CH 15 Structure and Function of Neurologic System Lecture Notes 2024 PDF

Structure and Function of the Nervous System Chapter 15 Rebecca Boeschel MSHS, PA-C 1 2 Functional Organization of PNS Somatic Nervous System – pathways that regulate voluntary motor control (e.g., skeletal muscle) Autonomic Nervous...

Structure and Function of the Nervous System Chapter 15 Rebecca Boeschel MSHS, PA-C 1 2 Functional Organization of PNS Somatic Nervous System – pathways that regulate voluntary motor control (e.g., skeletal muscle) Autonomic Nervous System – pathways that regulate the body’s internal environment (viscera) through the involuntary control of organ systems 1. Sympathetic Division 2. Parasympathetic Division What are the target cells (effectors) of the ANS? 3 Mosby items and derived items © 2006 by Mosby, Inc. 4 Cells of the Nervous System Neurons - primary Neuroglia - support cells found in information/communication cells; the CNS and PNS; provide working in parallel systems, structural support and nutrition for neurons can scan the neurons, remove debris and environment, integrate many increase the transmission speed systems at higher cognitive of nerve impulses levels, and maintain homeostasis; electrically excitable 5 Mosby items and derived items © 2006 by Mosby, Inc. Neurons  Variable size and structure throughout the nervous system  Fuel source is predominantly glucose; however, insulin is not required for glucose cellular uptake in the CNS  CNS starts out with more neurons than it needs and those that do not become involved in functional systems die; some neurons like olfactory neurons continue to divide after birth 6  Neuronal Structure Three common components: 1.Cell body (soma) densely packed cell bodies in the CNS are nuclei densely packed cell bodies in the PNS are ganglia or plexuses 2.Dendrites - receptive portion of the neuron 3.Axons - carry nerve impulses away from the cell body 7 Neuron Polarity Unipolar neuron Pseudounipolar neuron Bipolar neuron Multipolar neurons 8 Mosby items and derived items © 2006 by Mosby, Inc.  Myelinated Axons Myelin sheath Insulating lipid layer that increases conduction velocity Myelin in the PNS is formed and maintained by the Schwann cell; in the CNS it is formed by oligodendrocytes Regions of CNS with high levels of myelination constitute the white matter; regions lacking significant myelination are gray matter Nodes of Ranvier Regular interruptions of myelin sheath where nutrient/ion exchange can occur Saltatory conduction Mechanism allowing an action potential to leap between nodes of Ranvier facilitating rapid conduction 0f nerve impulses Mosby items and derived items © 2006 by Mosby, Inc. 10 Myelinated Axons in Sequence Principle of Convergence branches of multiple neurons converge on and influence a single neuron Principle of Divergence refers to the ability of axonal branches to influence many different neurons Functional Classification of Neurons Sensory Neurons - transmit Association neurons or impulses from sensory receptors interneurons - transmit impulses to the CNS from neuron to neuron and are located within the CNS Major Types of Sensory Receptors Motor Neurons -transmit Nociceptors impulses from the CNS to an Mechanoreceptors effector organ (i.e., skeletal Photochemical receptors muscle, smooth muscle, cardiac Chemoreceptors muscle, some glands) Thermoreceptors In skeletal muscle, the end processes of an axon form a Proprioception specialized structure called a Audition and balance neuromuscular junction 12 Mosby items and derived items © 2006 by Mosby, Inc.  Neuroglia (Glial Cells) – Support Cells for Neurons Astrocytes - form specialized contacts between the neuron and blood vessels and thought to form an essential component of blood brain barrier; provide rapid transport for nutrients and metabolites; appear to work with neurons in processing information and memory storage; appear to be scar forming cells of the CNS (my be foci for seizures) Microglia - clear cellular debris (phagocytic properties); the key immune cell in the CNS Ependymal cells - line ventricles and choroid plexuses involved in cerebrospinal fluid (CSF) production Oligodendrocytes – formation of myelin sheath in the CNS Schwann cell – formation of myelin sheath in the PNS; direct axonal regrowth and functional recovery in the PNS Non-myelinating Schwann cells - provide neuronal metabolic support and regeneration in the PNS