Some basic biology

The brain consists of neurons and glial cells.
1.1 The brain
The central nervous system (CNS) consists of two main parts: the spinal cord and the brain. 
 
The brain itself is partitioned in several areas. These are: the medulla oblongata, the pons, the cerebellum, the midbrain, the diencephalon, and cerebral hemispheres. Each of these consists of many parts or nuclei. For example, the diencephalon contains the thalamus and the hypothalamus. The cerebral hemispheres consist of the cerebral cortex and deep-lying structures that include the basal ganglia, the hippocampus and the amygdaloid nucleus. 
the brainstem is responsible for maintaining many of the functions necessary for life such as breathing and heart rate. The basal ganglia participate in regulating motor performance and the thalamus processes most of the information reaching the cerebral cortex from the rest of the CNS. The cortex consists of four lobes. The  frontal lobe is largely concerned with the planning and organization of future actions; the occipital lobe is concerned with vision; the pariental lobe receives sensory information; and the temporal lobe is involved with hearing and other aspects of language and memory.
Wernicke’s area, is important to the understanding of spoken language, while another area, called Broca’s area, is concerned with issuing specific commands that cause the mouth and tongue to form words. Hence, damage to Wernicke’s area makes it impossible for a person to understand speech, while damage to Broca’s area makes it impossible for a person to speak.
1.2 The neuron
The structure of a neuron can be divided into three parts: (i) the dendrites; (ii) the soma; and (iii) the axon. 
Although neurons share many common features, it has been estimated that there are as many as 10,000 different types.
The diameter of an axon may range from .2 to 20 µm and may extend for up to one meter. The action potential is initiated at a specialized region of the soma called the axon hillock. Neurons, like all other cells, maintain a potential difference of about 65 mV across their external membrane(到底是多少？)
Action potentials may last as short as 1 ms and may travel at rates that vary between about 1 and 100 meters per second.  After each action potential, there is a period during which a second impulse cannot be initiated. The is referred to as the refractory period. 
Stronger stimuli often produce higher frequencies of impulse firing. Since the shape and speed of each action potential does not depend on the stimulus, information about the stimulus is often carried in the frequency of firing. The frequency is limited by the refractory period. Some neurons are capable of continuously generating action potentials, even without inputs. 
1.3 The synapse
We have seen that chemical synapses can be either excitatory or inhibitory. They can also be classified as direct or indirect. In a direct synapse, the postsynaptic receptor contains both the transmitter binding site and the ion channel opened by the transmitter as part of the same receptor. In an indirect synapse, the transmitter binds to receptors that are not themselves ion channels. Direct synapses are typically much faster than indirect synapses. 
indirect synapses can be modified, through a process called neuromodulation, so that the efficacy of the synaptic response may change (through learning, for example). 
Each neuron in the human brain receives, on average, inputs from about 10000 other neurons.  Synaptic communication need not be in one direction; that is, neurons may make reciprocal synapses onto their presynaptic cells.