Neurons and Neurotransmitters
Explain neuron structure, action potentials, synapses, neurotransmitters, agonists, antagonists, and neural communication.
A lesson of AP Psychology, in Biological Bases of Behavior, about 10 minutes.
By the end of this lesson, you will be able to trace the action potential through a neuron, describe synaptic transmission, and explain how specific neurotransmitters affect behavior.
The Neuron
Neurons are specialized cells that transmit electrical and chemical signals.
Key structures:
- Dendrites: receive signals from other neurons; branch-like extensions
- Cell body (soma): contains the nucleus; integrates signals
- Axon: carries the electrical signal away from the cell body (can be very long — up to 1 meter for spinal neurons)
- Myelin sheath: fatty coating produced by glial cells; speeds transmission; gaps called Nodes of Ranvier
- Terminal buttons: release neurotransmitters into the synapse
The Action Potential
Resting potential: the neuron at rest is polarized (inside −70 mV relative to outside).
Threshold: when incoming signals cause the membrane potential to rise to about −55 mV, an action potential is triggered.
The action potential (firing sequence):
- Depolarization: sodium (Na⁺) channels open; Na⁺ rushes in; membrane becomes positive
- Repolarization: Na⁺ channels close; potassium (K⁺) channels open; K⁺ flows out; membrane returns to negative
- Refractory period: brief period where the neuron can't fire again; Na⁺/K⁺ pump restores resting potential
All-or-none principle: either the threshold is reached and the action potential fires at full strength, or it doesn't fire at all. The intensity of a stimulus is coded by the FREQUENCY of firing, not the size of individual action potentials.
Synaptic Transmission
- Action potential reaches the terminal buttons
- Calcium (Ca²⁺) channels open; calcium flows in
- Vesicles (sacs containing neurotransmitters) fuse with the membrane
- Neurotransmitters released into the synaptic cleft
- Neurotransmitters bind to receptors on the postsynaptic neuron
- Binding causes ion channels to open → neuron fires (excitatory) or is inhibited (inhibitory)
- Neurotransmitters are reuptaken (reabsorbed) into the terminal button, or broken down by enzymes
makes the postsynaptic neuron more likely to fire (depolarizes it)