Neurons

1. Neurons

2. Neurons • 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.

3. 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

5. 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

6. 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.

7. 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

8. 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

9. 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

10. 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.

11. Receptor sites

12. 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