THE NERVOUS SYSTEM

Chapter 33

STRUCTURE OF NERVOUS SYSTEM

Section 1

NEURONS

Neurons are specialized cells that help you gather information about your environment, interpret the information and react to it.

Neurons make up an enormous communication network in your body called your nervous system.

A neuron consists of 3 main regions:1. Dendrites: receive impulses from other

neurons and send impulses to the cell body. A single neuron may have many dendrites.

2. Cell Body: nucleus of the neuron3. Axon: carries the nerve impulse from the

cell body to other neurons and muscles

NEURON

NEURONS CONT…

There are 3 kinds of neurons:1. Sensory Neurons2. Interneurons3. Motor Neurons

A reflex arc is a nerve pathway that consists of a sensory neuron, an interneuron and a motor neuron. (The brain is not involved).

AN ACTION POTENTIAL AKA NERVE IMPULSE

The minimum stimulus to cause an action potential to be produced is a threshold.

Threshold: Not all stimuli cause an impulse. A stimulus below the threshold has no effect on

the neuron. Some people have higher thresholds for pain,

heat or other stimuli. This means they can tolerate a stronger stimulus before their nervous system reacts with an impulse.

Action potentials are described as being “all or nothing” meaning a nerve impulse is either strong enough to travel along the neuron or its not strong enough.

AN ACTION POTENTIAL CONTINUED… When a stimulus reaches threshold,

channels in the plasma membrane open.Na+ ions rapidly move into the cytoplasm

causing temporary reversal in electrical charges. The inside of the cell is now positive, which causes

other channels to open.K+ leave the cell through these channels,

restoring + charges outside the cell.

SPEED OF AN ACTION POTENTIAL

Depend upon the axons in a neuron.

Most neurons are covered with a lipid layer known as the myelin sheath.  The Myelin Sheath both insulates and speeds up

transmission of action potentials through the axon.

Other structures along the axon are gaps (nodes) in the myelin sheath known as nodes of ranvier. These gaps allow the impulses to travel faster

than if they travelled along the entire length of the neuron.

THE SYNAPSE A small gap exists

b/w the axon of one neuron and the dendrite of another neuron, this is called a synapse.

At the end of an axon, small sacs called vesicles carrying neurotransmitters fuse with the plasma membrane and release a neurotransmitter by exocytosis.

THE SYNAPSE CONT… A neurotransmitter is a chemical that diffuses

across a synapse and binds to receptors on the dendrite of a neighboring neuron.

An nt causes channels to open on the neighboring cell and creates a new action potential.

ORGANIZATION OF THE NERVOUS SYSTEM

Section 2

THE CENTRAL NERVOUS SYSTEM

The nervous system consists of 2 major divisions: Central Nervous System (CNS):

brain/spinal cordPeripheral Nervous System (PNS): sensory

neurons and motor neurons The CNS is made mostly of

interneurons. The CNS’s function is to coordinate all

the body’s activities. The CNS relays messages, processes

information, and analyzes responses.

THE BRAIN Over 1 billion neurons

are found in the brain. It is called the control

center of the body.

THE BRAIN—CEREBRUM The cerebrum is the

largest part of the brain Broken into 2 halves

called hemispheres. Hemispheres

connected by a bundle of nerves.

Functions: Thought Voluntary movement Language Reasoning Perception

MEMORY http://faculty.washington.edu/chudler/stm0.html Short Term Memory Trials 1-6

THE BRAIN—CEREBELLUM

Known as the “little brain”

Functions: Movement Balance Posture

The brain stem connects the brain to the spinal cord. Medulla Oblongata: relays

signals b/w the brain and spinal cord. Controls breathing rate, heart

rate and blood pressure. Pons: relays signals b/w the

cerebrum and cerebellum. Helps control breathing rate.

THE BRAIN—BRAIN STEM The brain stem is a general

term for the area of the brain b/w the thalamus and spinal cord.

Structures within the brain stem include the medulla oblongata, pons, etc.

