Neurons
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.
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