The Neuron

An everyday

(every second!)

use of active transport

The Nervous System

Controls and coordinates functions throughout the body and responds to internal and external stimuli.

What makes up the Nervous System?

• The basic unit of structure and function of the nervous system is a neuron (nerve cell).

Axon terminals

4. Myelin sheath

Nodes

1. Cell body

3. Axon

Nucleus

2. Dendrites

1. Cell body: Contains the nucleus and other cell organelles

2. Dendrites: Short, branched extensions of the cell body

*Function: receive stimuli*

3. Axon: long, thin extension of the cell body

*Function: sends nerve impulses away from the cell body*

(Remember “A” for away!)

Structure of a Neuron4. Myelin sheath: insulating membrane *increases speed of impulse*

3 Main Types of Neurons

1. Sensory Neurons (sensory = of the senses) -send impulse from sense organs to spinal cord & brain

2. Interneurons (inter- = between)-relay messages between sensory and motor neurons

3. Motor Neurons (motor = causing motion)-send impulse from brain and spinal cord to effectors

(glands or muscles)

Which are sensory? Which are motor neurons?

How do Neurons Communicate?

1. Dendrites receive the signal (action potential) which then travels down to the axon of the first neuron.

2. When the signal reaches the end of the axon, the axon releases chemicals called neurotransmitters.

ex. acetylcholine, norepinephrine, dopamine, serotonin

3. Neurotransmitters enter into the space between the 2 neurons, called the synaptic cleft.

4. Neurotransmitters bind to receptors on dendrites of the next neuron thereby passing on the signal.

The Neuron• The internal “solution” of the cell

has a characteristic concentration of sodium versus potassium ions

• called: RESTING POTENTIAL– More Na+ outside, more K+ inside– Takes Energy to maintain this

• A signal is produced when this internal “solution” changes

• Called: ACTION POTENTIAL

The Resting NeuronWhen a neuron is at rest

– There is a certain amount of ions inside & outside of cell– This difference in charges is called the resting potential (-70mV)

• The nerve cell membrane pumps sodium (Na+) out of the cell and potassium (K+) into the cell

• Works by active transport• Net result = more K+ inside the cell and more Na+ outside the cell

• The cell membrane leaks K+ back out of the cell– Diffusion – Causing the negative charge inside the cell– Also, organelles inside contribute to the negative charge

The Moving Impulse• An impulse begins when a neuron is stimulated by

another neuron or by the environment– Causes movement of ions across the membrane– Na + rushes in

What kind of transport is this?

• This is called an action potential• Also called “depolarization (+30mV)

• Impulse travels down axon away from the cell body to the axon terminal

• at 110 m/sec (225 mph)

• After impulse passes, K+ flows out of the cell • “repolarization” (-70mV)

What is an action potential?

• A sudden reversal of membrane potential.

• “Resting” is when the pump is active

• “Action” is when the sodium is diffusing into axon

http://www.youtube.com/watch?v=SCasruJT-DU

Threshold

• Strength of impulse is always the same – It is an all or nothing response

• Minimum level of stimulation to cause an impulse is call the thresholdthreshold

• What happens when the action potential What happens when the action potential reaches the end of the axon?reaches the end of the axon?

The Synapse

The area where an impulse is transferred between 2 neurons

• Synaptic cleft separates the axon terminal from the dendrites of the adjacent neuron

The Synapse• The terminals contain tiny sacs (vesicles) that

contain neurotransmitters chemical signals

• Impulse triggers release of neurotransmitters into the synaptic cleft (via exocytosis)– Neurotransmitters diffuse across gap & bind to

receptors on the adjacent neuron– Cause the impulse to continue

(if threshold is reached)

• Neurotransmitters are either broken down or recycled

• This is where drugs interfere

Vesicle

Axon

Axon terminal

Synaptic cleft

Neurotransmitter

Receptor

Dendrite of adjacent neuron

Direction of Impulse

Section 35-2

The Synapse = junction between two neurons