Functions: BreathingHeart Rate Blood Pressure

THE BRAIN—HYPOTHALAMUS

Its about the size of a fingernail.

Functions:Body TemperatureEmotionsHungerThirstCircadian Rhythms

(Sleep)

THE SPINAL CORD The spinal cord is a nerve column that

extends from the brain to the lower back. It is protected by the vertebrae. Spinal nerves extend from the spinal cord to

parts of the body and connect them to the CNS.

Reflexes are processed by the spinal cord.

THE PERIPHERAL NERVOUS SYSTEM

Contains all the nerves throughout the body. A nerve is actually a

bundle of axons. 12 cranial nerves

lead to and from the brain.

31 spinal nerves.

THE SOMATIC NERVOUS SYSTEM Nerves in the somatic

nervous system relay information from external sensory receptors (skin) to the CNS, and motor nerves relay info from the CNS to skeletal muscles.

Most reflexes (quick response to change in an environment) go only to the spinal cord, not to the brain.

THE AUTONOMIC NERVOUS SYSTEM The ANS carries

impulses from the CNS to the heart and other internal organs.

The body responds involuntary.

THE AUTONOMIC NERVOUS SYSTEM

There are two branches of the ANS: Sympathetic:

Activated in times of emergency or stress when the heart rate and breathing rate increases.

Fight or flight response

Parasympathetic Activated when the

body is relaxed.

Central Nervous System (CNS)

Peripheral NervousSystem (PNS)

Somatic (voluntary)

Relays info to and from skin to skeletal muscles.

Autonomic (involuntary)

Relays info to internal organs.

SympatheticControls organs in time of stress

ParasympatheticControls organs when the body is at rest.

THE SENSES

Section 3

TASTE & SMELL Caused by sensory neurons. Stimulated by chemicals. Specialized receptors located high in the

nose respond to chemicals in the air and send the info to the olfactory bulbs in the brain.

Taste buds are areas of specialized chemical receptors on the tongue that detect the tastes of sweet, sour, salty and bitter.

**Tip try holding your nose while eating; you’ll find out that your food looses much of its flavor.**

TASTE & SMELLTASTE SMELL

SIGHT Light enters the eye through the cornea.

Cornea: transparent durable layer of the eye. The cornea focuses the light that enters

through the pupil. The pupil is regulated by muscles in the iris, which is

the colored part of the eye. Behind the iris is the lens. The lens inverts the image and projects it

onto the retina. From the retina the image travels through

the vitreous humor. V.R. is a gelatin-like liquid b/w the lens and the retina.

The retina contains numerous receptors called rods and cones. Rods: light sensitive (black & white) Cones: bright light (colors)

These receptors send action potentials to the brain via the neurons in the optic nerve.

The brain interprets the signals from the retina and forms a visual image.

SIGHT

HEARING Vibrations called sound waves cause particles in the

air to vibrate. 1. Sound waves enter the auditory or ear canal

causing the eardrum to vibrate. 2. These vibrations travel through 3 bones in the

middle ear—malleus (hammer), incus (anvil), stapes (stirrup)

3. As the stapes vibrates it causes the oval window to move back & forth.

4. Now at the inner ear, the cochlea, which is filled with fluid and lined with tiny hair cells, vibrates the fluid along the hair cells.

5. The hair cells respond by generating nerve impulses in the auditory nerve and transmit them to the brain.

HEARING

BALANCE Found in the inner ear are structures that

control balance. Semicircular canals transmit information about

body position and balance to the brain. Like the cochlea they are filled with fluid and

contain hair cells. When the position of your head changes, fluid

moves through the canals. This causes the hair cells to bend, which sends nerve impulses to the brain.

The brain is able to determine your position and whether your body is still or in motion.

BALANCE

TOUCH Many sensory receptors that respond

to temperature, pressure and pain are found in the epidermis and dermis layers of the skin.

Distribution of receptors is not uniform in all areas of the body.

Fingers detect light touch Feet detect heavy pressure Pain receptors exist throughout the

body except for the brain.

