Download - Chapter 14, F 09
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Human Anatomy, Second EditionMcKinley & O'Loughlin
Chapter 14 Lecture Outline:Nervous Tissue
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The Nervous System The primary communication
and control system of the body.
Fast and homeostatic It can be divided into structural
and functional categories.
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Organization of the Nervous System
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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)
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Functional Organization: Sensory and Motor Nervous Systems
Three general functions Collecting information Processing and evaluating
information Responding to information
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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).
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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
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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
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Neurons Neurons have a high
metabolic rate. Neurons have extreme
longevity. Neurons typically are non-
mitotic.
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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
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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.
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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.
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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
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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.)
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Neuron Classification Neurons vary widely in morphology
and location. They can be classified according to
either their structure or their function.
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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
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Functional Classification Sensory neurons Interneurons or association
neurons Motor neurons
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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
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Functions of Glial Cells Form a structural network. Replace damaged neurons. Assist neuronal development.
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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.
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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
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Glial Cells of the CNS Ependymal cells line the
ventricles and spinal cord and produce and move CSF
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Glial Cells of the CNS Microglial cells are brain
macrophages Rarest and smallest of the glial
cells
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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
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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
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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)
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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
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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.
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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.
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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
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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
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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
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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
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Causes of Demyelination Heavy metal poisoning Diptheria Multiple sclerosis Guillain-Barre syndrome
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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
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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.
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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
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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.
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Synaptic Communication
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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
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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
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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.
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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.
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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.
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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)
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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
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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.
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Nervous System Disorders Amyotrophic lateral sclerosis Multiple sclerosis Parkinson disease Guillain-Barre syndrome
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Neural Tube Defects Serious developmental deformities
of the brain, spinal cord, and meninges
Anencephaly Spina bifida
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Development of the Nervous System Begins in the third week in the
embryo from the ectoderm.