1 structure and function of the nervous system

8/6/2019 1 Structure and Function of the Nervous System

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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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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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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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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.

St t & F ti f N S t


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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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Electrical and chemical impulses generated between

neurons


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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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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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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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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.

1 structure and function of the nervous system

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