Satellite glial cells - Surround sensory, sympathetic, and parasympathetic nerve cell bodies and ganglia to provide protection and promote cellular communication (similar to astrocytes in CNS) Figure 15-03. Types of Neuroglial Cells. Neuroglia of the central nervous system (CNS): (A) Astrocytes attached to the outside of a capillary blood vessel in the brain. (B) An oligodendrocyte with processes that wrap around nerve fibers in the CNS to form myelin sheaths. (C) Ciliated ependymal cells forming a sheet that usually lines fluid cavities in the brain. (D) A phagocytic microglial cell. Neuroglia of the peripheral nervous system (PNS): (E) A Schwann cell supporting a bundle of nerve fibers in the PNS. (F) Another type of Schwann cell encircling a peripheral nerve fiber to form a thick myelin sheath. (G) Satellite cells, another type of Schwann cell, surround and support cell bodies of neurons in the PNS. 14  Peripheral Nerve Injury and Regeneration  Mature neurons do not divide; therefore, injury can cause permanent loss of function  Neurons in the PNS can repair themselves through local, anterograde and retrograde changes – the process is very slow (about 1 mm per day) and limited to myelinated fibers in the PNS  Regeneration depends on location and type of injury, the presence of inflammatory responses, and the process of scarring  Injuries close to the neuron’s cell body often result in cell death - peripheral nerves injured close to the spinal cord recover poorly and slowly because of the long distance between the cell body and the peripheral termination of the axon  A crush injury allows for fuller recovery 15 compared with a cut injury  Nerve Impulse/Action Potential  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  Region between adjacent neurons is called a synapse; presynaptic neurons and postsynaptic neurons  Impulses are transmitted across the synapse by chemical and electrical conduction when a stimulus is strong enough; this is an all or none response; an unexcited neuron maintains a resting membrane potential  Chemical conducting substances, neurotransmitters, are synthesized in the neuron and stored in synaptic vessels in the presynaptic terminal  Neurons can synthesize more than one neurotransmitter and postsynaptic membranes can contain more than one type of neurotransmitter-specific receptor  Animation: Molecular Mechanisms of Synaptic Function 16 Mosby items and derived items © 2006 by Mosby, Inc. Figure 15-05. Neuronal Transmission and Synaptic Cleft. Details illustrate the synaptic bouton (knob) of a presynaptic neuron, the plasma membrane of a postsynaptic neuron, and a synaptic cleft. At step 1, the action potential arrives at the synaptic bouton. At step 2, the rapid exocytosis of neurotransmitter molecules from vesicles in the knob occurs. At step 3, neurotransmitter diffuses into the synaptic cleft and binds to receptor molecules (R) in the plasma membrane of the postsynaptic neuron. The postsynaptic receptors directly or indirectly trigger the opening of stimulus gated ion channels, initiating a local potential in the postsynaptic neuron. At step 4, the local potential may move toward the axon, where an action potential may begin.  Neurotransmitters  Binding of the neurotransmitter at the receptor site changes the permeability of the postsynaptic neuron and consequently its membrane potential resulting in either excitation (EPSP) or inhibition (IPSP)  Usually, a single postsynaptic potential cannot induce an action potential and the subsequent propagation of a nerve impulse  Summation refers to the number and frequency of potentials the postsynaptic neuron receives Temporal summation - successive, rapid impulses received from a single neuron on the same synapse Spatial summation - combined effects of impulses from a number of neurons on a single synapse at the same time  Efficient termination of the signal requires rapid degeneration of the neurotransmitter  The mechanisms of convergence (many neurons firing and converging on one neuron), divergence (one neuron firing and diverging on many neurons), summation, and facilitation allow for the integrative processes of the nervous system 18  Table 15.2 Neurotransmitters and Neuromodulators  Neuromodulators function to Acetylcholine raise/lower the membrane Norepinephrine potential – these chemicals facilitate or inhibit the effect of Serotonin neurotransmitters Dopamine  The plasma membrane