na2+ Receptor Na2+ Neural Communication • Neuronal pools Groups of interconnected neurons with specific functions • Divergence Spread of information from one neuron to several others • Convergence Several neurons send information to one other Neural Communication Two Common Types of Neuronal Pools Neural Communication Key Note: A synaptic terminal releases a neurotransmitter that binds to the postsynaptic cell membrane. The result is a brief, local change in the permeability of the postsynaptic cell. Many drugs affect the nervous system by stimulating neurotransmitter receptors and thus produce complex effects on perception, motor control, and emotions. The Central Nervous System Meninges—Layers that surround and protect the brain and spinal cord (CNS) • Dura mater (“tough mother”) • Tough, fibrous outer layer • Epidural space above dura of spinal cord • Arachnoid (“spidery”) • Subarchnoid space • Cerebrospinal fluid • Pia mater (“delicate mother”) • Thin inner layer The Meninges The Meninges The Central Nervous System Spinal Cord Basics: • Relays information to/from brain • Processes some information on its own (reflexes) • Divided into 31 segments • Each segment has a pair of: • Dorsal root ganglia • Dorsal roots • Ventral roots • Gray matter appears as horns • White matter organized into columns Anatomy of the Spinal Cord Anatomy of the Spinal Cord Sectional Anatomy of the Spinal Cord Sectional Anatomy of the Spinal Cord The Central Nervous System Key Note: The sensory and motor nuclei (gray matter) of the spinal cord surround the central canal. Sensory nuclei are dorsal, motor nuclei are ventral. A thick layer of white matter consisting of ascending and descending axons covers the gray matter. These axons are organized into columns of axon bundles with specific functions. This highly organized structure often enables predicting the impact of particular injuries. The Central Nervous System Brain Regions • • • • • • Cerebrum Diencephalon Midbrain Pons Medulla oblongata Cerebellum The Brain The Brain The Brain The Central Nervous System Brain—The four hollow chambers in the center of the brain filled with cerebrospinal fluid (CSF) • CSF produced by choroid plexus • CSF circulates • From ventricles and central canal • To subarachoid space • Accessible by lumbar puncture • To blood stream Ventricles of the Brain Circulation of Cerebrospinal Fluid The Central Nervous System Functions of the Cerebrum: • • • • Conscious thought Intellectual activity Memory Origin of complex patterns of movement Anatomy of Cerebral Cortex: • Highly folded surface • Elevated ridges (gyri) • Shallow depressions (sulci) The Central Nervous System Functions of the Cerebral Cortex • Hemispheres serve opposite body sides • Primary motor cortex (precentral gyrus) • Directs voluntary movement • Primary sensory cortex (postcentral gyrus) • Receives somatic sensation (touch, pain, pressure, temperature) • Association areas • Interpret sensation • Coordinate movement Surface of Cerebral Hemispheres The Central Nervous System Hemispheric Lateralization • Categorical hemisphere (usually left) • General interpretative and speech centers • Language-based skills • Representational Hemisphere (usually right) • Spatial relationships • Logical analysis Hemispheric Lateralization Brain Waves (Electroencephalogram) The Central Nervous System The Basal Nuclei • Lie deep within central white matter of the brain • Responsible for muscle tone • Coordinate learned movements • Coordinate rhythmic movements (e.g., walking) The Basal Nuclei The Central Nervous System Functions of the Limbic System • Establish emotions and related drives • Link cerebral cortex intellectual functions to brain stem autonomic functions • Control reflexes associated with eating • Store and retrieve long-term memories The Limbic System The Central Nervous System The Diencephalon • Switching and relay center • Integration of conscious and unconscious motor and sensory pathways • Components include: • Epithalamus • Choroid plexus • Pineal body • Thalamus • Hypothalamus The Central Nervous System The Diencephalon and Brain Stem Figure 8-24(a) The Central Nervous System Functions of the Thalamus • Relay and filter all ascending (sensory) information • Relay a small proportion to cerebral cortex (conscious perception) • Relay most to basal nuclei and brain stem centers • Coordinate voluntary and involuntary motor behavior The Central Nervous System Functions of the Hypothalamus • Produce emotions and behavioral drives • Coordinate nervous and endocrine systems • Secrete hormones • Coordinate voluntary and autonomic functions • Regulate body temperature The Central Nervous System Anatomy and Function of the Brain Stem • Midbrain • Process visual, auditory information • Generate involuntary movements • Pons • Links to cerebellum • Involved in control of movement • Medulla oblongata • Relay sensory information • Regulate autonomic function The Central Nervous System Functions of the Medulla Oblongata • Links brain and spinal cord • Relays ascending information to cerebral cortex • Controls crucial organ systems by reflex • Cardiovascular centers • Respiratory rhythmicity centers The Central Nervous System Anatomy and Function of the Cerebellum • • • • Oversees postural muscles Stores patterns of movement Fine tunes most movements Links to brain stem, cerebrum, spinal cord • Communicates over cerebellar peduncles The Central Nervous System Key Note: The brain, a large mass of neural tissue, contains internal passageways and chambers filled with CSF. The six major regions