1 Neurons and Neural Signaling
Learn how neurons receive and integrate signals, generate and propagate electrical activity, communicate at synapses, and depend on glial support.
Neural signaling at a glance
Neuroscience studies how the nervous system receives, processes, and sends information. Neurons and glial cells are its basic working units. Connected neurons form networks that use electrical and chemical signals to coordinate the nervous system's functions.
Neuron structure and signal flow
A typical neuron has four main regions, though neurons vary greatly in shape and connectivity. This layout is a useful model, not a rule for every neuron.
Dendrites receive many incoming signals.
The cell body (soma) contains the nucleus, supports the cell's metabolism, and integrates incoming signals.
The axon carries electrical signals away from the soma. Near its beginning, the axon initial segment is a common site where an starts.
Axon terminals form connections with target cells, often at synapses.
Neurons may signal to other neurons, muscles, or glands.
Membrane voltage and action potentials
The neuronal membrane separates fluids with different ion concentrations. Transport proteins help maintain these ion gradients. At rest, the membrane is especially permeable to potassium ions, written as , so the inside of a neuron is usually electrically negative relative to the outside. This voltage difference is the .
Inputs can produce : small, local changes in membrane voltage whose size depends on the input. can combine when inputs arrive close together in time or at different locations. If their combined effect brings the axon initial segment to threshold, voltage-gated channels trigger an .
An is a brief, all-or-none electrical event. Sodium ions, written as , move into the cell and rapidly depolarize the membrane. Sodium-channel inactivation and potassium ions moving out then repolarize it. A brief refractory period follows and helps limit immediate refiring.
A stronger stimulus generally does not make an individual larger. Instead, stimulus strength can be represented by changes such as the neuron's firing rate.
Propagation along the axon
Action potentials propagate along the axon. In myelinated axons, the insulating is interrupted by nodes of Ranvier. The electrical signal is regenerated at these nodes, allowing rapid .
Synaptic communication
At a , an arriving opens voltage-gated calcium channels in the axon terminal. Calcium entry triggers synaptic vesicles to release neurotransmitter into the synaptic cleft. The transmitter binds to receptors on the receiving cell, changing ion flow and therefore the postsynaptic cell's voltage.
Depending on the receptors and ions involved, the effect can make the receiving cell more or less likely to fire an . The transmitter is then cleared or recycled, ending or limiting its effect. A synapse's effect depends on its receptor and cellular context, not simply on the identity of the neurotransmitter.
A neuron integrates input from many synapses; if the combined influence is sufficient, it can generate an . Electrical synapses also occur: channels directly connect neighboring cells, allowing activity to transmit very quickly.
Glial cells and their roles
help create and maintain the conditions neurons need to function. They are not merely passive scaffolding: they support neural development, regulate the local chemical environment, influence signaling, and respond to injury.
Astrocytes help regulate ions and neurotransmitters around synapses, support neurons metabolically, and interact with blood vessels.
Oligodendrocytes make myelin in the central nervous system, consisting of the brain and spinal cord. Schwann cells make myelin in the peripheral nervous system.
Microglia act as resident immune cells, monitoring neural tissue and helping clear debris.
Ependymal cells line fluid-filled spaces in the brain and spinal cord and are associated with cerebrospinal-fluid functions.