Biology  5/e   Raven/Johnson
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Membrane transport: Cell membranes are lipid bilayers that are selectively permeable. Some substances simply diffuse across like water (osmosis) or by passing through a transmembrane protein channel. Some protein channels have carriers that help transport substances; this is facilitated diffusion. Some channels transport substances against their concentration gradients by expending energy and carrying out active transport. The sodium-potassium pump and the proton pump are examples of such channels and are often coupled to the transport of other substances or the synthesis of ATP.

Neuron: The nervous system is made up of cells called neurons. At rest, neurons are polarized as a result of an uneven concentration of Na⁺ and K⁺ ions inside and outside the cell membrane. This polarization is maintained by active transport. When a neuron receives a signal or stimulus above threshold value, it causes a sudden and massive movement of ions into and out of the neuron. This depolarization spreads along the neuron as an electric impulse. When it reaches the axon, it stimulates the vesicles there to release their neurotransmitter into the synaptic cleft. The neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane, which may be part of a muscle cell, another neuron, or an endocrine gland. Depending on the type of neurotransmitter, the recipient cell or organ will be stimulated or inhibited.

Central nervous system: The nervous system of a vertebrate is divided into two major components, the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord. It receives information from sensory neurons in the peripheral system, processes all the incoming information, and then sends out responsive commands to muscles and organs and glands via motor neurons in the peripheral nervous system.

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