is Histamine facilitated when the summation brings the membrane closer to GABA threshold potential and Glycine decreases the stimulus required Glutamate to generate an action potential Aspartate  The effect that a neurotransmitter has on the plasma membrane Endorphins depends on the balance of all of Enkaphalins these effects Substance P Vasoactive intestinal peptide 19 Mosby items and derived items © 2006 by Mosby, Inc. Central Nervous System: Brain Contained within the cranial vault and divided into three distinct regions, based on embryological origin (refer to Table 15.3) 1.Forebrain 2.Midbrain 3.Hindbrain Understanding functional specificity is useful when trying to understand pathological conditions What is neuroplasticity? 20 Forebrain Telencephalon (cerebral hemispheres) consists of cerebral cortex and the basal ganglia and is associated with conscious perception of internal and external stimuli, cognition and memory processes and voluntary control of skeletal muscles Diencephalon made up of the epithalamus, subthalamus, hypothalamus and thalamus which relay sensory information, control autonomic functions and links to the limbic system for memory and emotion Limbic system is a group of interconnected structures located between the telencephalon and the diencephalon involved in primitive behavioral responses, visceral reactions to emotions, motivation, mood, feeding behavior, rhythms, sense of smell 21 Figure 15-08. The Cerebral Hemispheres. (A) Left hemisphere of cerebrum, lateral view. (B) Functional areas of the cerebral cortex, midsagittal view. (C) Functional areas of the cerebral cortex, lateral view.  Figure 15-09. Primary Somatic Sensory (A) and Motor (B) Areas of the Cortex. (A) The motor homunculus shows proportional somatotopic representation in the main motor area. (B) The sensory homunculus shows proportional somatotopic representation in the somaesthetic cortex. Figure 15-10. Basal Ganglia. (A) The basal ganglia seen through the cortex of the left cerebral hemisphere. (B) The basal ganglia seen in a frontal (coronal) section of25 the brain. Midbrain (Mesencephalon) Mesencephalon connects the forebrain with the hindbrain and is made up of: tectum (the corpora quadrigemina) tegmentum (red nucleus, substangia nigra cerebral peduncles (link the cortex to the brainstem) Primarily a relay center for motor and sensory tracts, as well as a center for auditory and visual reflexes Mosby items and derived items © 2006 by Mosby, Inc. Hindbrain (Metencephalon and Myelencephalon) Metencephalon consists of the Myelencephalon is usually cerebellum and pons called the medulla Cerebellum is responsible for reflexive, oblongata, the lowest portion involuntary fine-tuning of motor control and for of the brainstem. maintaining balance and posture; allows Contains vital centers for reflex sampling and comparison of sensory data control of heart rate, respiration, from the periphery and motor impulses from blood pressure; coughing, the cerebral hemispheres for the purpose of sneezing, vomiting, swallowing coordination and refinement of skeletal Contains nuceli of the 9th -12th muscle movement; has 2 lobes/hemispheres cranial nerves with ipsilateral control of the body in contrast Major portion of the descending to the cerebral cortex which has contralateral control motor tracts decussate at the Pons (bridge) transmits information from the inferior medulla (other areas of crossover in the CNS as well) cerebellum to the brainstem and between the Contains elements of the reticular cerebellar hemispheres; the nuclei of the 5th through 8th cranial nerves are located in the activating system pons 27 Reticular Formation  Large network of diffuse nuclei that connect the brainstem to the cortex  Control vital reflexes, such as cardiovascular function and respiration  Essential for maintaining wakefulness/consciousness  Some nuclei within the reticular formation support specific motor movements, such as balance and posture Figure 15-07. Reticular Activating System. The reticular activating system consists of nuclei in the brainstem reticular formation plus fibers that conduct sensory information to the nuclei and fibers that conduct from the nuclei to widespread areas of the cerebral cortex. Functioning of the reticular activating system is essential for consciousness. 