3.12 Anatomy Chapter 12

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3.12 Anatomy Chapter 12
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2010-10-19 04:46:35
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Fundamentals Nervous System Nervous Tissue
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Functional organization, tissues, gross anatomy, and disorders of the nervous system, neuronal integration
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  1. Functions of the Nervous Sustem
    • 1. Sensory Input
    • 2. Integration
    • 3. Motor Output
  2. Basic Divisions of the Nervous System
    Central Nervous System Overview
    • Central Nervous System (CNS)
    • Brain and Spinal Cord

    Integrating and command center to the nervous system; receives incoming signals, interprets them and dictates motor responses based on past experiences, reflexes, and current conditions
  3. Basic Divisions of the Nervous System
    Peripheral Nervous System Overview
    • Peripheral Nervous System (PNS)
    • outside the CNS; mainly consists of cranial and spinal nerves
    • cranial nerves: carry signals to/from the brain
    • spinal nerves: carry signals to/from the spinal cord
    • *links all regions of the body to the CNS
    • ganglia: areas where cell bodies of neurons are clustered
  4. Peripheral Nervous System
    Types of signals and body regions
    • Signal Types:
    • 1. Sensory (afferent) signals: receptors located throughout the body and carried by nerve fibers of the PNS into the CNS afferent = "towards
    • 2. Motor (efferent) signals: carried away from the CNS by PNS nerve fibers to innervate muscles and glands

    • Body Regions:
    • 1. Somatic: structures external to the ventral body cavity (skin, skeletal musculature, bones)
    • 2. Visceral: contains viscera within the ventral body cavity (digestive tube, lungs, heart, bladder, etc.)
  5. Types of Fibers in the Peripheral Nervous System
    Somatic Sensory (SS)
    • Somatic Sensory
    • General Somatic Senses: senses whose receptors are spread widely throughout the outer tube (general = widespread); include touch, pain, pressure, vibration, and temperature
    • - proprioception: "sensing one's own body"; detects amount of stretch in muscles, tendons, joint capsules

    Special Somatic Senses: somatic senses whose receptors are confined to relatively small areas (special = localized); mostly in the head; include hearing, balance (equilibrium) from inner ear receptors, and vision
  6. Types of Fibers in the Peripheral Nervous System
    Visceral Sensory (VS)
    Visceral Sensory

    General Visceral Senses: include stretch, pain, and temperature; felt widely in the digestive/ urinary tracts, reproductive organs, etc. include hunger and nausea

    Special Visceral Senses: aka chemical senses; have receptors localized in tongue and nasal cavity
  7. Types of Fibers in the Peripheral Nervous System
    Somatic Motor (SM)
    • Somatic Motor
    • Stimulates contraction of the skeletal muscles

    "Voluntary Nervous System"
  8. Types of Fibers in the Peripheral Nervous System
    Visceral Motor (VM)
    • Visceral Motor
    • Regulates the contraction of smooth and cardiac muscle and secretion by the body's many glands;
    • Make up the autonomic nervous system (ANS) that controls the function of visceral organs

    "Involuntary Nervous System"
  9. Nervous Tissue (2 types of cells)
    1. neurons: excitable nerve cells that transmit electrical signals

    2. neuroglia: nonexcitable supporting cells that surround and wrap neurons

    **both develop from same embryonic tissues: neural tube and neural crest
  10. The Neuron
    Overview
    Basic Structural Units of the Nervous System

