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Structure and Function of the Neurologic System
Chapter 14
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Overview of the Nervous System
Central nervous system (CNS) Brain and spinal cord Peripheral nervous system (PNS) Cranial nerves Spinal nerves Pathways Afferent (ascending) Efferent (descending)
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Overview of the Nervous System
Peripheral nervous system (PNS) Somatic nervous system Motor and sensory pathways regulating voluntary motor control of skeletal muscle Autonomic nervous system Motor and sensory pathways regulating the body’s internal environment through involuntary control of organ systems Sympathetic Parasympathetic
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Cells of the Nervous System
Neuron Variable size and structure throughout the nervous system Cellular constituents Microtubules, neurofibrils, Nissl substances, and ribosomes Some neurons like olfactory neurons continue to divide
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Neuron Three components Cell body (soma) Dendrites
Located mainly in the CNS Densely packed cell bodies in the CNS are nuclei Densely packed cell bodies in the PNS are ganglia Dendrites Receptive portion of the neuron
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Neuron Axons Carry nerve impulses away from the cell body Axon Hillock
Cone-shaped, Nissl-free area, where the axon leaves the cell body Initial segment of the axon Area of the axon having the lowest threshold for stimulation, so the action potentials begin at this point
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Neuron Axons Myelin Endoneurium Neurilemma
Segmented layer of lipid material Insulating substance Formed and maintained by the Schwann cell Endoneurium Delicate layer of connective tissue around each axon Neurilemma Thin membrane between the myelin sheath and the endoneurium
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Neuron Axons Nodes of Ranvier Saltatory conduction Divergence
Regular interruptions of the myelin sheath Saltatory conduction The flow of ions between segments of myelin rather than along the entire length of the axon Divergence Ability of branching axons to influence many neurons Convergence Branches of numerous neurons converging on one or a few neurons
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Structural Classification of Neurons
Based on the number of processes extending from the cell body Unipolar Pseudounipolar Bipolar Multipolar
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Functional Classification of Neurons
Sensory Transmit impulses from sensory receptors to the CNS Associational Transmit impulses from neuron to neuron Motor Transmit impulses from the CNS to an effector organ
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Neuroglia
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Neuroglia “Nerve glue” Support the neurons of the CNS Astrocytes
Oligodendroglia (oligodendrocytes) Microglia Ependymal cells
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Neuroglia
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Nerve Injury and Regeneration
Wallerian degeneration Occurs distal to the cut Swelling appears Neurofilaments hypertrophy Myelin sheath shrinks and disintegrates Axon portion degenerates and disappears
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Nerve Injury and Regeneration
Proximal to the cut Swelling and dispersal of the Nissl substance Cell increases in metabolic activity, protein synthesis, and mitochondrial activity New terminal sprouts project from the proximal segment Process limited to myelinated axons Generally only in the PNS Depends on location, type of injury, the inflammatory response, and scar tissue formation
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Neuroglia
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Nerve Impulse Neurons generate and conduct electrical and chemical impulses by selectively changing the electrical portion of their plasma membranes and influencing other nearby neurons by the release of neurotransmitters
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Synapses Region between adjacent neurons is called a synapse
Impulses are transmitted across the synapse by chemical and electrical conduction Presynaptic neurons and postsynaptic neurons Connections between pre- and postsynaptic neurons Axoaxonic Axosomatic Axodendritic Dendrodentritic
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Synapses
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Synapses Neurotransmitters Synaptic boutons Synaptic cleft Summation
>30 substances Excitatory (excitatory postsynaptic potential) Inhibitory (inhibitory postsynaptic potential) Synaptic boutons Synaptic cleft Summation Temporal summation Spatial summation
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Central Nervous System
Forebrain Cerebral hemispheres Midbrain Corpora quadrigemina, tegmentum, and cerebral peduncles Hindbrain Cerebellum, pons, and medulla
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Central Nervous System
Reticular formation Network of connected nuclei that regulate viral reflexes, such as cardiovascular and respiratory function Together with the cerebral cortex is referred to as the reticular activating system
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Central Nervous System
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Central Nervous System
Forebrain Telencephalon Cerebrum Gyri, sulci, and fissures Gray matter and white matter Cerebral nuclei (basal ganglia)
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Forebrain
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Forebrain
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Central Nervous System
Diencephalon Epithalamus Thalamus Hypothalamus Subthalamus
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Central Nervous System
Midbrain Corpora quadrigemina (tectum) Superior and inferior colliculi Tegmentum Red nucleus and substantia nigra Basis pedunculi
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Central Nervous System
