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Text Box: CHAPTER
12

Nervous Tissue   (download here)  More detailed notes here

Nervous System

      The master controlling and communicating system of the body

      Functions:

    Sensory input monitoring stimuli occurring inside and outside the body 

    Integration interpretation of sensory input

    Motor output response to stimuli by activating effector organs 

Organization of the Nervous System

      Central nervous system (CNS)

    Brain and spinal cord

    Integration and command center

      Peripheral nervous system (PNS)

    Paired spinal and cranial nerves

    Carries messages to and from the spinal cord and brain

Peripheral Nervous System (PNS): 
Two Functional Divisions

      Sensory (afferent) division

    Sensory afferent fibers carry impulses from skin, skeletal muscles, and joints to the brain

    Visceral afferent fibers transmit impulses from visceral organs to the brain

      Motor (efferent) division

    Transmits impulses from the CNS to effector organs

Motor Division: Two Main Parts

      Somatic nervous system

    Conscious control of skeletal muscles

      Autonomic nervous system (ANS)

    Regulate smooth muscle, cardiac muscle, and glands

    Divisions sympathetic and parasympathetic

Histology of Nerve Tissue

      The two principal cell types of the nervous system are:

    Neurons excitable cells that transmit electrical signals

    Supporting cells cells that surround and wrap neurons

Supporting Cells: Neuroglia

      The supporting cells (neuroglia or glia):

    Provide a supportive scaffolding for neurons

    Segregate and insulate neurons

    Guide young neurons to the proper connections

    Promote health and growth

Astrocytes

      Most abundant, versatile, and highly branched glial cells

      They cling to neurons and cover capillaries

      Functionally, they:

    Support and brace neurons

    Anchor neurons to their nutrient supplies

    Guide migration of young neurons

    Control the chemical environment

Microglia and Ependymal Cells

      Microglia small, ovoid cells with spiny processes

    Phagocytes that monitor the health of neurons

      Ependymal cells squamous- to columnar-shaped cells

    They line the central cavities of the brain and spinal column

Oligodendrocytes, Schwann Cells, 
and Satellite Cells

      Oligodendrocytes branched cells that wrap CNS nerve fibers

      Schwann cells (neurolemmocytes) surround fibers of the PNS

      Satellite cells surround neuron cell bodies with ganglia

Neurons (Nerve Cells)

      Structural units of the nervous system

    Composed of a body, axon, and dendrites

    Long-lived, amitotic, and have a high metabolic rate

      Their plasma membrane functions in:

    Electrical signaling

    Cell-to-cell signaling during development

Nerve Cell Body (Perikaryon or Soma)

      Contains the nucleus and a nucleolus

      Major biosynthetic center

      Focal point for the outgrowth of neuronal processes

      There are no centrioles (hence its amitotic nature)

      Well developed Nissl bodies (rough ER)

      Axon hillock cone-shaped area from which axons arise

Processes

      Armlike extensions from the soma

      Called tracts in the CNS and nerves in the PNS

      There are two types: axons and dendrites

Dendrites of Motor Neurons

      Short, tapering, and diffusely branched processes

      They are the receptive, or input, regions of the neuron

      Electrical signals are conveyed as graded potentials (not action potentials)

Axons: Structure

      Slender processes of uniform diameter arising from the hillock

      Long axons are called nerve fibers

      Usually there is only one unbranched axon per neuron

      Rare branches, if present, are called axon collaterals

      Axonal terminal branched terminus of an axon

Axons: Function

      Generate and transmit action potentials

      Secrete neurotransmitters from the axonal terminals

Myelin Sheath

      Whitish, fatty (protein-lipid), segmented sheath around most long axons

      It functions in:

    Protection of the axon

    Electrically insulating fibers from one another

    Increasing the speed of nerve impulse transmission

Myelin Sheath and Neurilemma: Formation

      Formed by Schwann cells in the PNS

      A Schwann cell:

    Envelopes an axon in a trough

    Encloses the axon with its plasma membrane

    Concentric layers of membrane make up the myelin sheath

      Neurilemma remaining nucleus and cytoplasm of a Schwann cell

Nodes of Ranvier (Neurofibral Nodes)

