Transcript Nervous Tissue - Chiropractor Manhattan | Chiropractor New

Nervous Tissue
Dr. Michael P. Gillespie
Structures of the Nervous
System
 Brain
 Spinal cord
 Nerves
 Cranial nerves
 Ganglia
 Sensory receptors
Functions of the Nervous
System
 Sensory function – afferent neurons
 Integrative function - interneurons
 Motor function – efferent neurons
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The cells contacted by these neurons are called
effectors
Organization of the Nervous
System
 Central nervous system
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Brain
Spinal cord
Organization of the Nervous
System
 Peripheral nervous system
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Cranial nerves and their branches
Spinal nerves and their branches
Ganglia
Sensory receptors
Somatic nervous system
Autonomic nervous system
Enteric nervous system
Somatic Nervous System (SNS)
 Sensory neurons.
 Motor neurons located in skeletal muscles.
 The motor responses can be voluntarily
controlled; therefore this part of the PNS is
voluntary.
Autonomic Nervous System
(ANS)
 Sensory neurons from the autonomic sensory
receptors in the viscera.
 Motor neurons located in smooth muscle,
cardiac muscle and glands.
 These motor responses are NOT under
conscious control; Therefore this part of the
PNS is involuntary.
ANS Continued…
 The motor portion of the ANS consists of
sympathetic and parasympathetic divisions.
 Both divisions typically have opposing
actions.
Enteric Nervous System (ENS)
 “The brain of the gut”.
 Functions independently of the ANS and
CNS, but communicates with it as well.
 Enteric motor units govern contraction of the
GI tract.
 Involuntary.
Nervous Tissue
 Neurons.
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Sensing.
Thinking.
Remembering.
Controlling muscular activity.
Regulating glandular secretions.
 Neuroglia.
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Support, nourish, and protect neurons.
Neurons
 Have the ability to produce action potentials
or impulses (electrical excitability).
 Action potentials propagate from one point
to the next along the plasma membrane.
Parts of a Neuron
 Cell body.
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Contains the nucleus surrounded by cytoplasm which
contains the organelles.
 Dendrites (= little trees).
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The receiving (input) portion of a neuron.
 Axon.
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Each nerve contains a single axon.
The axon propagates impulses toward another neuron,
muscle fiber, or gland cell.
Synapse
 The site of communication between two
neurons or between a neuron and an effector
cell.
 Synaptic end bulbs and varicosities contain
synaptic vesicles that store a chemical
neurotransmitter.
Axonal Transport
 Slow axonal transport.
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1-5 mm per day.
Travels in one direction only – from cell body
toward axon terminals.
 Fast axonal transport.
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200 – 400 mm per day.
Uses proteins to move materials.
Travels in both directions.
Structural Classifications of
Neurons
 Multipolar neurons.
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One axon and several dendrites.
Most neurons of the brain and spinal cord.
Structural Classifications of
Neurons
 Bipolar neurons.
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One axon and one main dendrite.
Retina of the eye, inner ear, and the olfactory area of the
brain.
 Unipolar neurons.
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The axon and the dendrite fuse into a single process that
divides into two branches.
The dendrites monitor a sensory stimulus such as touch
or stretching.
Neuroglia
 Half the volume of the CNS.
 Generally, they are smaller than neurons, but
5 to 50 times more numerous.
 They can multiply and divide.
 Gliomas – brain tumors derived from glia.
Types of Neuroglia
 CNS
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Astrocytes
Oligodendrocytes
Microglia
Ependymal cells
 PNS
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Schwann cells
Satellite cells
Myelination
 The myelin sheath is a lipid and protein covering.
It is produced by the neuroglia.
 The sheath electrically insulates the axon of a
neuron.
 The sheath increases the speed of nerve impulse
conduction.
 Axons without a covering are unmyelinated.
Axons with a covering are myelinated.
Myelination Continued…
 Two types of neuroglial cells produce
myelination.
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Schwann cells – located in the PNS.
Oligodendrocytes – located in the CNS.
Gray and White Matter
 The white matter consists of aggregations of
myelinated and unmyelinated axons.
 The gray matter consists of neuronal cell
bodies, dendrites, unmyelinated axons, axon
terminals, and neuroglia.
