Do Now Name any parts of the nervous system that you know. What do you think some of the functions of the nervous system are?

Neurons are cells specialized to transmit electrical signals, called nerve impulses Your senses, thoughts, and movements are the result of the electrical activity of neurons. .

There are three categories of neurons Sensory neurons respond to stimuli like light, touch, and sound, and send information to either the brain or spinal cord. Motor neurons carry signals from the brain or spinal cord to your muscles, organs, and glands. Interneurons connect neurons to create pathways through the body or brain. Connects sensory and motor neurons together.

Numbers of neurons in each category differ How many? Sensory neurons ~ 10 million Interneurons ~ 20 billion Motor neurons ~ 500,000 Why do you think there are so many more interneurons than other types? Why are there so many fewer motor neurons?

Cellphone Charger as Analogy for Neuron

The cell body is what keeps a neuron alive The cell body contains most of the same organelles that are in other cells. If the cell body dies, the neuron dies.

The nucleus is inside the cell body Neurons rarely divide to make new cells They still use DNA to direct the making of proteins.

Dendrites branch out from the cell body Dendrites are the neuron’s “receivers.” Signals or stimuli reach the cell through the dendrites. Why would you want lots of dendrites in the neurons in your brain but fewer for motor neurons?

Axons are the conductors of the neuron Axons resemble long tails. Nerve impulses—also called action potentials— start off in the axon and spread toward its end, away from the cell body.

Many axons are covered by a myelin sheath The myelin sheath protects the axon and makes the action potential move down the axon more quickly. Why would it be more important to insulate nerves in your arms and legs than nerves in your brain and spinal cord?

Axon terminals contain neurotransmitters

Nerve signals travel only in one direction through every neuron A nerve signal always travels from the “receiver” dendrites through the “conductor” axon and finally to the axon terminals.

Neurons meet at synapses At the synapse, the axons of one neuron pass the signal to the dendrites of another neuron.

Axon terminals have tiny sacs called vesicles that contain neurotransmitters Neurotransmitters carry the nerve impulse across the synapse, from one cell to the next.

Neurotransmitters diffuse across the cleft and bind with receptor proteins on the other side When neurotransmitters bind the receptor proteins, the proteins change shape, opening ion channels and letting sodium ions into the cell. What is an ion? What roles do ions play in our bodies?

What type of neuron do we have most in the brain? Do Now What do the axons do? What type of neuron do we have most in the brain? What is electricity? What do neurotransmitters do?

Sodium ions rush across the cell membrane, changing the charge inside The influx of sodium ions makes the inside of the cell membrane more positively charged in the area near the synapse. This shift in charge is called depolarization.

If enough sodium crosses the membrane, it will trigger an action potential Like the initial push that makes a row of dominos fall, a strong enough depolarization at the synapse will cause the depolarization to spread down the cell. This propagation of the shift in charge is called the action potential or the nerve impulse. It is an all- or-nothing response, and always goes in the same direction.

Potassium ions then move out of the cell, making the charge inside the cell negative again After depolarization, the membrane lets potassium ions out. This restores the negative-inside, positive-outside state of the cell membrane, a process called repolarization. Repolarization propagates in the same direction as the action potential.

Sodium and potassium return to their rightful sides of the cell membrane via active transport This whole process happens quickly enough to fire, reset, and fire your neurons over and over in cycles fast enough to allow you to move smoothly. Do you remember the definition of active transport? Why is it necessary across a cell membrane?