1. Most neurons possess a profusion of dendrites to receive simultaneous input from several different sources. A neuron's cell body integrates information received from many different dendrites. The impulse travels from dendrite to cell body to axon. The axon then transmits the neuron's impulse to a target organ or gland, or another neuron.
2. Neuroglia cells provide structural and nutritional support to neurons and are non-nervous transmission components of the nervous system. Only Schwann cells produce the myelin sheath, which provides insulation. The nodes of Ranvier are breaks in the myelin sheath where an axon is in direct contact with the intercellular fluid.
3. K+ is more highly concentrated inside the cell; Na+ is more highly concentrated outside the cell membrane. These concentration differences are maintained by active transport during a nerve cell's resting potential. At resting potential, the plasma membrane is most permeable to K+.
4. A voltage-gated ion channel is a channel that will open or close when triggered by the appropriate membrane potential. When an action potential is generated, membrane proteins are changed so that sodium is freely admitted into the cell. This causes other channels to open, and so on, which passes the depolarization along the length of the fiber. As the action potential progresses, sodium channels close and postassium channels open second. This allows potassium ions to diffuse out of the cell which serves to restore the resting potential of the cell. Any stimulus strong enough to cause depolarization of the membrane will open both kinds of channels.
5. The "all-or-none" law states that a nerve cannot conduct a partial impulse; it is an all-or-nothing process. The threshold value of an impulse is the amount of stimulation required to open enough sodium channels to initiate depolarization. The action potential is the shift of ions and consequent shift in electrical charge; not all neurons possess the same action potential. It is dependent upon the density of the sodium ion channels. The refractory period is the time required to restore the original ion concentrations, during which another series of depolarization events cannot occur.
6. Action potentials remain the same throughout transmission of the impulse, creating the "domino effect." Myelination and increase in nerve diameter, in general, will both increase the velocity at which an action potential is conducted. The region between Schwann cells is the node of Ranvier. These nodes are in direct contact with the intercellular fluid, and the action potential jumps from node to node. This type of conduction (called saltatory conduction) is advantageous because it travels faster than an impulse passed in continuous fashion, and requires less metabolism than depolarizing and repolarizing an entire length of neuron.
7. Neurotransmitters are chemicals stored in vesicles in the axon terminal. A synapse is the junction between an axon and another cell. The axon side is the presynaptic membrane and the target cell side is the postsynaptic membrane. The nerve impulse crosses the synapse by the release of neurotransmitter chemicals from the vesicles at the presynaptic membrane, triggered by membrane depolarization, which combine with receptor molecules on the postsynaptic membrane, effecting a response. Neurotransmitters can have a stimulatory or inhibitory effect on the post-synaptic membrane.
8. A reflex is a response that is an automatic consequence of nerve stimulation, exerts no control over complex actions, and shows little coordination. The simplest reflex arc involves a sensory neuron, a motor neuron, and possibly an interneuron.
9. The diencephalon contains the thalamus-a relay station and component of visceral control-and the hypothalamus-interprets primitive sensations such as hunger, fear, thirst. The hypothalamus is also responsible for secreting hormones that impact on the pituitary gland.
10. The primary motor cortex and primary somatosensory cortex are located on the frontal lobe and the parietal lobes, respectively, where they come into contact with each other. The right halves of these cortices serve to coordinate the actions of, and interpret information from, the left side of the body.
11. The reticular activating system is located in the pons and serves as an "alert system," activating the nervous system to full capacity when required.
12. Short-term memory is transient and easily lost, so its storage in the brain may be in the form of a transient impulse. Long-term memory, on the other hand, probably involves a structural change within the cortex. Long-term potentiation occurs when a synapse is used intensively for a short period of time. The neurotransmitter may "sensitize" the receptor, making it perhaps more important as a mechanism of memory storage.
13. Sympathetic axons leave the top and bottom of the spinal cord; parasympathetic ones come off the spinal cord in the chest and abdominal areas. In both systems, the preganglionic neurons are in the CNS, and the postganglionic neurons synapse with the viscera. The parasympathetic system secretes acetylcholine as a neurotransmitter, while the sympathetic system secretes norepinephrine. If the sympathetic system is stimulatory for one particular organ, the parasympathetic system will most likely be inhibitory.