The Neuron An everyday (every second!) use of active transport OPENER  Do the wave!

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).

Structure of a Neuron 4. Myelin sheath: insulating membrane *increases speed of impulse* 1. Cell body: Contains the nucleus and other cell organelles Axon terminals 4. Myelin sheath Nodes 1. Cell body 3. Axon Nucleus 2. Dendrites 3. Axon: long, thin extension of the cell body *Function: sends nerve impulses away from the cell body* (Remember “A” for away!) 2. Dendrites: Short, branched extensions of the cell body *Function: receive stimuli*

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? Top arrows = sensory; bottom arrows = motor

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 Neuron When 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) Passive transport (high to low)

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 threshold What happens when the action potential 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

Dendrite of adjacent neuron The Synapse = junction between two neurons Section 35-2 Direction of Impulse Dendrite of adjacent neuron Axon Receptor Vesicle Axon terminal Synaptic cleft Neurotransmitter