Chapter Summary

• The cells of the nervous systems include many types of neurons and glia.

• All neurons can generate and conduct ionic electrical signals, and most can generate action potentials (APs). Glia support and modulate the activities of neurons but do not generate APs.

• A neuron generally receives information via its dendrites, of which there can be many, and transmits information via its single axon, which ends in axon terminals. Review Figures 44.1, 44.2

• Where neurons and their target cells meet, information is transmitted across specialized junctions called synapses.

• Glia include Schwann cells and oligodendrocytes, both of which generate myelin sheaths on axons. Review Figure 44.3

• Glia also include astrocytes, which support neurons metabolically, modulate synaptic signaling, and contribute to the blood–brain barrier.

• Neurons have an electric charge difference across their cell membranes. This membrane potential is generated by ion gradients (due to ion transporters) and ion channels. When a neuron is not stimulated, its membrane potential is referred to as the resting potential. Review Figure 44.4, Animation 44.1

• The sodium–potassium pump concentrates K⁺ on the inside of a neuron and Na⁺ on the outside. Leak K⁺ channels in the cell membrane allow K⁺ to diffuse out leaving behind unbalanced negative charges, creating the resting potential. Review Figure 44.5

• Patch clamping allows the study of single ion channels. Review Figure 44.6

• The resting potential is perturbed when ion channels open or close, changing the permeability of the cell membrane to charged ions. Through this mechanism, the cell membrane can become depolarized or hyperpolarized and therefore have a graded membrane potential response to input. Review Figure 44.7

• An AP is a rapid transient reversal in charge across a portion of the cell membrane resulting from the sequential opening and closing of voltage-gated Na⁺ and K⁺ channels. These changes in voltage-gated channels occur when the cell membrane depolarizes to a threshold level. Review Figure 44.8, Animation 44.2

• APs are all-or-none, self-regenerating events. They are conducted down axons because local current flow depolarizes adjacent regions of membrane and brings them to threshold. Review Figure 44.9

• In myelinated axons, APs appear to jump between nodes of Ranvier, areas of axonal cell membrane that are not covered by myelin. Review Figure 44.10

• Neurons communicate with each other and with other cell types by transmitting information over electrical synapses or by the transmission of molecular signals called neurotransmitters over chemical synapses.

• The neuromuscular junction is a well-studied chemical synapse between a motor neuron and a skeletal muscle cell. Its neurotransmitter is acetylcholine (ACh), which causes depolarization of the postsynaptic membrane when it binds to its receptor at the motor end plate. Review Focus: Key Figure 44.11, Animation 44.3

• When an AP reaches an axon terminal, it causes the release of neurotransmitters, which diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane. See Activity 44.1

• Synapses between neurons can be either excitatory or inhibitory. A postsynaptic neuron integrates information by summing excitatory and inhibitory postsynaptic potentials both spatially and temporally. Review Figure 44.12

• There are many different neurotransmitters and even more types of receptors. The action of a neurotransmitter depends on the type of receptor to which it binds. See Activity 44.2

• Neurotransmitter molecules cannot be allowed to accumulate in a synapse but must be cleared to turn off responses in the postsynaptic cell. This may be done by enzymatic degradation, simple diffusion, or reuptake of the neurotransmitter.

• In vertebrates, the brain and spinal cord form the central nervous system (CNS), which communicates with the rest of the body via the peripheral nervous system (PNS). The CNS increases in complexity from invertebrates to vertebrates and from fish to mammals. Review Figures 44.14, 44.15

• Neural networks include afferent neurons and efferent neurons, generally connected through interneurons.

• A spinal reflex is an example of a simple neural network that integrates information and controls a response. Review Figure 44.15, Animation 44.4