Hindbrain (metencephalon) Cerebellum Pons Myelencephalon Medulla oblongata
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Central Nervous System
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Spinal Cord Lies within the vertebral canal and is protected by the vertebral column Connects the brain and the body Conducts somatic and autonomic reflexes Provides motor pattern control centers Modulates sensory and motor function
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Spinal Cord
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Spinal Cord
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Spinal Cord
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Reflex Arc Receptor Afferent (sensory) neuron Efferent neuron Effector
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Reflex Arc
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Upper and Lower Motor Neurons
Upper motor neurons Efferent pathways primarily relaying information from the cerebrum to the brain stem or spinal cord Synapse with interneurons Lower motor neurons Neurons having direct influence on muscles Cell bodies originate in the gray matter of the spinal cord, but their axons extend into the PNS
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Neuromuscular Junction
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Motor Pathways Lateral corticospinal Corticobulbar Basal ganglia
Vestibulospinal
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Sensory Pathways Anterior spinothalamic Lateral spinothalamic
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Protective Structures
Cranium Eight bones Frontal Occipital Temporal (2) Parietal (2) Sphenoid Ethmoid Galea aponeurotica
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Protective Structures
Meninges Protective membranes surrounding the brain and spinal cord Dura mater Arachnoid Pia mater
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Meninges
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Protective Structures
Cerebrospinal fluid (CSF) and the ventricular system CSF is a clear, colorless fluid similar to blood plasma and interstitial fluid 125 to 150 mL Produced by the choroid plexuses in the lateral, third, and fourth ventricles Reabsorbed through the arachnoid villi
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Protective Structures
Vertebral column 33 vertebrae 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal Intervertebral disks Nucleus pulposus
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Vertebral Column
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Vertebral Column
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Blood Supply to the Brain
800 to 1000 mL per minute CO2 is the primary regulator for CNS blood flow Internal carotid and vertebral arteries Arterial circle (circle of Willis)
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Blood Supply to the Brain
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Blood Supply to the Brain
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Blood Supply to the Brain
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Blood Supply to the Brain
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Blood Supply to the Spinal Cord
Vertebral arteries Anterior spinal arteries Posterior spinal arteries Aorta
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Blood Supply to the Spinal Cord
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Peripheral Nervous System
31 pairs of spinal nerves Names correlate with the vertebral level from which they exit Mixed nerves Arise from the anterior and posterior horn cells of the spinal cord 12 pairs of cranial nerves Sensory, motor, and mixed
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Peripheral Nervous System
Fascicles Bundles of myelinated axons in the PNS Rami Plexuses Brachial plexus Lumbar plexus Sacral plexus Dermatomes
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Peripheral Nervous System
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Autonomic Nervous System
Located in both the CNS and PNS Coordinates and maintains a steady state among the visceral (internal) organs Neurons Preganglionic (myelinated) Postganglionic (unmyelinated) Two divisions Sympathetic Parasympathetic
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Sympathetic Nervous System
Mobilizes energy stores in times of need “Fight or flight response” Receives innervation from cell bodies located from the first thoracic through the second lumbar Thoracolumbar division Sympathetic (paravertebral) ganglia
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Sympathetic Nervous System
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Parasympathetic Nervous System
Functions to conserve and restore energy “Rest or repose response” Receives innervation from cell bodies located in the cranial nerve nuclei and sacral region of the spinal cord Craniosacral division Preganglionic neurons travel to ganglia close to the organs they innervate
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Parasympathetic Nervous System
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Neurotransmitters and Neuroreceptors of the ANS
Sympathetic preganglionic fibers Acetylcholine and cholinergic receptors Sympathetic postganglionic fibers Norepinephrine and adrenergic receptors Parasympathetic pre- and postganglionic fibers
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Neurotransmitters and Neuroreceptors of the ANS
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Aging and the Nervous System
Decrease in the number of neurons Decreased brain weight and size Senile plaques Neurofibrillary tangles Slowing of neurologic responses
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Tests of Nervous System Function
Skull and spine roentgenograms Computed tomography Magnetic resonance imaging Magnetic resonance angiography Positron emission tomography scan Brain scan Cerebral angiography
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Tests of Nervous System Function
Myelography Echoencephalography (ultrasound) Electroencephalography Evoked potentials Cerebrospinal fluid analysis
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