    • Characteristics of Neurons
    • 1. Highly Specialized cells that conduct electrical signals (nerve impulses; action potentials)
    • 2. Extreme Longevity: neurons can live and function for a lifetime; over 100 years
    • 3. Do Not Divide: neurons lose their ability to undergo mitosis; cannot be replaced if destroyed (except neural stem cells in some areas of the CNS)
    • 4. High Metabolic Rate: requires continuous and abundant supplies of oxygen and glucose
  11. The Neuron
    Cell Body
    • Cell Body (Soma) 5-140um diameter
    • Contains usual cellular organelles, along with:
    • chromatophilic bodies: "color loving"; large clusters of rough endoplasmic reticulum and free ribosomes that stain darkly with basic dye; continually renew membranes of the cell and the protein part of the cytosol
    • neurofibrils: bundles of intermediate filaments (neurofilaments) that run in a network between the chromatophilic bodies; adds structural support
    • **most neuron cell bodies are located in the CNS; clusters of cell bodies = ganglia lie along nerves in the PNS
  12. The Neuron
    Neuron Processes
    Dendrites: processes that branch from the cell body like the limbs on a tree; contains most organelles in cell body; function as receptive sites, providing enlarged surface area for signal reception; conduct electrical signals toward the cell body

    Axons: a neuron only has one axon that arises from a cone-shaped region called the axon hillock; thin processes of uniform diameter; impulse generators and conductors that transmit nerve impulses away from the cell body; contain no chromatophilic bodies or golgi apparatus; lack ribosomes and all organelles involved in protein synthesis; must receive protein from cell body

    Neurofilaments, actin microfilaments, and microtubules are evident to provide structural support and aid in transportation of substances to and from cell body (axonal transport); axonal cytoplasm is continually renewed

    • nerve fiber: any long axon
    • *diameters vary; larger diameters conduct impulses faster (less resistance)
    • axon collaterals: perpendicular branches along axon lengths
    • terminal branches: branches at the terminus of an axon
    • axon terminals: knobby endings of terminal branches where neurotransmitters are released

    Nerve impulse: axon hillock --> axon --> axon terminals--> release NTs ==> excite/inhibit neurons or target organ
  13. The Neuron
    Synapses
    The site at which neurons communicate

    Mostly through chemical messengers; sometimes through electrical signals via gap junctions; Mostly unidirectional

    • presynaptic neuron: conducts signals toward synapse
    • postsynaptic neuron: transmits signals away from synapse
    • *most neurons in CNS function as both

    • axodendritic synapses: most neurons; a synapse that occurs between the axon terminals of one neurons and the dendrites of another neuron
    • axosomatic synapses: between axons and neuron cell bodies
    • presynaptic terminal contains synaptic vesicles (membrane-bound sacs filled with NTs) and mitochondria are abundant
    • synaptic cleft: separation between plasma membranes of pre and post synaptic neurons
  14. Classification of Neurons
    Structural Classification
    Structurally grouped according to the number of processes that extend from the cell body

    multipolar neurons: most neurons; have more than two processes; usually have numerous dendrites and a single axon (some have no axons)

    bipolar neurons: have two processes that extend from opposite sides of the cell body; occur in some special sensory organs ie. inner ear, olfactory, retina of the eye; mostly serve as sensory neurons

    • unipolar neurons: have a short, single process that emerges from the cell body and divides like a T into two long branches; most start out as bipolar neurons whose processes fuse together during development (pseudounipolar neurons); found in sensory ganglia in the PNS functioning as sensory neurons
    • - central process: branch that runs centrally into the CNS
    • - peripheral process: branch that extends peripherally to the receptors; no dendrites
    • **central process is definitely an axon; peripheral process is ambiguous, but referred to as axon
  15. Classification of Neurons
    Functional Classification
    Functionally grouped according to the direction of nerve impulse relative to the CNS

    sensory neurons: afferent neurons; make up sensory division of the PNS; transmit impulses toward the CNS from sensory receptors in the PNS; virtually all are psudounipolar, and their cell bodies are in ganglia outside the CNS: peripheral process extends from sensory receptor, central process terminates in the CNS

    motor neurons: efferent neurons; make up the motor division of the PNS; carries impulses away from the CNS to effector organs; multipolar, with cell bodies located in the CNS (except those of the ANS); junction with effector cells, stimulating muscle contraction or gland secretion

    interneurons: association neurons; lie between motor and sensory neurons; confined entirely to CNS; link together into chains that form complex neuronal pathways; multipolar
  16. Supporting Cells
    Neuroglia
    • Neuroglia or glial cells
    • six types of glial cells: four in the CNS, two in the PNS

