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Structure & Function of Human Nervous System 1
Nervous system is divided into:
1.Central nervous system (CNS)
2.Peripheral nervous system (PNS)
The central nervous system is composed of:
Brain, which act as the command and integration
centers of the nervous system (Figure 1, below), andwhere information or stimuli are analyzed andresponses generated.
CNS is also the site of thought, reasoning, andmemory. The information that the CNS receives isdivided into 12 cranial nerves and the peripheral orspinal nerves (Figure 2, below).
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Structure & Function of Nervous System 1
1. Central nervous system (CNS)
2. Peripheral nervous system (PNS)
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Brain, Spinal Cord, & Nerves
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Structure & Function of Nervous System
Spinal cord,
The cranial nerves convey impulses to and from thebrain.
The peripheral nerves convey impulses to and from
the spinal cord. Both sets of nerves serve as communication lines
linking all parts of the body to the central nervous
system, and carry impulses from the sensoryreceptors to the CNS and from the CNS to the
appropriate glands or muscles.
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Structure & Function of Nervous System Peripheral Nervous System(PNS) has an enormous
number of receptors, are used to gather information
about the outside world.
Peripheral nerves are divided intosensory (afferent/
ascending)pathways that carry impulses toward the
CNS; andmotor (efferent/descending)pathways
that carry impulses away from the CNS to skeletal
muscles, glands, and effector organs.
Effector organs such as the heart or pancreas areinnervated by specific components of the nervous
system.
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Reflex Arc Afferent and Efferent Nerve Fibers
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Structure & Function of Nervous System Functionally-the peripheral nervous system is
divided into:
1. Somatic nervous system (SNS)
2. Autonomic nervous system (ANS)
Thesomatic nervous system consists of pathwaysthat regulate voluntary control (such as that
needed to lift objects) of skeletal muscles.
Autonomic nervous system regulates automaticor involuntary control of organ systems (such as
cardiac muscle and glands).
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Structure & Function of Nervous System The autonomic nervous system can be further
subdivided into:
1. Sympathetic nervous system (SNS)
2. Parasympathetic nervous system (PSNS)
NERVOUS TISSUE Nervous Tissuetwo principal types of cells
make up nervous tissue:
I.Neurons, andII.Supporting cells.
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Structure & Function of Nervous System
I.Neuronsnerve cells are called neurons and
specialized to transmit nerve impulses (messages)
from one part of the body to another. They work
alone or in units to detect environmental changes
and to initiate body responses to maintain an active,
steady state. Neurons differ structurally, yet havecommon features (Figure 3, below).
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Structure & Function of Nervous System
All neurons consist of:
Cell body- contains a nucleus that regulates cell
functions and one or more processes or fibersextending from the cell body
Dendrites- neuron processes that conduct electrical
currents toward the cell body Axons- single fibers that carry nerve impulses away
from the cell body. Axons occasionally give offcollateral branches along their length, but all end inmultiple branches known asaxonal terminals.
Neurons-have only one axon, but they may havehundreds of branching dendrites, depending on their
type.
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Structure & Function of Nervous System When an impulse reaches the axonal terminals, it
stimulates the release of chemicals into the
extracellular space (the synapse). They either help an impulse to cross the synapse or
stop it from crossing. Neurons are very close
together but never actually touch each other. Synaptic cleft- the tiny space
that separates one neuron from
another neuron.
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Structure and Function of the Nervous System What occurs in the synaptic
cleft.
The synapse is the functional
junction that joins one
neuron to another. It is
usually a chemical type of
synapse. Some neurons are
physically joined by gap
junctions, where electricalcurrents are able to flow
directly from one neuron to
the next neuron.
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Structure & Function of Nervous System The synapse is the functional junction that joins
one neuron to another. It is usually a chemical type
of synapse.
Some neurons are physically joined by gap
junctions, where electrical currents are able to flow
directly from one neuron to the next neuron.
II. Supporting Cellsin the central nervous system
areneuroglia, which generally support, insulate, and
protect the neurons.
Each type of neuroglia has special functions.
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Structure & Function of Nervous System Myelin is the whitish,
fatty material that covers
most long nerve fibers. Itprotects and insulates thenerve fibers and increasesthe transmission rate ofnerve impulses.
Axons outside of the CNSare myelinated by
Schwann cells, whichform the myelin sheath.
f N S
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Structure & Function of Nervous System The neurilemma is the part of
the Schwann cell cytoplasm that
ends up beneath the outermostpart of the plasma membraneexternal to the myelin sheath.
The myelin sheath hasindentations called nodes ofRanvier, which are formed bythe individual Schwann cells.
i.e. Myelin sheath & neurilemma Neurilemma plays an important
role in fiber regeneration if it
remains intact when a peripheralnerve fiber is dama ed.
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Structure & Function of Nervous System The velocity of nerve impulses increases where
myelin is present.
The increased speed occurs because the myelin actsas an insulator that allows ions to flow betweensegments rather than along the entire length of the
membrane. Movement of the electrical impulse along the nodes
ofRanvier is call Saltatory movement ortransmission.