      Gaps in the myelin sheath between adjacent Schwann cells

      They are the sites where collaterals can emerge

Unmyelinated Axons

      A Schwann cell surrounds nerve fibers but coiling does not take place

      Schwann cells partially enclose 15 or more axons

Axons of the CNS

      Both myelinated and unmyelinated fibers are present

      Myelin sheaths are formed by oligodendrocytes

      Nodes of Ranvier are widely spaced

      There is no neurilemma

Regions of the Brain and Spinal Cord

      White matter dense collections of myelinated fibers

      Gray matter mostly soma and unmyelinated fibers

Neuron Classification

      Structural: 

    Multipolar

    Bipolar

    Unipolar

      Functional: 

    Sensory (afferent)

    Motor (efferent)

    Interneurons (association neurons)

Membrane Potentials: Signals

      Used to integrate, send, and receive information

      Membrane potential changes are produced by:

    Changes in membrane permeability to ions

    Alterations of ion concentrations across the membrane

      Types of signals graded potentials and action potentials

Action Potentials (APs)

      A brief reversal of membrane potential with a total amplitude of 100 mV

      Action potentials are only generated by muscle cells and neurons

      They do not decrease in strength over distance

      They are the principal means of neural communication

      An action potential in the axon of a neuron is a nerve impulse

Synapses

      A junction that mediates information transfer from one neuron:

    To another neuron

    To an effector cell

      Presynaptic neuron conducts impulses toward the synapse

      Postsynaptic neuron transmits impulses away from the synapse

Chemical Synapses

      Specialized for the release and reception of neurotransmitters

      Typically composed of two parts:

    Axonal terminal of the presynaptic neuron, which contains synaptic vesicles

    Receptor region on the dendrite(s) or soma of the postsynaptic neuron

Synaptic Cleft

      Fluid-filled space separating the presynaptic and postsynaptic neurons

      Prevent nerve impulses from directly passing from one neuron to the next

      Transmission across the synaptic cleft:

    Is a chemical event (as opposed to an electrical one)

    Ensures unidirectional communication between neurons

Synaptic Cleft: Information Transfer

      Nerve impulse reaches axonal terminal of the presynaptic neuron

      Neurotransmitter is released into the synaptic cleft

      Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron

      Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect

Neurotransmitters

      Chemicals used for neuronal communication with the body and the brain

      50 different neurotransmitter have been identified

      Classified chemically and functionally

 

 
This is your video Lesson for the Nervous System Introduction

Please download this worksheet to discuss in class

Study Guide:

Nervous Tissue

  1. More precise Study Guide:
  2. Know the general architecture of the nervous system (chart below)
  3. Know the anatomy of a neuron
  4. Know the location and functions of the glial cells
  5. Know the difference between the CNS and PNS
  6. Know how information travels between neurons
  7. Describe a synapse

CNS

  1. Describe the structural features which protect the brain and spinal cord
  2. What is CSF, what is it's function
  3. What is the location and very general functions of the medulla oblongata, pons, thalamus, corpora quadrigemina, cerebelllum, hypothalamus
  4. Describe the surface anatomy and very general functions of the areas on the surface of the cerebral cortex and cerebellum
  5. What are white matter, gray matter, nucleus, ganglia, tract, nerve
  6. Know the anatomy of the spinal cord
  7. Describe a reflex arc

PNS

  1. Describe the name, number and effector organ(s) of CN I-XII
  2. Describe the brachial, lumbar and sacral plexuses and the MAJOR nerves emanating from them

ANS

  1. Understand the structural and functional differences between the ANS and somatic efferent systems
  2. Understand what dual innervation is and it's function
  3. Explain the structural and functional differences between the sympathetic and parasympathetic divisions

Special Senses

  1. Describe what stimuli the following receptors respond to and what sensory systems incorporate each type:  mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors and photoreceptors
  2. Describe the structure and location of taste buds
  3. Describe the structure and function of the eyeball including cornea, ciliary body, retina, lacriminal system, sclera, ciliary, focea centralis, suspensory ligaments, choroid, lens, optic disc
  4. What are rods and cones?
  5. Describe the anatomy of the ear including:  the auricle, ossicles, oval and round windows, acoustic canal, bony labyrinth, tympanic membrane and auditory tube
  6. Describe the anatomy, location and function of the organ of Corti including: scala vestibule, scala tympani, scala edia (cochlear duct), basilar membrane, techtorial membrane and hair cells
  7. Describe the anatomy , location and function of the otolith organs (maculae)
  8. Describe the anatomy, location and function of the semicircular canals and the cristae ampullares

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