    • Neuroglia in the CNS
    • (nuclei of neuroglia stain darker than neurons)
    • Outnumber neurons 10 to 1; make up ~half brain mass
    • *can divide throughout life

    • astrocytes: star-shaped; most abundant type in the CNS; have radiating processes with bulbous ends that cling to neurons or capillaries
    • functions:
    • 1. regulating NT levels by increasing rate of uptake
    • 2. signaling increased blood flow in active regions
    • 3. controlling the ionic environment around neurons
    • help synapses form in developing neural tissue; produce molecules necessary from neural growth; propagate calcium signals that may be involved in memory

    microglia: smallest and least abundant type in CNS; have elongated cell bodies and processes with many projections; are phagocytes the "macrophages of the CNS": migrate to and engulf invading microorganisms and injured/dead neurons; derived from blood cells (monocytes)

    Ependymal cells: form a simple epithelium lining the central cavity of the spinal cord and brain; bear cilia that help circulate the cerebrospinal fluid

    oligodendrocytes: have fewer branches than astrocytes; line up in small groups and wrap cell processes around thicker axons in the CNS, producing insulating myelin sheaths

    • Neuroglia in the PNS
    • satellite cells: surround neuron cell bodies within ganglia
    • schwann cells: surround all axons in the PNS, forming myelin sheaths
  17. Myelin Sheaths
    in the PNS and the CNS
    • produced by oligodendrocytes in the CNS and Schwann cells in the PNS
    • *segmented structures composed of lipoprotein myelin and consists of the plasma membrane of a supporting cell rolled in concentric layers around the axon
    • *prevents leakage of electrical current from the axon, increases speed of impulse conduction, makes impulse propagation more energy-efficient.

    • Myelin Sheaths in the PNS
    • formed by Schwann cells
    • develop during the fetal period and first year of life:
    • Schwann cells wrap repeatedly around an axon, loosely at first, then cytoplasm is squeezed outwards; many concentric laters ensheathe an axon; nucleus and cytoplasm end up external to the myelin layers, called neurilemma

    • nodes of Ranvier: gaps in the myelin sheaths where nerve impulses jump, skipping myelin sheaths
    • thinnest axons are unmyelinated

    • Myelin Sheaths in the CNS
    • formed by oligodendrocytes; have multiple processes that coil around several different axons
    • Nodes of Ranvier are more widely spaced than those in PNS; thinnest axons are unmyelinated, but covered by processes of glial cells like astrocytes.
  18. Gross Anatomy of the Nervous System: Overview
    Nerves
    • A nerve is a cablelike organ in the PNS that consists of many axons (nerve fibers) arranged in parallel bundles enclosed in connective tissue.
    • Within a nerve, each axon is surrounded by a Schwann cell.
    • Covering the Schwann cells is a delicate layer of loose connective tissue called endoneurium.
    • Fascicles are groups of axons bound into bundles
    • Perineurium refers to the connective tissue wrapping fascicles
    • Epineurium is a tough fibrous sheath surrounding the whole nerve
    • CT also contains blood vessels that nourish the axons and Schwann cells

    • **Terms to note:
    • a neuron is a nerve cell
    • a nerve fiber is a long axon
    • a nerve is a collection of nerve fibers in the PNS
  19. Gross Anatomy of the Nervous System: Overview
    Gray and White Matter of the CNS
    The brain and spinal cord have distinct regions of gray and white matter that reflect the arrangement of their neurons.

    Gray matter: gray colored zone that surrounds the hollow cavity of the CNS; butterfly shaped in the spinal cord: dorsal half contains cell bodies of interneurons, ventral half contains cell bodies of motor neurons; gray matter is the site where neuron cell bodies are clustered; also contains dendrites, short unmyelinated neurons and neuroglia.

