The Nervous System - National Paralegal College · Introduction to Life Processes - SCI 102 1 Lesson 14. ... Structures and Functions of the Human Nervous System •The nervous system

The Nervous System

Introduction to Life Processes - SCI 102 1

Lesson 14

Structures and Functions of Nerve Cells

• The nervous system has two principal cell types:

Neurons (nerve cells)

Glia

• The functions of a neuron are localized in separate parts of the cell

A neuron has four distinct parts: dendrites, a cell body, an axon, and

synaptic terminals

These parts of the neuron perform four functions

• Neurons receive information from the environment

• Neurons process information and produce an electrical signal

• Neurons conduct signals over distances

• Neurons transmit signals to other neurons, or the cells of muscles or glands



• Dendrites respond to stimuli

These structures receive information, usually in the form

of chemicals called neurotransmitters

• The cell body processes signals from the dendrites

These electrical signals are positive or negative

If a signal is sufficiently positive, the neuron produces an

electrical signal called an action potential



• The axon conducts action potentials over long distances

Axons are typically bundled into nerves

• At synapses, signals are transmitted from one cell to

another

The synapse is the site where one neuron communicates with another

cell

A typical synapse consists of a synaptic terminal, a dendrite or cell

body, and a small gap separating the two cells


Neurons


• Section 1: the dendrites, which

“receive information”, signals from

the environment

• Section 2: area of

neurotransmitters production

• Path a nerve impulse travels within

a single neuron

Dendrite → cell body → axon →

synaptic terminal

Neurons Produce and Transmit Information

• Information within a single neuron is carried by

electrical signals

Most neurons have a negative resting potential, which is

the difference in voltage across its membrane

If the membrane potential reaches the threshold

potential, the neuron will generate an action potential



• Myelin speeds up the conduction of action potentials

Many vertebrate axons have myelin covering them

Between each myelin wrapping are nodes of exposed axon

• Action potentials jump from node to node, speeding up their

transmission

The thicker the axon, the faster the action potential will move

When an axon is covered in fatty myelin, the action potential

can travel even faster



• Myelin sheath

The myelin which protects an axon and helps speed up an action

potential

The myelin sheath enables nerve conduction to occur at a much more

rapid rate than without it

People with multiple sclerosis develop areas in which the myelin

sheath on their neurons has disintegrated.

• This disease often results in paralysis and extreme weakness.

