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Chapter 14, F 09 Chapter 14, F 09 Presentation Transcript

  • Human Anatomy, Second Edition McKinley & O'Loughlin Chapter 14 Lecture Outline: Nervous Tissue
  • The Nervous System
    • The primary communication and control system of the body .
    • Fast and homeostatic
    • It can be divided into structural and functional categories.
  • Organization of the Nervous System 14-
  • Structural Organization
    • Central nervous system (CNS)
      • brain and spinal cord
    • Peripheral nervous system (PNS)
      • cranial nerves (to and from the brain)
      • spinal nerves (to and from the spinal cord)
      • ganglia (clusters of neuron cell bodies located outside the CNS)
  • Functional Organization: Sensory and Motor Nervous Systems
    • Three general functions
      • Collecting information
      • Processing and evaluating information
      • Responding to information
  • Sensory Division
    • Somatic sensory components are the general somatic senses—touch, pain, pressure, vibration, temperature, and proprioception .
    • Visceral sensory components transmit nerve impulses from blood vessels and viscera to the CNS. The visceral senses primarily include temperature and stretch (of the organ wall).
  • Motor Division
    • The somatic motor component (somatic nervous system; SNS) conducts nerve impulses from the CNS to skeletal muscles.
      • also known as the voluntary nervous system
    • The autonomic motor component (autonomic nervous system; ANS) innervates internal organs, regulates smooth muscle, cardiac muscle, and glands.
      • also known as the visceral motor system or involuntary nervous system
  • Cytology of Nervous Tissue
    • Two distinct cell types form nervous tissue.
      • Neurons are excitable cells that initiate and transmit nerve impulses. Structural units of the nervous system.
      • Glial cells are nonexcitable cells that support and protect the neurons
  • Neurons
    • Neurons have a high metabolic rate.
    • Neurons have extreme longevity.
    • Neurons typically are non-mitotic.
  • New Neurons in Adults?
    • A population of immature progenitor cells in the hippocampus known as neural stem cells can under certain circumstances mature into neurons
  • Neuron Structure
    • Neurons come in all shapes and sizes , but all neurons share certain basic structural features.
      • Some are tiny, less than 1 mm in the CNS
      • Some are some of the largest cells in the body
    • A typical neuron has a cell body, dendrites, and axons.
  • Neuron Structure – Dendrites
    • Dendrites tend to be shorter, smaller processes that branch off the cell body.
    • Some neurons have only one dendrite, while others have many (100’s often).
    • Dendrites conduct nerve impulses toward the cell body; they receive input and then transfer it to the cell body for processing.
    • The more dendrites a neuron has, the more nerve impulses that neuron can receive from other cells.
  • Neuron Structure – Cell Body
    • The cell body serves as the neuron’s control center and is responsible for receiving, integrating, and sending nerve impulses.
    • Has most of the organelles found in other cells, but no centrioles (no mitosis or regeneration)
    • Chromatophilic substance, Nissl bodies - rough ER and ribosomes
    • Lipofuscin inclusions - probably lysosomal wastes. Yellowish-brown. Probably harmless.
    • Neurofibrils and intermediate filaments form cytoskeleton
  • Neuron Structure – Axon
    • The larger, typically longer nerve cell process emanating from the cell body is the axon, sometimes called a nerve fiber.
    • Most neurons have one axon, but no more than one. May have collaterals.
    • The axon transmits a nerve impulse away from the cell body toward another cell.
    • May be short, absent, or long (3-4 feet)
    • Axon originates at the axon hillock and then tapers. The nerve impulse, action potential, starts at the first part, the initial segment.
    • Branches a lot at the end usually, axon terminals, telodendria. May be over 10,000 . Ends at synaptic knobs (end bulbs, etc.)
  • Neuron Classification
    • Neurons vary widely in morphology and location.
    • They can be classified according to either their structure or their function.
  • Structural Classification
    • By the number of processes extending from the cell body .
      • Unipolar
        • PNS mainly: sensory neurons
      • Bipolar
        • Rare: sense organs’ receptor cells in the eye, ear, and nose
      • Multipolar
        • Most common in humans
        • Major type in CNS
        • Most motor neurons and association neurons
  • Functional Classification
    • Sensory neurons
    • Interneurons or association neurons
    • Motor neurons
  • Interneurons
    • Interneurons, or association neurons, lie entirely within the CNS and are multipolar .
    • They receive nerve impulses from many other neurons and carry out the integrative function of the nervous system.
    • Thus, interneurons facilitate communication between sensory and motor neurons .
