Chapter 35: The Nervous
System
Section 1: The Human Nervous
System
Neuron: basic unit of nervous system
Impulses: electrical signals
In most animals, neurons are clustered into bundles of
fibers called nerves.
Neuron
Dendrites: carry impulses toward the cell body
Axon: carries impulses away from the cell body
Cell body: largest part, metabolic activity of the cell
Nucleus
Axon terminals
Myelin: Schwann cells wrap axon in layers of their own cell
membrane. They leave gaps called nodes. When an impulse
moves down an axon covered with myelin, the action
potential jumps from one node to the next.
3 Types of Neurons
Sensory neurons: carry impulses from the sense organ
to the brain and spinal cord
Motor neurons: carry impulses from the brain and
spinal cord to muscles and other organs
Interneurons: connect sensory and motor neurons and
carry impulses between them
Resting/Action potential
Resting potential: A neuron not carrying an impulse is said
to be at rest. The resting neuron has electrical potential
across its membrane. The inside of the cell is negative
and the outside is positive. The difference in electrical
charges is the resting potential.
Action potential: rapid
change in voltage on
the inside of an axon.
How do impulses move?
An impulse begins when a neuron is stimulated by another neuron
or by the environment. Once it begins, the impulse travels
down the axon rapidly and away from the cell body.
The flow of positive charges into one region of the axon causes the
membrane just ahead of it to open up and let positive charges
flow across the membrane. This continues.
Synapse
Synapse: small space between the axon of one neuron and
the dendrites of the next neuron. It contains tiny sacs
filled with neurotransmitters (chemicals used by one
neuron to signal another cell)
When an impulse reaches the end of an axon it makes
contact with another cell.
Synapse
When an action potential arrives at the end of an axon, the
sacs release the neurotransmitters into the synapse
between the two cells. Neurotransmitter molecules
attach to receptors on the neighboring cell. This causes
positive ions to rush across the cell membrane,
stimulating the cell. If the stimulation is great enough,
a new impulse begins.
Section 2: Organization of the
Nervous System
The human nervous system
is divided into 2 partsthe central and the
peripheral nervous system.
The Central Nervous System
Consists of the brain and the spinal cord
Brain and spinal cord share structural similarities. They
are both cushioned by three layers of tough elastic
tissues called meninges. Between the meninges is a
space filled with cerebrospinal fluid which cushions
the brain and spinal cord.
Brain
Contains about 100 billion cells
Uses 25% of the body’s energy
Parts of the Brain
Cerebrum:
Largest part of the human brain
Responsible for functions such as learning, intelligence
and judgment
Divided into a left and right hemisphere by a deep
groove
Parts of the Brain
Cerebral Cortex:
Surface of cerebrum
Processes information from the senses and controls
body movements
Cerebellum:
Second-largest part of the brain
Coordinates and balances actions of muscles
Parts of the Brain
Brainstem:
Connects the brain to the spinal cord
Controls blood pressure, breathing, swallowing and
heart rate
Thalamus:
Receives messages from sense organs before they are
relayed to the cerebral cortex
The Peripheral Nervous
System
Includes all the nerves and associated cells that
connect the brain and the spinal cord to the rest of the
body
Receives information from the environment and relays
commands from the central nervous system to organs
throughout the body
Divided into the sensory division and the motor
division
Sensory/Motor Division
Sensory division: carries information from the sense organs
to the central nervous system
Motor division: transmits messages from the central
nervous system to the rest of the body
divided into the somatic nervous system and the
autonomic nervous system
Somatic/Autonomic Nervous
System
Somatic: controls voluntary movements (Ex. turning a page
of a book)
Autonomic: regulates activities that are not under
conscious control, including the beating of the heart
and the contraction of muscles surrounding the
digestive system
Section 3: The Senses
Each of our five senses begins with specialized sense organs
that respond to the environment.
Sensory neurons carry impulses from these sense organs
back to the central nervous system.
Vision
Light enters the eye through the cornea, a tough
transparent layer at the surface of the eye.
Iris: disk of tissue
Pupils: tiny muscles adjust the size of the opening in the
iris to regulate the amount of light that enters the eye
Lens: flexible structure filled with a transparent protein
Hearing and Balance
Hearing: Vibrations enter the ear through the auditory
canal, causing the tympanum to vibrate. The vibrations
are picked up by three tiny bones, the hammer, the
anvil and the stirrup. These bones transmit the
vibrations to a thin membrane called the oval window.
Vibrations of the oval window crate pressure waves in
the fluid-filled cochlea. The cochlea is lined with tiny
hair cells that are pushed back and forth by these
pressure waves. In response to these movements, the
hair cells produce nerve impulses that are sent to the
brain through the auditory nerve.
Balance: three semicircular canals enable the nervous
system to sense changes in the position of the human
head.
Smell
Special cells in the upper part of the nasal passageway act
as receptors for a variety of chemicals. When
stimulated these cells produce nerve impulses that
travel to the central nervous system.
Taste
Tastebuds: chemical receptors- identify salty, bitter, sweet
and sour taste
Touch
The skin contains different receptors for touch, pain, heat
and cold. Each receptor responds to its particular
stimulus and produces nerve impulses that signal the
central nervous system.
Section 4: Nerve Impulses and
Drugs
Resting Potential:
•
Sodium-potassium pump: uses energy from ATP to
pump sodium (Na+) ions out of the cell while at the
same time pumping potassium (K+) ions into the cell.
Significant numbers of potassium ions do manage to leak
across the membrane. Large members of positive ions
leak out of the cell. The inside of the cell is now
negatively charged. The great difference in charges
between that two sides of the membrane produces the
resting potential.
Resting potential
Sodium gates open
Potassium gates open
Action Potential
Voltage-sensitive gates allow either sodium/potassium to
pass through
Sodium moves. The inside of the cell is more positive.
Potassium gates open and allow potassium to flow out of the
cell.
The rapid opening and closing of sodium and potassium
gates makes the impulse possible. When an action potential
reaches the synapse, it triggers the release of a
neurotransmitter. The neurotransmitter molecules diffuse
across the gap and bind to receptors in the dendrites of the
next neuron. The receptors cause the ion gates to open and
the impulse continues.
Drugs
A drug is any substance that causes a change in the
body
Drugs can affect the body in a variety of ways, causing
changes in the brain, the nervous system, and the
synapses between nerves.
Stimulants/Depressants
Stimulants: Increase the release of neurotransmitters at
some synapses in the brain
Ex. Cocaine is a powerful stimulant that increases the
heart rate and blood pressure
Depressants: decrease the rate of brain activity
Opiates
Mimic natural chemicals in the brain such as
Endorphins, which normally help to overcome
sensations of pain.
If the user attempts to stop taking these drugs, the
body cannot produce enough of the natural
endorphins that are needed to prevent the user from
the uncontrollable pain and sickness that accompany
withdrawal from the drug.