• When the myelin sheath disintegrates, impulses travel more slowly to and

from the brain, contributing to the symptoms of multiple sclerosis



• Creating action potential

Electrical signals travel down the dendrites and converge on the neuron’s

cell body

The cell body performs the “processing information” function

The cell body adds up the electrical signals it receives from the dendrites

• Some signals are positive while some are negative

When the sum of electrical signals is sufficiently positive, the neuron

produces an action potential

• Action potential: a large, rapid electrical signal

The action potential is carried down the axon to be transmitted through

synaptic terminals to the dendrites of other nerve cells

• Caffeine lowers the threshold for an action potential

That means the nerve needs less electrical signals to stimulate an action

potential



• Neurons use chemicals to communicate with one

another at synapses

The presynaptic neuron transmits a signal by releasing

neurotransmitter molecules that are stored in vesicles in

the presynaptic terminal into the synaptic cleft

The postsynaptic neuron receives the signal because it

has specialized receptor molecules that bind to the

neurotransmitter and cause changes in the cell



• Synapses produce inhibitory or excitatory

postsynaptic potentials

A postsynaptic potential is a small brief change in voltage of

the neuron

Inhibitory postsynaptic potentials (IPSPs) move the resting

potential further from the threshold potential and make the

cell less likely to generate an action potential

Excitatory postsynaptic potentials (EPSPs) move the resting

potential closer to the threshold potential and make the cell

more likely to generate an action potential



• Neurotransmitter action is usually brief

• Integration of postsynaptic potentials

determines the activity of a neuron

The dendrites and cell body of a neuron receive EPSPs

and IPSPs from thousands of presynaptic neurons

• The voltages of all of these are added up by the postsynaptic

neuron through a process called integration


Processing Information and Controlling Behavior

• Most behaviors are controlled by pathways composed of

four elements

Sensory neurons: respond to a stimulus, either internal or external to

the body

Interneurons: receive signals from many different sources and often

activate motor neurons

Motor neurons: receive messages from sensory neurons and

interneurons and activate muscles or glands

Effectors, such as muscles and glands: perform the response directed

by the nervous system



• The four elements carry out the basic operations required of

any nervous system

Nervous systems must determine the type of stimulus

Nervous systems must determine and signal the intensity of a

stimulus

Nervous systems must integrate information from many sources

Nervous systems must initiate and direct appropriate responses



• The nature of a stimulus is encoded by

specialization of sensory neurons and their

connections to specific parts of the brain

Sensory neurons respond to different stimuli

• Information gathered by the senses is converted to action

potentials, which are sent to the brain

• All nervous systems interpret what a stimulus is by monitoring

which neurons are firing action potentials



• The intensity of a stimulus is encoded by the

frequency of action potentials

Intensity can also be coded by the number of similar

neurons that fire at the same time

• The nervous system processes information from

many sources

The brain receives sensory stimuli from many different

sources



• The nervous system produces outputs to muscles and

glands

The nervous system stimulates activity in muscles and glands,

which produce behaviors

• Behaviors are controlled by networks of neurons in the

nervous system

Simple behaviors such as the reflex may be controlled by

activity in as few as two or three neurons

Complex behaviors are organized by interconnected neural

pathways


Nervous System Organization

• All animals have two basic types of nervous systems

Radially symmetrical animals have a diffuse network of

neurons called a nerve net

Bilaterally symmetric animals have a centralized nervous

system in which cell bodies tend to be clustered in the

“head” region and along the central axis of the animal


Structures and Functions of the Human Nervous System

• The nervous system of all mammals is divided into the

peripheral nervous system (PNS) and the central nervous

system (CNS)

• The PNS links the CNS with the rest of the body

The PNS consists of three major categories of axons

• Axons of sensory neurons that carry sensory information from all parts of

the body

Sensory neurons carry this information to sensory organs (eyes, ears, nose, etc.)

• Axons of motor neurons that control the skeletal muscles, regulating

voluntary movements

• Axons of motor neurons that control involuntary movements



• Motor neurons make up the somatic and autonomic

nervous systems

The somatic nervous system controls voluntary movement

• Most neurons of this system synapse with skeletal muscles

The autonomic nervous system controls involuntary actions

• The sympathetic division of the autonomic nervous system

prepares the body for “fight or flight”

• The parasympathetic division of the autonomic nervous system

dominates during “rest and digest”



• Part of the sympathetic nervous system

• Activate target organs of the sympathetic nervous system

• Also known as adrenaline and noradrenaline

• When you experience something very scary, the automatic response of the body is to produce epinephrine. This may cause you to feel your heart racing, lungs pumping, blood rushing.

Epinephrine and

norepinephrine


Important Neurotransmitters


• Found in many areas of the brain and spinal cord

• Tasks: block the transfer of pain sensations and alter mood

Endorphins

• Found in the midbrain, pons, and medulla

• Influences mood and sleepSerotonin

• Found in many areas of the brain and spinal cord

• Is the major excitatory neurotransmitter in the CNS

Glutamate


Important Neurotransmitters


• The CNS consists of the spinal cord and brain

First line of defense around the CNS: the skull and backbone

Second line of defense around the CNS: A triple layer of connective

tissue, called the meninges

Cerebrospinal fluid between the meninges aids in cushioning the CNS

The blood–brain barrier, a wall of brain capillaries that are much

less permeable than capillaries in the rest of the body, forms the

third line of defense

Tasks of the CNS:

• Receive and process information

• Initiates action



• The spinal cord controls many reflexes and conducts

information to and from the brain

The spinal cord has axons of both sensory neurons and motor

neurons

Gray matter:

