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
CELL BODY
Dendrites
Myelin sheath
AXON
Schwann cell
Node of Ranvier
Synaptic terminals
Nucleus
Synapses