28 Mosby items and derived items © 2006 by Mosby, Inc. Figure 15-15. Examples of Somatic Motor and Sensory Pathways. (A) Motor tracts. The pyramidal pathway through the lateral corticospinal tract and the extrapyramidal pathways through the rubrospinal, reticulospinal, and vestibulospinal tracts. Note that the pathways from the motor cortex cross over to the opposite side of the body, demonstrating contralateral control. (B) Sensory tracts. 1, The dorsal column-medial lemniscal pathway for transmitting critical types of tactile signals: touch/proprioception. Note the lateral corticospinal tract decussation; the point where it crosses to the other side is in the lower medulla. 2, Anterior and lateral divisions of the 29 anterolateral spinothalamic sensory tract: pain/temperature. Note the decussation is in the spinal cord.  Spinal Cord Lies within the vertebral canal and is protected by the vertebral column; continues from the medulla oblongata and ends at the conus medullaris (level of the first and second lumbar vertebra in adults) Spinal nerves extend from the conus medullaris and form a nerve bundle called the cauda equina Transmits long motor and sensory tracts that originate in the brain and synapse with cell bodies in gray matter of the spinal cord before exiting to the body Conducts somatic and autonomic reflexes and modulates sensory and motor function 30 31 Reflex Arcs  Form basic units that respond to stimuli and provide protective circuitry for motor output  The motor effects of reflex arcs generally occur before the event is perceived in the brain’s higher centers  Much internal environmental regulation is mediated by reflex activity involving the ANS (cardiac and smooth muscle contraction/relaxation and glandular responses) 32 Mosby items and derived items © 2006 by Mosby, Inc.  Reflex Arc Components  Receptor  Afferent (sensory) neuron  Interneuron  Efferent (motor) neuron  Effector muscle or gland Upper and Lower Motor Neurons Upper motor neurons (completely contained within the CNS) Efferent pathways primarily relaying information from the cerebrum to the brain stem or spinal cord Synapse with interneurons which then synapse with lower motor neurons Control fine motor movement and influence/modify reflex arcs and circuits Lower motor neurons (extend into the periphery) Have direct influence on muscles Cell bodies originate in the gray matter of the spinal cord, but their axons extend out of the CNS into the PNS, terminating at the neuromuscular junction 34 Mosby items and derived items © 2006 by Mosby, Inc. Neuromuscular Junction Figure 15-14. Normal Neuromuscular Junction. This figure shows how the distal end of a motor neuron fiber forms a synapse, or “chemical junction,” with an adjacent muscle fiber. Neurotransmitters (specifically, acetylcholine [ACh]) are released from the neuron's synaptic vesicles and diffuse across the synaptic cleft. There they stimulate receptors in the motor end-plate region of the sarcolemma  Protective Structures  Galea aponeurotica - thick band of fibrous tissue overlying the cranium  Subgaleal space - has venous connections with the dural sinuses (If there is increased intracranial pressure, blood can be shunted to the space, thus reducing pressure in the intracranial cavity; the subgaleal space is also a common site for wound drains after intracranial surgery)  Cranium - eight bones  Meninges x 3  Vertebral Column  Cerebral spinal fluid and the ventricular system 36 Mosby items and derived items © 2006 by Mosby, Inc. Meninges CSF and the Ventricular System CSF is a clear, colorless fluid similar to blood plasma and interstitial fluid (see Table 15.4); protects intracranial and spinal cord structures from jolts and blows 125 to 150 ml circulating at any given time (approx. 600 ml produced daily) within the ventricles and subarachnoid space; formed continuously but does not accumulate Produced by the ependymal cells lining the choroid plexuses in the lateral, third, and fourth ventricles Tight junctions of the choroid blood vessels provide a limiting barrier between the CSF and blood that functions similarly to the blood- brain barrier CSF is reabsorbed into the venous circulatory system through the arachnoid villi which act as one-way valves directing CSF outflow into the blood Exerts pressure within the brain and the spinal cord Lumbar puncture, spinal anesthesia procedures provide access to CSF 38 Figure 15-16. Flow of Cerebrospinal Fluid and Meninges of the Brain. (A) Ventricles highlighted in blue within a