• The center of the spinal cord

• Consists mostly of cell bodies of neurons

White matter:

• The outer part of the spinal cord

• Consists mostly of the myelin-coated axons of the neurons



• The spinal cord controls many reflexes and conducts

information to and from the brain

The neurons that control many reflexes reside in the spinal

cord and PNS

• A reflex is an involuntary movement of a body part in response to

a stimulus

• The pain-withdrawal reflex is a good example

Some complex actions are coordinated within the spinal cord

• Neurons needed for a variety of complex actions, such as walking

and running, are found in the spinal cord



• The brain consists of many parts that perform specific

functions

• All vertebrate brains consist of three main parts:

The hindbrain

The midbrain

The forebrain

• In nonmammalian vertebrates, the three divisions are

prominent

• In mammals, the brain regions are significantly modified

• The human brain has a variety of structures



• The hindbrain consists of the medulla, pons, and cerebellum

The medulla controls several automatic functions

The pons plays a role in the transition from sleep to wakefulness, as

well as between the stages of sleep

The cerebellum is crucial in coordinating movements of the body

• The midbrain contains clusters of neurons that contribute to

movement, arousal, and emotion

The midbrain is quite small in humans

The reticular formation is an important relay and filtering station

that extends from the medulla into the lower regions of the

forebrain



• The forebrain includes the thalamus, hypothalamus, and

cerebrum

The thalamus channels sensory information from all parts of the

body to the limbic system and cerebral cortex

The hypothalamus, through its hormone production and neural

connections, helps maintain homeostasis

The cerebrum consists of two cerebral hemispheres

• Each hemisphere is composed of an outer cerebral cortex

• Structures in the Interior of the cerebrum

The amygdala produces sensations of pleasure, fear, or sexual arousal

The hippocampus is critical for the formation of long-term memory

The basal ganglia are important in the overall control of movement



• The forebrain includes the thalamus, hypothalamus, and

cerebrum

The limbic system is a group of structures that produce our

most basic emotions, drives, and behaviors

The cerebral cortex

• The cerebral cortex is folded into convolutions

• The cerebral cortex is divided into two hemispheres connected by

a large band of axons, called the corpus callosum

• Each hemisphere is divided into four anatomical regions: the (1)

frontal, (2) parietal, (3) occipital, and (4) temporal lobes, which all

have specific functions



• Cerebral cortex

Thin outer layer of each cerebral hemisphere

Neurons in the cortex:

• Receive and process sensory information

Sound, sight, smell, taste, touch

• Direct voluntary movements

• Create memories

• Allow us to be creative

• Envision the future

Is made up of four parts called lobes

• Occipital lobe: visual association area

• Parietal lobe: sensory association area, primary sensory area

• Frontal lobe: premotor area, area of higher intelligence functions, speech motor area,

primary motor area

• Temporal lobe, primary auditory area, memory, language comprehension



• How do neuroscientists learn about the functions of

brain regions?

The functions of different parts of the brain were

discovered by examining the behaviors and abilities of

people who suffered brain injuries, such as the case of

Phineas Gage



• The left and right sides of the brain are specialized for

different functions

Studies in which the corpus callosum has been cut have shown

the differing functions of each hemisphere

• Axons from the eyes go to opposite sides of the brain

Further studies have confirmed the different functions of each

hemisphere

• Example: It is possible that brain damage from a stroke or other

injury could leave a person with the ability to comprehend written

language but not spoken language (assume the person's hearing is

fine) because different aspects of language comprehension are

controlled by different parts of the brain



• Learning and memory involve biochemical and

structural changes in specific parts of the brain

Learning has two phases

• Short-term memory

• Long-term memory

The frontal and parietal lobes and some of the basal ganglia

deep in the cerebrum are the primary sites of short-term

memory

Short-term memory probably requires repeated activity,

whereas long-term memory involves structural changes


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The Nervous System - National Paralegal College · Introduction to Life Processes - SCI 102 1 Lesson 14. ... Structures and Functions of the Human Nervous System •The nervous system