of the brain have specific functions. As you ascend from the medulla oblongata to the cerebrum, those functions become more complex and variable. Conscious thought and intelligence are provided by the cerebral cortex. The Peripheral Nervous System PNS Basics • Links the CNS with the body • Carries all sensory information and motor commands • Axons bundled in nerves • Cell bodies grouped into ganglia • Includes cranial and spinal nerves The Peripheral Nervous System The Cranial Nerves • 12 Pairs • Connect to brain not the cord • Olfactory (CN I) • Sense of smell • Optic (CN II) • Sense of vision • Oculomotor (CN III) • Eye movement The Peripheral Nervous System The Cranial Nerves (continued) • Trochlear (CN IV) • Eye movement • Trigeminal (CN V) • Eye, jaws sensation/movement • Abducens (CN VI) • Eye movement • Facial (CN VII) • Face, scalp, tongue sensation/movement • Vestibulocochlear (CN VIII) • Hearing, balance The Peripheral Nervous System The Cranial Nerves (continued) • Glossopharyngeal (CN IX) • Taste, swallowing • Vagus (CN X) • Autonomic control of viscera • Accessory (CN XI) • Swallowing, pectoral girdle movement • Hypoglossal (CN XII) • Tongue movement The Cranial Nerves The Cranial Nerves The Peripheral Nervous System Key Note: The 12 pairs of cranial nerves are responsible for the special senses of smell, sight, and hearing/balance, and control movement of the eye, jaw, face, tongue, and muscles of the neck, back, and shoulders. They also provide sensation from the face, neck, and upper chest and autonomic innervation to thoracic and abdominopelvic organs. The Peripheral Nervous System The Spinal Nerves • 31 Pairs • 8 Cervical • 12 Thoracic • 5 Lumbar • 5 Sacral • Dermatome—Region of the body surface monitored by a pair of spinal nerves The Peripheral Nervous System Nerve Plexus—A complex, interwoven network of nerves • Four Large Plexuses • Cervical plexus • Brachial plexus • Lumbar plexus • Sacral plexus The Peripheral Nervous System Reflex—An automatic involuntary motor response to a specific stimulus • The 5 steps in a reflex arc • Arrival of stimulus and activation of receptor • Activation of sensory neuron • CNS processing of information • Activation of motor neuron • Response by effector (muscle or gland) Arrival of stimulus and activation of receptor Activation of a sensory neuron Receptor Sensation relayed to the brain by collateral Dorsal root REFLEX ARC Stimulus Effector Ventral root Activation of a motor neuron Response by effector Reflex Action Information processing in CNS KEY Sensory neuron (stimulated) Excitatory interneuron Motor neuron (stimulated) The Peripheral Nervous System Examples of Reflexes • Monosynaptic reflex—Simplest reflex arc; sensory neuron synapses directly on motor neuron • Stretch reflex—Monosynaptic reflex to regulate muscle length and tension (example: patellar reflex) • Muscle spindle—Sensory receptor in a muscle that stimulates the stretch reflex Stretching of muscle tendon stimulates muscle spindles Muscle spindle (stretch receptor) Stretch Spinal cord REFLEX ARC Contraction Activation of motor neuron produces reflex muscle contraction Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings The Peripheral Nervous System Polysynaptic reflex—A reflex arc with at least one interneuron between the sensory afferent and motor efferent • Has a longer delay than a monosynaptic reflex (more synapses) • Can produce more complex response • Example: flexor reflex, a withdrawal reflex • Brain can modify spinal reflexes The Flexor Reflex The Peripheral Nervous System Key Note: Reflexes are rapid, automatic responses to stimuli that “buy time” for the planning and execution of more complex responses that are often consciously directed. The Peripheral Nervous System The Autonomic Nervous System Autonomic Nervous System Branch of nervous system that coordinates cardiovascular, digestive, excretory, and reproductive functions The Autonomic Nervous System Divisions of the ANS • Sympathetic division • Preganglionic neurons in the thoracic and lumbar segments of the spinal cord • “Fight or flight” system • Parasympathetic division • Preganglionic neurons in the brain and sacral segments • “Rest and digest” system The Autonomic Nervous System Key Note: The two divisions of the ANS operate largely without our awareness. The sympathetic division increases alertness, metabolic rate, and muscular abilities; The parasympathetic division reduces metabolic rate and promotes visceral activities such as digestion. The Sympathetic Division The Autonomic Nervous System Effects of Sympathetic Activation • • • • • • Generalized response in crises Increased alertness Feeling of euphoria and energy Increased cardiovascular activity Increased respiratory activity Increased muscle tone The Parasympathetic Division The Autonomic Nervous System Effects of Parasympathetic Activation • • • • Relaxation Food processing Energy absorption Brief effects at specific sites The Autonomic Nervous System Relationship between the Two Divisions: • Sympathetic division reaches visceral and somatic structures throughout the body • Parasympathetic division reaches only visceral structures via cranial nerves or in the abdominopelvic cavity • Many organs receive dual innervation • In general, the two divisions produce opposite effects on the their target organs Aging and the Nervous System Age-Related Changes • • • • Reduction in brain size and weight Loss of neurons Decreased brain blood flow Changes in synaptic organization of the brain • Intracellular and extracellular changes in CNS neurons