    • Thousands of types
      • Purkinje cells in the cerebellum
      • Renshaw cells in the spinal cord
      • Pyramidal cells in the cerebral cortex
  • Functions of Glial Cells
    • Form a structural network.
    • Replace damaged neurons.
    • Assist neuronal development.
  • Glial Cells (Neuroglia)
    • Occur within both the CNS and the PNS.
      • 4 of the 6 types are within the CNS.
    • Smaller and capable of mitosis.
    • Do not transmit nerve impulses.
    • Physically protect and help nourish neurons, and provide an organized, supporting framework for all the nervous tissue.
    • Far outnumber neurons (about 10x).
    • Account for roughly half the volume of the nervous system.
    • Almost 1/2 of brain tumors are gliomas.
    • Astrocytes exhibit a starlike shape due to projections from their surface.
    • Astrocytes are the most abundant and largest glial cells in the CNS, and they constitute over 90% of the tissue in some areas of the brain.
      • Help from the blood-brain barrier (BBB) that strictly controls substances entering the nervous tissue in the brain from the bloodstream.
      • Regulate tissue fluid composition (ionic balance).
      • Forming a structural network.
      • Replacing damaged neurons.
      • Assisting neuronal development.
    Glial Cells of the CNS 14-
  • Glial Cells of the CNS
    • Ependymal cells line the ventricles and spinal cord and produce and move CSF
  • Glial Cells of the CNS
    • Microglial cells are brain macrophages
    • Rarest and smallest of the glial cells
  • Glial Cells of the CNS
    • Oligodendrocytes myelinate fibers in the CNS
    • One oligodendrocyte may help myelinate 60 axons
    • Cell body not involved in wrapping (like an octopus)
    • No neurilemma
    • Large nodes of Ranvier
  • Glial Cells of the PNS
    • Satellite cells are found in the ganglia around the cell bodies.
    • Physically separate cell bodies from the surrounding interstitial fluid and regulate exchanges
  • Glial Cells of the PNS
    • Neurolemmocytes or Schwann cells myelinate in the PNS
    • Have neurilemma around myelin sheath
    • Many Schwann cells/axon
    • Smaller nodes of Ranvier (neurofibril nodes)
  • Myelination of Axons
    • Main activity of axons is nerve impulse conduction.
    • Nerve impulse (action potential) is the rapid movement of an electrical charge along a neuron’s plasma membrane.
      • A voltage (potential) change that moves along the plasma membrane
      • The ability to travel along an axon is affected by myelination
  • Myelination
    • Myelination is the process by which part of an axon is wrapped with a myelin sheath, a protective fatty coating that gives it glossy-white appearance ( white matter ).
    • The myelin sheath supports, protects, and insulates an axon.
    • No change in voltage can occur across the membrane in the insulated portion of an axon.
  • Nerve Impulse Conduction
    • In a myelinated axon, the nerve impulse “jumps” from neurofibril node to neurofibril node and is called saltatory conduction .
    • In an unmyelinated axon, the nerve impulse must travel the entire length of the axon and is called continuous conduction and is slower. Pain stimuli carried this way.
    • A myelinated axon produces a faster nerve impulse and it requires less energy.
  • Development of the Myelin Sheath
    • Starts during late fetal development and first year
    • Increases to maturity
    • Increase in conduction rate with increase myelination
    • Explains why infants are slower and less coordinated in their responses
  • Regeneration of PNS Axons
    • PNS axons are vulnerable to cuts, crushing injuries, and other trauma.
    • A damaged axon can regenerate, however, if at least some neurilemma remains.
    • PNS axon regeneration depends upon three factors.
      • the amount of damage
      • neurolemmocyte secretion of nerve growth factors to stimulate outgrowth of severed axons
      • the distance between the site of the damaged axon and the effector organ
  • Regeneration of CNS Axons
    • Very limited
    • Oligodendrocoytes do not release a nerve growth factor
    • Large number of axons crowded together
    • Astrocytes and connective tissue coverings may form some scar tissue that obstructs axon regrowth
    • No neurilemma to act as a regeneration tube
  • Demyelination
    • Progressive destruction of myelin sheath is accompanied by inflammation, axon damage, and scarring of neural tissue
    • Usually results in a gradual loss of sensation and motor control
  • Causes of Demyelination
    • Heavy metal poisoning
    • Diptheria
    • Multiple sclerosis
    • Guillain-Barre syndrome
  • Nerves
    • A nerve is a bundle of parallel axons .
    • Like a muscle, a nerve has three successive connective tissue wrappings.
      • endoneurium - a delicate layer of loose connective tissue
      • perineurium - a cellular and fibrous connective tissue layer that wraps groups of axons into bundles called fascicles
      • epineurium - a superficial connective tissue covering
        • A thick layer of dense irregular fibrous connective tissue encloses the entire nerve, providing both support and protection
  • Nerves
    • Nerves are a component of the peripheral nervous system.
