NeuronsNeurons

Definition: An individual nerve cell

100 billion neurons make up the brain

Neurons link to each other in tight clusters and long “chains”

Millions of neurons must send messages at the same time to produce a single thought.

Each neuron in the brain is linked to as many as 15,000 other neurons.

Neurons

Neurons have 4 basic parts:

Dendrites – neuron fibers that receive incoming messages

Soma (cell body) – main body of a neuron

Axon – thin fiber that carries information away from the cell body of a neuron

Axon terminals, or terminal buttons – branching fibers at the end of axons that link to the dendrites and somas of other neurons so information can be passed from neuron to neuron

The Nerve Impulse Electrically charged chemical molecules,

called ions, are found in and around neurons

Some ions are negatively charged, and some are positively charged

The inside of a human neuron has an electrical charge of about -70 millivolts

(Inside the neuron is a negative charge, outside the neuron is a positive charge.)

Resting potential – the electrical charge of a neuron at rest

The Nerve Impulse cont’d…

Messages received from other neurons alter the resting potential.

If the resting potential changes enough, the cell reaches its threshold, or trigger point, for firing.

The threshold for human neurons is approx -50 millivolts

When a neuron reaches its threshold, an action potential, or nerve impulse, moves down the axon at up to 200 mph.

The Action Potential The axon membrane is pierced by tiny molecular

channels called ion channels

These channels are blocked when the neuron is at its resting potential.

During the action potential, the channels open, allowing sodium ions (Na+) to rush into the axon, beginning at the soma and moving toward the axon terminals, briefly creating a positive charge within the axon.

This is an all-or-nothing event – the impulse occurs completely, or not at all

Definitions

Synapse – microscopic space between 2 neurons over which messages pass

Neurotransmitters – any chemical released by a neuron that alters activity in other neurons

Synaptic vesicles – tiny sacs in the axon terminal that store neurotransmitters

Receptor sites – tiny areas on the surface of neurons and other cells that are sensitive to neurotransmitters or hormones

Definitions cont’d… Acetylcholine – neurotransmitter released by neurons to

activate the muscles

Neuropeptides – brain chemicals that regulate the activity of neurons

Enkephalins – opiate-like brain chemicals that regulate reactions to pain and stress

Endorphins – chemicals that are similar in structure and pain-killing effect to opiate drugs such as morphine; released by the pituitary gland

Dopamine – neurotransmitter involved in the control of bodily movements (involved in Parkinson’s Disease and Alzheimer’s)

Serotonin – mood control

Communication Between Neurons

Communication between neurons is chemical.

When an action potential reaches the tip of the axon terminal, the synaptic vesicles move to the surface and release neurotransmitters into the synaptic gap

Neurotransmitter molecules cross the synaptic gap and attach to receptor sites on the receiving neuron.

Neurotransmitters either excite or inhibit the receiving neuron

A neuron needs to receive several “exciting” messages at about the same time to fire the action potential.

Receptor sites

Nerves

• Nerves are large bundles of axons and dendrites

• Nerves have a whitish color b/c they contain axons coated with a fatty layer of tissue called myelin.

• Neurilemma – thin layer of cells that encases most axons outside of the brain and spinal cord – creates a tunnel damaged fibers can follow to repair

themselves

CELL BODY

AXON

Myelin sheath

Schwann cellNode of Ranvier

Synaptic terminals

Dendrites NucleusSynapses