TOUCH

Light Touch

Vibration & pressure

EFFECTS OF DRUGS

Section 4

HOW DRUGS WORK A drug is a substance, natural or artificial,

that alters the function of the body. Examples include: antibiotics, cocaine,

marijuana, caffeine, nicotine and alcohol. Not all drugs affect the nervous system. However those that affect the nervous

system affect it by: Increase the amount of a neurotransmitter Block the receptor site on a dendritePrevent a neurotransmitter from leaving a

synapse Imitate a neurotransmitter

HOW DRUGS WORK Many drugs affect the nervous system

influence the level of a neurotransmitter called Dopamine.

Dopamine is a neurotransmitter found in the brain that is involved with the control of body movements and other functions.

CLASSES OF COMMONLY ABUSED DRUGS Stimulants are drugs that increase

alertness and physical activity. Nicotine:

Is found in cigarette and cigar smoke which increases the amount of dopamine released in a synapse.

Nicotine also constricts blood vessels, raising blood pressure and causes the heart to work harder.

It also causes lung cancer. Caffeine:

Is the most used and abused stimulant Found in coffee, tea, soft drinks and chocolate Makes users feel awake and alert. Temporarily raises epinephrine (adrenaline) level

in the body giving quick bursts of energy, but quickly wears off.

CLASSES OF COMMONLY ABUSED DRUGS

Depressants: drugs tend to slow down the CNS. These drugs can lower blood pressure, interrupt breathing and slow heart rate.Alcohol

Most widely abused drug Results in feeling of relaxation and sluggishness Short term use impairs judgment, coordination and

reaction time. Long term use includes reduction in brain mass, liver

damage, stomach and intestinal ulcers as well as high blood pressure.

Inhalants Chemical fumes that influence the nervous system Short term use effect intoxication (nausea and vomiting) Long term use effect memory loss, hearing loss, vision

problems, peripheral nerve damage, and brain damage.

CLASSES OF COMMONLY ABUSED DRUGS

Illegal Drugs Amphetamines and cocaine prevent dopamine from

being reabsorbed so it remains in the synapse. Cocaine

Results in disturbances in heart rhythm, heart attacks, chest pains, respiratory failure, strokes, seizures, headaches, abdominal pain and nausea.

Amphetamines Results in rapid heart rate, irregular heartbeat,

increased blood pressure, and irreversible stroke producing damage.

Marijuana Most used illegal drug in the U.S. Tetrahydrocannabinol (THC) binds to receptors on

neurons in the brain, which produces an effect of intense pleasure.

Short term problems include memory and learning loss, anxiety, paranoia and panic attacks.

TOLERANCE & ADDICTION Tolerance occurs when a person needs

more and more of the same drug in order to get the same effect. The dosage increases because the body

becomes less responsive to the drug. Addiction is the psychological and/or

physiological dependence on a drug. Emotionally disturbed Physical health deteriorates

SENSORY NEURONS

Send impulses from receptors in the skin and sense organs to the brain and spinal cord. (Sensory signal interneurons)

INTERNEURONS

Found in the spinal cord and brain. They carry the impulse to motor neurons. (Interneurons signal motor neurons)

MOTOR NEURONS

Carry the impulse away from the brain and spinal cord to a gland or muscle which creates a response.

A NERVE IMPULSE

A nerve impulse is an electrical charge traveling the length of a neuron.

A NEURON AT REST When a neuron is not

sending a signal, it is "at rest"

At rest the inside of the neuron is negative relative to the outside.

The concentrations of the different ions attempt to balance out on both sides of the membrane, but cannot because of the cell’s selective permeability.

At rest, Potassium ions (K+) can cross through the membrane easily. Chloride ions (Cl-)and Sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane. 

A NEURON AT REST In addition to these selective ion channels, there is

a pump that uses energy to move 3 Sodium (Na+) ions out of the neuron for every 2 Potassium (K+) ions it puts in.

Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential.

The resting potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside.

At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside the neuron.