    • White matter: contains no cell neuron cell bodies but millions of axons; white color comes from myelin sheaths around many of the axons; the axons run to/from the brain and spinal cord; white matter consists of axons running between different parts of the CNS; axons traveling to similar destinations form axon bundles called tracts
    • **white matter is generally external to the gray matter in the CNS, except in the cerebrum and cerebellum, there is an additional superficial layer of gray matter (cortex)
  20. Integration Between the PNS and CNS
    The PNS is composed of axons of sensory (afferent) and motor (efferent) neurons (nerve fibers) bundled together as nerves

    Afferent fibers respond to sensory stimuli and carry that information to the CNS; efferent fibers transmit motor stimuli from the CNS to muscles and glands.

    The CNS is composed of interneurons in the gray matter that 1. process sensory information, 2. direct this information to specific regions of the CNS, and 3. initiate the appropriate motor response, and axons in the white matter that 4. transport information (sensory and motor) from one region of the CNS to another.
  21. Reflex Arc
    Reflex arcs are simple chains of neurons that cause our simplest, reflexive behaviors and reflect the basic structural plan of the nervous system.

    Reflexes are rapid, automatic motor responses to stimuli that are unlearned, unpremeditated, and involuntary.

    • 5 Essential Components of a Reflex Arc:
    • 1. The receptor is the site where stimuli act; located at the terminal end of the peripheral process of a sensory neuron
    • 2. The sensory neuron transmits the afferent impulses to the CNS
    • 3. The integration center consists of one or more synapses in the gray matter of the CNS (can be just one synapse, or a long chain of interneurons with many synapses)
    • 4. The motor neuron conducts efferent impulses from the integration center to an effector
    • 5. The effector is the muscle or gland cell that responds to the impulses by contracting or secreting

    monosynaptic reflex: no interneuron between sensory neuron and motor neuron; only contains one synapse; simplest and fastest of reflexes, example is the "knee-jerk" reflex.

    polysynaptic reflex: more common; one or more interneurons involved; examples of three-neuron polysynaptic reflex (with just one interneuron) is the withdrawal reflex: where we pull away from danger
  22. Neuronal Circuits
    Neuronal Circuits occur when a single neuron synapses with many other neurons and interneurons are interconnected in many ways.

    Diverging circuits: one presynaptic neuron synapses with several other neurons; ex. stretch reflex; information is distributed through multiple neuronal pathways

    Converging circuits: when many neurons synapse on a single postsynaptic neuron; ex. when a single motor neuron receives both excitatory and inhibitory impulses

    Reverberating circuit: when one neuron in the circuit receives feedback from another neuron in the same circuit; involved in the control of many rhythmic activities like breathing and swinging arms when walking

    Serial processing: neurons that synapses one-on-one in a sequence; linked along a single pathway from one neuron to the next; ex. reflex arc

    Parallel processing: information from a single neuron is sent along two or more parallel pathways; when a single sensory stimulus results in multiple perceptions; ex. retina cells associate with neurons evaluating color, shape, spatial location, and movement, as well as recognition and memory
  23. Multiple Sclerosis and Neuronal Regeneration
    Multiple Sclerosis: A progressive disease that destroys patches of myelin in the brain and spinal cord, disrupting neuronal signals in the CNS and leading to sensory disorders and weakened musculature.

    Autoimmune: lymphocytes break down the myelin and macrophages consume the remains; accompanied by inflammation that can destroy the axons themselves

    • Neuronal Regeneration: no effective replacement of dead/damaged neurons after fetal period
    • Axons may regenerate if cell bodies are intact.

    ex. if nerve is severed in PNS, macrophages destroy distal axon; axon filaments can grow peripherally at 1.5mm a day in regeneration tubes formed by Schwann cells; reinnervation of the target organ with partial functional recovery is possible in the PNS

    neuroglia in the CNS never form bands to guide regrowing axons and even secrete inhibitory chemicals: no effective regeneration to brain/spinal cord injury

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