Chapter 17

The nervous system

The Nervous System• Nerves communicate with each other by

sending chemical and electrical signals to each other.

• Nerve receptors (part of your peripheral nervous system PNS) collect information and send it to your central nervous system (CNS) (which is your brain and spinal cord). This is then sent back to the PNS so your body can respond

Neuron structure• Dendrites send signals toward the cell body• Cell body contains all the cell organelles and the

nucleus• Axon conducts nerve impulses along its length

• Information always moves

dendrites cell body axon

• Axons are filled with a fluid called axoplasm. It is slightly different from normal cytoplasm. Axoplasm is filled with molecules that conduct electricity.

• Axons are also surrounded by an axomembrane. Very similar to a regular cell membrane, but it has a lot of sodium and potassium pumps.

3 Types of Neurons• Sensory Neuron part of PNS. Has receptors. This

sends a signal to the interneuron• Interneuron part of CNS. Receives information

from the PNS or from other parts of the CNS. It will transmit information to other CNS parts or to motor neurons.

• Motor Neuron part of PNS. Receives messages from the CNS to perform an action.

Myelin Sheath• Axons are covered by a protective myelin sheath. • The myelin sheath is actually made from other cells

called Schwann Cells.• This protects the axon and also makes signals travel

faster down the axon.• Between the myelin sheaths are spaces. These

spaces are called the Nodes of Ranvier

Nerve Impulse• A nerve impulse is how neurons transmit

information• Step 1. when the axon is not sending a

message the inside of the axon is negative compared to the outside the axon. This is called the Resting Potential

• The potassium K+ has greater concentrations inside the axon than outside the axon.

• The sodium Na+ has a greater concentrations outside the axon than inside the axon.

• The reason why there is a difference between the inside and the outside of an axon is because of sodium-potassium pumps. These pumps use ATP to and actively move potassium and sodium against a concentration gradient

• Resting potential

Step 2: Action Potential• This is a rapid change in polarity across the

axomembrane. • Sodium gates open first and Na+ flows into the axon.

This causes the axon to become depolarized. Going from negative to positive.

• This changes the charge of the axon from -65mV to +40mV

• Potassium gates then open and K+ flows from the inside to the outside of the axon. This causes it to become repolarized. Or for the axon to go back to being -65mV

Step 3: Propagation of an action potential

• This means how to make it keep moving down the axon• When an action potential reaches a large enough

depolarization it is called a threshold• This will make the next section undergo depolarization.• After a section has done its action potential then there is

a short period of time for the sodium and potassium pumps to reset. This is called the refractory period. It makes sure that the action potential can only move in one direction.

• This only occurs at the nodes of Ranvier, and when this action potential reaches the end of the node of Ranvier it is able to induce an action potential at the next node of ranvier.

• This is how the mylenated sheath increases the speed of a signal.

• The electricity that is generated at one node of Ranvier is enough to excite the next node of Ranvier. This is how mylenated sheaths speed up the signals.

• This is called saltatory transmission

Transmission across a Synapse• Axon branches and ends in an axon bulb. • The axon bulbs come into close contact with the

dendrites of the next neuron. • The membrane of the axon bulb is referred to as the

presynaptic membrane. • The membrane of the dendrite is referred to as the

postsynaptic membrane.• The space between the axon bulb and the dendrite

is called the synaptic cleft.

The Threshold• If a nerve impulse is above the threshold then

the impulse and the message will be sent. If it is below the threshold then it won’t be sent.

• This is called the “all or none” response– If its above the threshold then it is “ALL”– If its below the threshold then it is “None”

How does a message cross the synapse?

Step 1: Nerve impulse reaches the axon bulb, and this causes the calcium gates to open and Ca2+ ions move into the axon bulb.

Step 2: This causes vesicles carrying a chemical called neurotransmitters to merge with the presynaptic cleft and release its neurotransmitter in the synaptic cleft.

Step 3: the neurotransmitter moves across the synaptic cleft and binds with a receptor on the dendrite.

Neurotransmitters• These are chemicals that cross the synaptic cleft.• There are many types of neurotransmitters.• Acetylcholine and norepinephrine are two

examples of neurotransmitters. • These bind with the receptor on the post-synaptic

membrane. An enzyme takes the neurotransmitter out of the receptor.

• Acetylcholinesterase is an enzyme that removes acetylcholine

The Reflex Arc

• This refers to the reaction to a stimulus that is painful or scary or that surprises you.

• Sensory neuron receives a stimulus and sends the signal to the interneuron in the spinal cord. This makes a decision and sends a message down your motor neuron to an effector muscle.

Drugs. Are they a Disease or a Choice?

• What is a disease?• A disease is any disturbance or anomaly in the

normal functioning of the body that probably has a specific cause and identifiable symptoms.

• People who take drugs can quit anytime, or can they?