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.

NeuronDendrites: 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 parts-

the 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 PotentialVoltage-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.