The Blood-Brain Barrier
THOUGHT QUESTION The fact that the mitochondria in our cells were originally microorganisms that infected our very remote ancestors points out that evolution can involve interactions between two or more species. Most species have other organisms living inside them; in fact, the bacteria in our intestines are necessary for our good health. Some microorganisms can exchange genetic information, so adaptive mutations developed in one species can be adopted by another. Is it possible that some of the features of the cells of our nervous system were bequeathed to our ancestory by other species? KEY CONCEPTS Neurons have soma, dendrites, an axon, and terminal buttons. Circuits of interconnected neurons are responsible for the functions performed by the nervous system. Neurons are supported by glia and by Schwann cells, which provide myelin sheaths, houskeeping services, adn physical support. The bloof-brain barrier helps to regulate the chemicals that reach the brain.
Neural Communication: An Overview
Measuring Electrical Potentials of Axons
The Membrane Potential: Balance of Two Forces
The Action Potential
Conduction of the Action Potential
THOUGHT QUESTION The evolution of the human brain, with all its complexity, depended on many apparently trivial mechanisms. For example, what if cells had not developed the ability to manufacture myelin? Unmyelinated axons must be very large if they are to transmit action potentials rapidly. How big would the human brain have to be if oliodendcytes did not produce myelin? Could the human brain as we know it have evolved without myelin? KEY CONCEPTS The action potential occurs when the membrane potential of an axon reaches the threshold of excitation. Although the action potential is electrical, it is caused by the flow of sodium and potassium ions through voltage-dependent ion channels in the membrane. Saltatory conduction, which takes place in myelinated axons, is faster and more efficient than conduction in unmyelinated axons.
Structure of Synapses
Release of Neurotransmitters
Activation of Receptors
Termination of Postsynaptic Potentials
Effects of Postsynaptic Potentials: Neural Integration
Nonsynaptic Chemical Communication
THOUGHT QUESTIONS Why does synaptic transmission involve the release of chemicals? Direct electrical coupling of neurons is far simpler, so why do our neurons not use it more extensively? (A tiny percentage of synaptic connections in the human brain do not use electrical coupling). Normally, nature uses the simplest means possible to a given end, so there must be some advantages to chemical transmission. What do you think they are? Consider the control of the withdrawal reflux illustrated in Figure 11. Could you design a circuit using electrical synapses that would accomplish the same task? KEY CONCEPTS Neurons communicate by means of synapses, which enable the presynaptic neuron to produce excitatory or inhibitory effects on the postsynaptic neuron. These effects increase or decrease the rate at which the axon of the postsynaptic neuron sends action potentials down to its terminal buttons. When an action potential reaches the end of an axon, it causes some synaptic vesicles to release a neurotransmitter into the synaptic cleft. Molecules of the neurotransmitter attach themselves to receptors in the postsynaptic membrane. When they become activated by molecules of the neurotransmitters, postsynaptic receptors produce either excitatory or inhibitory postsynaptic potentials by opening neurotransmitter-dependent sodium, potassium, or chloride ion channels. The postsynaptic potential is terminated by the destruction of the neurotransmitter or by its reuptake into the terminal button. Autoreceptors help to regulate the amount of neurotransmitter that is released. Axoaxonic synapses consist of junctions between two terminal buttons. Release of neurotransmitter by the first terminal button increases or decreases the amount of neurotransmitter released by the second. Neuromodulators and hormones have actions similar to those of neurotransmitters: They bind with and activate receptors on or in their target cells.