Nearby areas of membrane are depolarized to the threshold for Na + channel activation, propagating a wave of depolarization from the initial site. If a depolarizing stimulus raises the membrane potential to about −45 mV, voltage-gated Na + channels open, allowing influx of Na + and further depolarization toward E Na (±50 mV). Binding sites for neurotransmitters such as glutamate, γ-aminobutyric acid (GABA), glycine, and acetylcholine exist on ligand-gated channels and, when occupied, induce a conformational change to open the channel.Īn action potential arises primarily from voltage-dependent changes in membrane permeability to Na + and K + ( Figure 7–2). An array of charged amino acids within voltage-dependent channels detects changes in voltage and induces a conformational change in the channel to alter ion permeability. In general, channels are selective for a particular species of ion. These channels are composed of protein complexes embedded in the lipid membrane to form aqueous pores to the inside of the cell. This is accomplished by ligand-gated and voltage-gated ion channels that allow the passage of Na +, K +, Ca 2+, or Cl − ions in response to electrical or chemical stimuli. Neurons are highly specialized to use rapid changes in membrane potential to generate electrical signals. The membrane potential may be altered by increasing the permeability of the membrane to another ion, which drives the resting membrane potential toward the equilibrium potential for that ion. Synapses between neurons most often occur between axons and dendrites but may occur between an axon and a cell body, between two axons, or between two dendrites. Each branch of the axon terminates on the next cell at a synapse, which is a structure specialized for information transfer from the axon to muscle, to glands, or to another neuron.
Axon branching allows several target cells to simultaneously receive a message from one neuron. Dendritic branching can be very complex, with the result that a single neuron may receive thousands of inputs. Axons and dendrites usually branch extensively at their ends. Spinal sensory ganglia contain pseudounipolar neurons that have a single process that emanates from the cell body and divides into two branches, one extending to the spinal cord and the other extending to the periphery. Bipolar neurons have one dendrite and one axon and are found in the cochlear and vestibular ganglia, retina, and olfactory mucosa. Most neurons are multipolar, containing one axon and several dendrites. Neurons consist of three parts: dendrites, which are elongated processes that receive information from the environment or from other neurons the cell body, which contains the nucleus and the axon, which may be up to 1 m long and conducts impulses to muscles, glands, or other neurons ( Figure 7–1). The major function of neurons is to receive, integrate, and transmit information to other cells.