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Figure 7.2 Regions of the left cerebral cortex in lateral view.

Although specific regions of the brain are associated with specific functions, if one region is damaged, the brain can reorganize to take over its function, which is called plasticity. In phantom limb syndrome, a somewhat unfortunate example of plasticity, reorganization of the somatosensory cortex leads to experiencing sensations where a missing limb used to be.

Structure and Function of the Neuron

Your extraordinarily complex brain is composed of trillions of neurons and glial cells. Glial cells guide the growth of developing neurons, help provide nutrition for and get rid of wastes of neurons, and form an insulating sheath around neurons that speeds conduction. The neuron is the basic unit of structure and function of your nervous system. Neurons perform three major functions: receive information, process it, and transmit it to the rest of your body. Three major regions of a neuron enable the cell to communicate with other cells (see Figure 7.3). The cell body (a.k.a. cyton or soma) contains cytoplasm and the nucleus, which directs synthesis of such substances as neurotransmitters. The dendrites are branching tubular processes capable of receiving information. The axon emerges from the cyton as a single conducting fiber (longer than a dendrite) which branches and ends in tips called terminal buttons, axon terminals, or synaptic knobs. The axon is usually covered by an insulating myelin sheath (formed by glial cells). Neurogenesis, the growth of new neurons, takes place throughout life.

Figure 7.3 Typical neurons.

Neurotransmitters are chemicals stored in structures of the terminal buttons called synaptic vesicles. Different neurotransmitters have different chemical structures and perform different functions. For example, acetylcholine (ACh) causes contraction of skeletal muscles, helps regulate heart muscles, is involved in memory, and also transmits messages between the brain and spinal cord. Lack of ACh is associated with Alzheimer’s disease. Dopamine stimulates the hypothalamus to synthesize hormones and affects alertness and movement. Lack of dopamine is associated with Parkinson’s disease; too much dopamine is associated with schizophrenia. Glutamate is a major excitatory neurotransmitter involved in information processing throughout the cortex and especially memory formation in the hippocampus. Both schizophrenia and Alzheimer’s may involve glutamate receptors. Serotonin is associated with sexual activity, concentration and attention, moods, and emotions. Lack of serotonin is associated with depression. Opioid peptides such as endorphins are often considered the brain’s own pain killers. Gamma-aminobutyric acid (GABA) inhibits firing of neurons. Benzodiazepine (Valium) and anticonvulsant drugs increase activity of GABA. Huntington’s disease is associated with insufficient GABA-producing neurons in parts of the brain involved in coordination of movement. Seizures are associated with malfunctioning GABA systems. Other chemicals, such as drugs, can interfere with the action of neurotransmitters. Agonists may mimic a neurotransmitter and bind to its receptor site to produce the effect of the neurotransmitter. Antagonists block a receptor site inhibiting the effect of the neurotransmitter or agonist.

Figure 7.4 Action potential.

Neuron Functions

All your behavior begins with the actions of your neurons. A neuron gets incoming information from its receptors spread around its dendrites. That information is sent to its cell body, where it’s combined with other incoming information. Neural impulses are electrical in nature along the neuron. The neuron at rest is more negative inside the cell membrane relative to outside of the membrane. The neuron’s resting potential results from the selective permeability of its membrane and the presence of electrically charged particles called ions near the inside and outside surfaces of the membrane in different concentrations. When sufficiently stimulated (to threshold),