Structure and Function of the Neuron
6 Biological Bases of Behavior
STEP 4 Review the Knowledge You Need to Score High
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 6.3). 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) that 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 6.3 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 transmits messages between the brain and spinal cord. A lack of ACh is associated with Alzheimer’s disease. Dopamine stimulates the hypothalamus to synthesize hormones and affects alertness and movement. A 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. A lack of serotonin is associated with depression. Opioid peptides such as endorphins are often considered the brain’s own painkillers. 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. Norepinephrine, also known as noradrenaline, is associated with attentiveness, sleeping, dreaming, and learning. It is also released as a hormone into the blood where it contracts blood vessels and increases heart rate. 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.
All your behavior begins with the actions of your neurons. A neuron receives 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), a net flow of sodium ions into the cell causes a rapid change in potential across the membrane, known as the action potential (see Figure 6.4). If stimulation is not strong enough, your neuron doesn’t fire. The strength of the action potential is constant whenever it occurs. This is the all-or-none principle.
Figure 6.4 Action potential.
The wave of depolarization and repolarization is passed along the axon to the terminal buttons, which release neurotransmitters. Spaces between segments of myelin are called nodes of Ranvier. When the axon is myelinated, conduction speed is increased since depolarizations jump from node to node. This is called saltatory conduction. Chemical neurotransmitters are released into the synapse where they attach to specific receptor sites on membranes of dendrites of your postsynaptic neurons, like a key fitting into the tumbler of a lock (the lock-and-key concept). Some of your synapses are excitatory, the neurotransmitters cause the neuron on the other side of the synapse to generate an action potential (to fire); other synapses are inhibitory, reducing or preventing neural impulses. The sum of all excitatory and inhibitory inputs determines whether your next neuron will fire and at what rate. The constant flow of these neurochemical impulses gives your behavior its amazing complexity. It regulates your metabolism, temperature, and respiration. It also enables you to learn, remember, and decide.
The simplest form of your behavior, called a reflex, involves impulse conduction over a few (perhaps three) neurons. The path is called a reflex arc. Sensory or afferent neurons transmit impulses from your sensory receptors to the spinal cord or brain. Interneurons, located entirely within your brain and spinal cord, intervene between sensory and motor neurons. Motor or efferent neurons transmit impulses from your sensory or interneurons to muscle cells that contract or gland cells that secrete. Muscle and gland cells are called effectors. Examples of your reflexes include your pupillary reflex, knee jerk, sneezing, and blinking. Neural impulses travel one way along the neuron from dendrites to axons to terminal buttons and among neurons from the receptor to the effector.