Electrical Signals in Neurons
 Neurons are electrically excitable and communicate
with one another using 2 types of electrical signals.
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Action potentials.
Graded potentials.
 The plasma membrane exhibits a membrane
potential. The membrane potential is an electrical
voltage difference across the membrane.
Electrical Signals in Neurons
 The voltage is termed the resting membrane
potential.
 The flow of ions produces the electrical
current.
Ion Channels
 The plasma membrane contains many
different kinds of ion channels.
 The lipid bilayer of the plasma membrane is
a good electrical insulator.
Ion Channels
 The main paths for flow of current across the
membrane are ion channels.
Ion Channels
 When ion channels are open, they allow specific
ions to move across the plasma membrane down
their electrochemical gradient.
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Ions move from greater areas of concentration to lesser
areas of concentration.
Positively charged cations move towards negatively
charged area and negatively charged anions move
towards a positively charged area.
As they move, they change the membrane potential.
Ion Channel “Gates”
 Ion channels open and close due to the
presence of “gates”.
 The gate is part of a channel protein that can
seal the channel pore shut or move aside to
open the pore.
Types of Ion Channels
 There are 4 types of ion channels.
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Leakage channels – gates randomly alternate between
open and closed positions.
Voltage-gated channels – opens in response to c change
in membrane potential (voltage).
Ligand-gated channels – opens and closes in response to
a specific chemical stimulus.
Mechanically gated channels – opens or closes in
response to mechanical stimulation.
Resting Membrane Potential
 The resting membrane potential occurs due
to a buildup of negative ions in the cytosol
along the inside of the membrane and
positive ions in the extracellular fluid along
the outside of the membrane.
 The potential energy is measured in
millivolts (mV).
Resting Membrane Potential
 In neurons, the resting membrane potential
ranges from –40 to –90 mV. Typically –70
mV.
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The minus sign indicates that the inside of the
cell is negative compared to the outside.
 A cell that exhibits a membrane potential is
polarized.
Electrochemical Gradient
 An electrical difference and a concentration
difference across the membrane.
Graded Potentials
 A graded potential is a small deviation from
the resting membrane potential.
 It makes the membrane either more polarized
(more negative inside) or less polarized (less
negative inside).
 Most graded potentials occur in the dendrites
or cell body.
Graded Potentials
 Hyperpolarizing graded potential.
 Depolarizing graded potential.
 Graded potentials occur when ligand-gated or
mechanically gated channels open or close.
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Mehcanically gated channels are present in sensory
neurons.
Ligand-gated channels are present in interneurons and
motor neurons.
Action Potentials
 An action potential is known as an impulse.
 Depolarizing phase – the resting membrane
potential decreased towards zero.
 Repolarizing phase – restores the resting
membrane potential.
Action Potentials
 Threshold – depolarization reaches a certain
level (about –55 mV), voltage gated channels
open.
 Action potentials arise according to an all or
none principal.
Comparison of Graded
Potentials and Action Potentials
 See table 12.2 p. 404
Depolarizing Phase
 A depolarizing graded potential or some other
stimulus causes the membrane to reach threshold.
 Voltage-gated ion channels open rapidly.
 The inflow of positive Na+ ions changes the
membrane potential from –55mv to +30 mV.
 About 20,000 Na+ enter through the gates.
Millions are present in the surrounding fluid.
 Na-k pumps bail them out.
Repolarizing Phase
 While Na+ channels are opening during
depolarization, K+ channels are opening,
although slowly.
 K+ channels allow outflow of K+ ions.
 The closing of Na+ channels and the slow
opening of K+ channels allows for
repolarization.
Refractory Period
 The period of time after an action potential begins
during which an excitable cell cannot generate
another action potential.
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Absolute refractory period – a second action potential
cannot be initiated, even with a very strong stimulus.
Relative refractory period – an action potential can be
initiated, but only with a larger than normal stimulus.
Propagation of Nerve Impulses
 The impulse must travel from the trigger
zone to the axon terminals.
 This process is known as propagation or
conduction.
 As Na+ ions flow in, they trigger
depolarization which opens Na+ channels in
adjacent segments of the membrane.
Neurotoxins & Local
Anesthetics
 Neurotoxins produce poisonous effects upon
the nervous system.