translucent brain in a left lateral view. (B) Flow of cerebral spinal fluid. The fluid produced by filtration of blood by the choroid plexus of each ventricle flows inferiorly through the lateral ventricles, interventricular foramen, third ventricle, cerebral aqueduct, fourth ventricle, and subarachnoid 39 space to the blood. (C) Meninges of the brain in relation to cerebrospinal fluid and venous blood flow Protective Structure of the Spinal Cord 33 vertebrae 7cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal Intervertebral disks Nucleus pulposus Functions to absorb shocks, preventing damage to the vertebrae Common source of back problems See Fig. 15-18 Figure 15-17. Vertebral Column. (A) The normal curves and regions of the vertebral column. The vertebrae in each 40 region are numbered. (B) Lateral view of several vertebrae Mosby items and derived items © 2006 by Mosby, Inc. showing how they articulate. Blood Supply to the Brain  800 to 1000 mL per minute or 20% of CO  CO2 is the primary regulator for CNS blood flow; potent vasodilator in the CNS and its effects ensure an adequate blood supply  Paired carotid and vertebral arteries supply blood to the brain and connect to form the circle of Willis  Drainage from the brain is accomplished through the venous sinuses and jugular veins 41  Arterial Circle of Willis  Provides an alternative route for blood flow when one of the contributing arteries is obstructed  Formed by the posterior cerebral arteries, posterior communicating arteries, internal carotid arteries, anterior cerebral arteries, and anterior communicating arteries Figure 15-20. Arteries at the Base of the Brain. (A) View of the arteries at the base of the brain. (B) Circle of Willis. The arteries that compose the circle of Willis are the two anterior cerebral arteries, joined to each other by the anterior communicating artery and two short segments 42 of the internal carotids, off of which the posterior communicating arteries connect to the posterior cerebral arteries. Figure 15-21. Areas of the Brain Affected by Occlusion of the Anterior, Middle, and Posterior Cerebral Artery Branches. ACA, Gray area affected by occlusion of branches of anterior cerebral artery; MCA, pink area affected by occlusion of branches of middle cerebral artery; PCA, orange area affected by occlusion of branches of posterior cerebral artery. Occlusions can occur in the cortical or deep areas of the border zone. 43 Blood Supply to the Brain  Drainage from the brain is accomplished through the venous sinuses and jugular veins  Cerebral venous drainage does not parallel its arterial supply – these veins are classified as superficial and deep and drain into venous plexuses and dural sinuses and eventually join the jugular veins at the skull base  Adequacy of venous outflow can significantly affect intracranial pressure 44  Blood Brain Barrier  Describes cellular structures that selectively inhibit certain potentially harmful substances in the blood from entering the interstitial spaces of the brain or CSF  Endothelial cells in brain capillaries (with intracellular tight junctions) are the site of the BBB; supporting cells include astrocytes, pericytes, and microglia  Some substances, including glucose, lipid-soluble molecules, electrolytes, and chemicals, can cross into and out of the brain Figure 15-23. Blood-Brain Barrier. facilitated by transport molecules (A) Cellular structure of brain capillary.  Has implications for drug therapy Endothelial cell membranes with tight junctions create a physical barrier between capillary blood and the brain, restricting movement of Predict what could happen if the BBB bacteria or neurotoxic substances. The pia 45 mater is present only in larger vessels. breaks down? Peripheral Nervous System 31 pairs of spinal nerves Names correlate with the vertebral level from which they exit Mixed nerves containing sensory and motor neurons 12 pairs of cranial nerves Sensory, motor, and mixed Animation: Cranial Nerves 46 Mosby items and derived items © 2006 by Mosby, Inc. Peripheral Nervous System Cranial and spinal nerves, including their branches and ganglia, constitute the PNS Spinal nerves originate in the spinal cord Cranial nerves originate in the brain and pass out of the skull A peripheral nerve is composed of individual axons/dendrites, with most wrapped in a myelin sheath; these individual fibers are arranged in bundles called fascicles (Fig. 15.25B) Coverings provide structural support, a blood supply, and interstitial compartments necessary to deliver essential electrolytes to support nerve impulse conduction. 