    • Sensory (afferent) nerves convey sensory information to the CNS .
    • Motor (efferent) nerves convey motor impulses from the CNS to the muscles and glands.
    • Axons terminate as they contact other neurons, muscle cells, or gland cells.
    • An axon transmits a nerve impulse at a specialized junction called a synapse .
  • Nerve Regeneration and Spinal Cord Injuries
    • Spinal injuries at the neck used to be fatal because of respiratory failure
    • Much better survival now from early steroid use and antibiotics
    • Reconnection and partial restoration of function of severed cords in rats
      • Olfactory nerve bridge spans injured area and acts as a guide (not in humans yet)
      • Neural stem cells may be able to regenerate CNS axons
    • Christopher Reeve, paralyzed below the second cervical vertebra, made tremendous progress and raised funds and awareness
  • Synapses
    • Presynaptic neurons transmit nerve impulses along their axonal membranes toward a synapse.
    • Postsynaptic neurons conduct nerve impulses through their dendritic and cell body membranes away from the synapse.
    • Axons may establish synaptic contacts with any portion of the surface of another neuron, except those regions that are myelinated.
  • Synaptic Communication 14-
  • Electrical Synapses
    • Electrical synapses are not very common in mammals.
    • In humans, these synapses occur primarily between smooth muscle cells where quick, uniform innervation is essential.
    • Electrical synapses are also located in cardiac muscle and the developing embryo.
    • Use gap junctions .
    • Advantages
      • Faster communication
      • Can synchronize the activity of a group of neurons or muscle fibers
  • Chemical Synapses
    • The most numerous type of synapse is the chemical synapse.
    • It facilitates most of the interactions between neurons and all communications between neurons and effectors.
    • At these junctions, the presynaptic membrane releases a signaling molecule called a neurotransmitter , such as acetylcholine (ACh), into the synaptic cleft.
    • Other types of neurons use other neurotransmitters.
    • Synaptic delay of about 0.5 msec
  • Neurotransmitters
    • Are released only from the plasma membrane of the presynaptic cell.
    • It then binds to receptor proteins found only on the plasma membrane of the postsynaptic cell.
    • A unidirectional flow of information and communication takes place.
    • Two factors influence the rate of conduction of the impulse: the axon’s diameter and the presence (or absence) of a myelin sheath.
  • Graded Potentials
    • Graded potentials occur on the dendrites and cell body (no polarity reversal, a local depolarization)
    • Spreads from the receptive zone to the axon hillock (the trigger zone). It decreases in strength as it travels.
    • If sufficient graded potentials at the initial segment, then it will initiate an action potential (all or none) if the voltage threshold is exceeded.
  • Synapses
    • The typical CNS neuron receives input from 1,000 to 10,000 synapses!
    • Synapses may be excitatory and/or inhibitory.
    • Integration of inputs on post-synaptic neuron can result in:
      • 1 or more nerve impulses if the threshold is reached or surpassed
      • No impulse if inhibitory effects are greater than the excitatory effects
      • Facilitated: if subthreshold stimulation. More easily can generate an impulse.
  • Synapses
    • The neurotransmitter affects the post-synaptic neuron as long as it remains in the synaptic cleft.
    • Three ways to remove
      • Diffusion
      • Destroyed by enzymes
      • Reuptake into presynaptic neuron or transport to neighboring neuron
        • Example, Prozac is a selective serotonin reuptake inhibitor (SSRI)
  • Synapses
    • Synapses are essential for homeostasis by transmitting certain impulses and inhibiting others.
    • Often brain and psychiatric disorders result from disruption of synaptic communication.
    • Also site for most drugs that affect brain, therapeutic and addictive
  • Neural Integration and Neuronal Pools
    • Billions of interneurons within the CNS are grouped in complex patterns called neuronal pools (or neuronal circuits or pathways).
    • Neuronal pools are defined based upon function , not anatomy, into four types of circuits:
      • converging
      • diverging
      • reverberating
      • parallel-after-discharge
    • A pool may be localized, or its neurons may be distributed in several different regions of the CNS.
  • Nervous System Disorders
    • Amyotrophic lateral sclerosis
    • Multiple sclerosis
    • Parkinson disease
    • Guillain-Barre syndrome
  • Neural Tube Defects
    • Serious developmental deformities of the brain, spinal cord, and meninges
    • Anencephaly
    • Spina bifida
  • Development of the Nervous System
    • Begins in the third week in the embryo from the ectoderm.