 Local anesthetics are drugs that block pain
and other somatic sensations.
 These both act by blocking the opening of
voltage-gated Na+ channels and preventing
propagation of nerve impulses.
Continuous and Saltatory
Conduction
 Continuous conduction – step-by-step
depolarization and repolarization of adjacent
segments of the plasma membrane.
 Saltatory conduction – a special mode of
impulse propagation along myelinated axons.
Continuous and Saltatory
Conduction
 Few ion channels are present where there is
myelin.
 Nodes of Ranvier – areas where there is no
myelin – contain many ion channels.
 The impulse “jumps” from node to node.
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This speeds up the propagation of the impulse.
This is a more energy efficient mode of
conduction.
Effect of Axon Diameter &
Myelination
 Larger diameter axons propagate impulses
faster than smaller ones.
 Myelinated axons conduct impulses faster
than unmyelinated ones.
Effect of Axon Diameter &
Myelination
 A fibers.
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Largest diameter.
Myelinated.
Convey touch, pressure, position, thermal
sensation.
Effect of Axon Diameter &
Myelination
 B fibers.
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Smaller diameter than A fibers.
Myelinated.
Conduct impulses from the viscera to the brain
and spinal cord (part of the ANS).
Effect of Axon Diameter &
Myelination
 C fibers.
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Smallest diameter.
Unmyelinated.
Conduct some sensory impulses and pain
impulses from the viscera.
Stimulate the heart, smooth muscle, and glands
(part of ANS).
Encoding Intensity of a
Stimulus
 A light touch feels different than a firmer
touch because of the frequency of impulses.
 The number of sensory neurons recruited
(activated) also determines the intensity of
the stimulus.
Signal Transmission at
Synapses
 Presynaptic neuron – the neuron sending the
signal.
 Postsynaptic neuron – the neuron receiving
the message.
 Axodendritic – from axon to dendrite.
 Axosomatic – from axon to soma.
 Axoaxonic – from axon to axon.
Types of Synapses
 Electrical synapse
 Chemical synapse
Electrical Synapses
 Action potentials conduct directly between
adjacent cells through gap junctions.
Electrical Synapses
 Tubular connexons act as tunnels to connect the
cytosol of the two cells.
 Advantages.
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Faster communication than a chemical synapse.
Synchronization – they can synchronize the activity of a
group of neurons or muscle fibers. In the heart and
visceral smooth muscle this results in coordinated
contraction of these muscle fibers.
Chemical Synapses
 The plasma membranes of a presynaptic and
postsynaptic neuron in a chemical synapse do not
touch one another directly.
 The space between the neurons is called a synaptic
cleft which is filled with interstitial fluid.
 A neurotransmitter must diffuse through the
interstitial fluid in the cleft and bind to receptors on
the postsynaptic neuron.
 The synaptic delay is about 0.5 msec.
Removal of Neurotransmitter
 Diffusion.
 Enzymatic degradation.
 Uptake by cells.
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Into the cells that released them (reuptake).
Into neighboring glial cells (uptake).
Spatial and Temporal
Summation of Postsynaptic
Potentials
 A typical neuron in the CNS receives input
from 1000 to 10,000 synapses.
 Integration of these inputs is known as
summation.
Spatial and Temporal
Summation of Postsynaptic
Potentials
 Spatial summation – summation results from
buildup of neurotransmitter released by
several presynaptic end bulbs.
 Temporal summation – summation results
from buildup of neurotransmitter released by
a single presynaptic end bulb 2 or more times
in rapid succession.
Summary of Neuronal Structure
and Function
 Table 12.3
 P. 408
Neural Circuits
Neurogenesis in the CNS
 Birth of new neurons.
 From undifferentiated stem cells.
 Epidermal growth factor stimulates growth
of neurons and astrocytes.
 Minimal new growth occurs in the CNS.
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Inhibition from glial cells.
Myelin in the CNS.
Damage and Repair in the PNS
 Axons and dendrites may undergo repair if
the cell body is intact, if the Schwann cells
are functional, and if scar tissue does not
form too quickly.
 Wallerian degeneration.
 Schwann cells adjacent to the site of injury
grow torwards one another and form a
regeneration tube.