47 Mosby items and derived items © 2006 by Mosby, Inc. Figure 15-25. Cranial and Peripheral Nerves and Skin Dermatomes. (A) Ventral surface of the brain showing attachment of the cranial nerves. The red lines indicate motor function, and the blue lines indicate sensory function. (B) Peripheral nerve trunk and coverings. (C) Dermatome map, 48 anterolateral view (left), and posterolateral view (right). Autonomic Nervous System (ANS)  Coordinates and maintains a steady state among the body organs and regulates cardiac muscle, smooth muscle and some glands  Components are located both the PNS and CNS; however, the ANS is considered to be part of the efferent division of the PNS  Two divisions – effects are usually antagonistic Sympathetic nervous system Parasympathetic nervous system 49 Sympathetic Nervous System  Mobilizes energy stores in times of need  Predominates during emergency “fight or flight’ reactions and physical exercise  Receives innervation from cell bodies located from the first thoracic through the second lumbar; sometimes referred to as the thoracolumbar division (T1-L2)  Paravertebral sympathetic chain ganglia or collateral ganglia  Short preganglionic fibers and long adrenergic postganglionic fibers; ratio of preganglionic fibers to postganglionic fibers is 1:20  Divergence coordinates activity of neurons at multiple levels of the spinal cord  Activity often involves mass discharge of the entire system  Primary neurotransmitter of postganglionic neuron is norepinephrine 50 Mosby items and derived items © 2006 by Mosby, Inc. Parasympathetic Nervous System  Functions to conserve and restore energy  Predominates during quiet resting conditions  Originate in the brainstem (CNs III, VII, IX, and X) and sacral region of spinal cord (S2-S4)  Long cholinergic preganglionic fibers and short cholinergic post- ganglionic fibers; ration of preganglionic to postganglionic fibers is 3:1  Limited divergence  Activity normally to discreet organs  Primary neurotransmitter of postganglionic neurons is acetylcholine 51 Mosby items and derived items © 2006 by Mosby, Inc.  52 Figure 15-26. Sympathetic and Parasympathetic Divisions of the Autonomic Nervous System  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  Acetylcholine and cholinergic receptors Animation: Autonomic Neurotransmitters 53 55 Geriatric Considerations: Aging and the Nervous System  CNS mechanisms involved in the aging brain are complex and many questions are yet to be answered concerning the neurologic effects of aging  Ambiguous distinctions between mechanisms of normal aging and those that are pathologic 57 Mosby items and derived items © 2006 by Mosby, Inc. Geriatric Considerations: Aging and the Nervous System Structural Changes with Aging Decreased brain weight and size, particularly frontal regions Increase in ventricular volume Fibrosis and thickening of the meninges Narrowing of gyri and widening of sulci Increase in size of ventricles Cerebrovascular Changes with Aging Arterial atherosclerosis (may cause infarcts and scars) Increased permeability of blood-brain barrier Decreased vascular density 58 Mosby items and derived items © 2006 by Mosby, Inc.  Geriatric Considerations: Aging and the Nervous System Cellular Changes with Aging Decrease in number of neurons not consistently related to changes in mental function Decreased myelin Lipofuscin deposition (a pigment resulting from cellular autodigestion) Decreased number of dendritic processes and synaptic connections Intracellular neurofibrillary tangles; significant accumulation in cortex associated with Alzheimer dementia Imbalance in amount and distribution of neurotransmitters Decrease in glucose metabolism 59 Mosby items and derived items © 2006 by Mosby, Inc.  Geriatric Considerations: Aging and the Nervous System Functional Changes with Aging Decreased tendon reflexes Progressive deficit in taste and smell Decreased vibratory sense Decrease in accommodation and color vision Decrease in neuromuscular control with change in gait and posture Sleep disturbances Memory impairments Cognitive alterations associated with chronic disease Functional changes and nervous system aging have significant individual variation 60 Mosby items and derived items © 2006 by Mosby, Inc.