Disorders of the myelin sheath such as multiplesclerosis and Guillain-Bare` syndrome, provideexamples of the important role myelin plays in nerve
function.
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Structure &Function of Nervous System
III. Neuroglia are structurally very similar toneurons, but are not able to conduct nerve impulses
and they never lose their ability to divide. Becauseneurogliacan divide, most brain tumors are formedby neuroglia and are known as gliomas.
The Schwann cellsand thesatellite cellsare theprimary supporting cells of theperipheral nervous
system. Schwann cells form the myelin sheaths
around nerve fibers found in the PNS, and the
satellite cells protect and cushion cells.
Clusters of neuron cell bodies found in the CNS are
called nuclei.
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Structure & Function of Nervous System
The Nuclei are well protected within the bony skull
or vertebral column. This protection is essential
since these neurons do not undergo cell division
after birth. Cell body carries out most of the
metabolic functions of these neurons. If the cell
body is damaged and dies, it is not replaced. Ganglia are small collections of cell bodies found
outside the CNS in the PNS.
Tractsare nerve bundles in the CNS. In the PNS tracts are called nerves.
White matteris composed of dense collections of
myelinated tracts.
S d F i f h N S
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Structure and Function of the Nervous System Gray matterconsists mostly of unmyelinated fibers
and cell bodies.
Functionally, neurons are classified according to thedirection the nerve impulse is traveling in relation tothe CNS.
Sensory or afferent neurons carry impulses fromsensory receptors in the internal organs or the skin to
the CNS.
Pain receptors are the least specialized. Cutaneous receptors are the most numerous and are
actually bare dendrite endings.
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Structure & Function of Nervous System Motor or efferent neurons carry impulses from the
CNS to the viscera and/or muscles and glands. The
motor neurons cell bodies are always located in theCNS.
Association neurons or interneurons connect motor
and sensory neurons in the neural pathways. Likemotor neurons, their cell bodies are always located
in the CNS.
Nerve ImpulsesElectrical and chemical impulsesare generated and conducted by neurons, which
selectively change the electrical potential of the
plasma membrane and influence other nearby
neurons b the release of neurotransmitters.
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Structure & Function of Nervous System
Electrical and chemical impulses generated between
neurons
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Structure & Function of Nervous System
A nerve impulse is a self-propagated electrical
charge transmitted along the membrane of a nerve
fiber. It is much like the electrical impulses that are
carried along a telephone wire (see previous figure).
More than 30 neurotransmitters have been
identified.
Common selected neurotransmitters are described in
Table 1.
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Table 1:Selected Common Neurotransmitters
Transmitter Location Action
Acetylcholine CNS, autonomic Excitationspeeds
Nervous system impulse transmission
(ANS), neuromus-cularjunctions
Serotonin CNS Inhibition controls
body heat, hunger,behavior, and sleep
Structure and Function of the Nervous
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Structure and Function of the Nervous
System
Dopamine CNS, ANS Inhibitioncontrolsbehavior and fine
movement
Norepinephrine CNS, ANS Excitationchieftransmitter of
sympathetic nervous
system
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For electrical impulses to flow through the nervoussystem, a stimulus must occur.
The stimulus raises a potential response, called theaction potential (Figure ).
If the stimulus is too weak, the membrane remains atrest (unexcited). This is often referred to as the all-or none responseit either is conducted over theentire axon or it does not happen at all. The eventsthat involve nerve impulses are:
Polarization, which is the normal state of the restingneuron
Depolarization and generation of the action potential
Repolarization.
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Structure & Function of Nervous System
Three steps describe the movement of a nerve
impulse along unmyelinated fibers. Fibers that have
a myelin sheath conduct impulses much faster. The
nerve impulse literally leaps from node to node
along the length of the fiber.
Reflex Arc
Reflexes are rapid, predictable, and involuntary
responses to stimuli. Once initiated, a reflex always
goes in the same direction and occurs over neural
pathways called reflex arcs (Figure ). Reflexes can
be classified as either autonomic or somatic reflexes.
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Structure & Function of Nervous System
Autonomic reflexes regulate the activity of smooth
muscles, the heart, and glands. Autonomic reflexes
regulate body functions such as digestion,
elimination, blood pressure, and sweating.
The sympathetic nervous system responds, that is,
activates the fight-or-flight response, to get the
body moving in emergency or exciting situations.
The parasympathetic nervous system calms and
restores the body. The parasympathetic nervous
system returns the body to normal balance.
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Structure & Function of Nervous System
Figure of the Reflex Arc illustrates effects on organs
of the sympathetic and parasympathetic nervous
system. Somatic reflexes are the reflexes that
stimulate the skeletal muscles.
Clinical Observation: Testing of reflexes is a
valuable assessment tool in evaluating the condition
of the nervous system. Nervous system disorders are
indicated whenever reflexes are exaggerated,
distorted, or absent. Often reflex changes occurbefore the pathologic condition has become obvious
in other ways.
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