[Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them. The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone! While variable in size and shape, all neurons have three parts. Open your book to Fig. 16.2 Page 280 before proceeding]

Dendrites receive information from another cell and transmit the message to the cell body. The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon conducts messages away from the cell body and will usually display axon bulbs in a diagram to distinguish them from dendrites. Be able to label the the three parts on a digram.

Three types of neurons occur. Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the central nervous system. Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the effector (muscles or glands). Interneurons have short dendrites and long or short axons and are found only within the central nervous system where they connect neuron to neuron. Sometimes a sensory neuron is refered to as an afferent neuron and a motor neuron is called the efferent neuron.

[Turn to figure 16.5 Page 283 before proceeding] A nerve impulse is the way a neuron transmits information. The plasma membrane of neurons (the axomembrane), like all other cells, has an unequal distribution of ions and electrical charges between the two sides of the membrane. The outside of the membrane has a positive charge, inside has a negative charge. This charge difference is a resting potential and is measured in millivolts. Passage of ions across the cell membrane passes the electrical charge along the cell. The voltage potential is -65mV (millivolts) of a cell at rest (resting potential). Resting potential results from differences between sodium and potassium positively charged ions and negatively charged ions in the cytoplasm. Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane. This imbalance is maintained by the active transport of ions to reset the membrane known as the sodium potassium pump. The sodium-potassium pump maintains this unequal concentration by actively transporting ions against their concentration gradients. The sodium-potassium pump pumps out sodium and pumps in potassium. It must work continuously because the membrane is somewhat permeable to both substances. The membrane is slightly more permeable to potassium than sodium which accounts for the resting potential of -65 mV. Changed polarity of the membrane, the action potential, results in propagation of the nerve impulse along the membrane. An action potential is a temporary reversal of the electrical potential along the membrane for a few milliseconds when the voltage difference inside the cells jumps to + 40 mV. Sodium gates and potassium gates open in the membrane to allow their respective ions to cross. Sodium and potassium ions reverse positions by passing through membrane protein channel gates that can be opened or closed to control ion passage. Sodium crosses first and the voltage difference inside the cells jumps to + 40 mV (depolarization). At the height of the membrane potential reversal, potassium channels open to allow potassium ions to pass to the outside of the membrane. Potassium crosses second, resulting in changed ionic distributions, which must be reset by the continuously running sodium-potassium pump. Eventually enough potassium ions pass to the outside to restore the membrane charges to those of the original resting potential (repolarization).The cell begins then to pump the ions back to their original sides of the membrane.

The action potential begins at one spot on the membrane, but spreads to adjacent areas of the membrane, propagating the message along the length of the cell membrane. After passage of the action potential, there is a brief period, the refractory period (recovery period), during which the membrane cannot be stimulated. This prevents the message from being transmitted backward along the membrane. threshold ("all-or-none response") - Wether or not a neuron fires (conducts a nerve impulse) depends on summation, the net effect of all the excitatory and inhibitory stimuli it is recieving. If enough sodium ion channels open, excitation is sufficient to raise the membrane above the threshold level and the neuron fires. Otherwise it does not fire. It is an all or none response because either the stimuli are sufficient to make the neuron fire or they are not sufficient. A stimulus that is half the intensity of one which was just able to make the neuron fire will not cause the neuron to fire or to half fire. It is all or nothing. To make an analogy there are no "half marks". It's all or nothing.

1. At rest the outside of the membrane is more positive than the inside.

2. Sodium moves inside the cell causing an action potential, the influx of positive sodium ions makes the inside of the membrane more positive than the outside.

3. Potassium ions flow out of the cell, restoring the resting potential net charges.

4. Sodium ions are pumped out of the cell and potassium ions are pumped into the cell, restoring the original distribution of ions. (recovery period)

[Turn to Figure 16.3 Page 280 before continuing] Some axons are wrapped in a myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann cells. Schwann cells serve as supportive, nutritive, and service facilities for neurons. The myelin serves as an excellent electrical insulator. The gap between Schwann cells is known as the node of Ranvier, and serves as points along the neuron for generating a signal. Signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon. This allows your brain to communicate with your toes in a few thousandths of a second. [Because of the manner in which the Schwann cells wrap themselves around the nerve fiber two sheaths are formed. The inner is the above mentioned myelin sheath and the outer is the neurilemma. In the peripheral nervous system the neurilemma plays an important role in nerve regeneration.]

[Turn to figure 16.7 Page 285 before continuing] The junction between a nerve cell and another cell is called a synapse. Messages travel within the neuron as an electrical action potential. But neurons do not have an electrical connection with each other. They are in fact seperated by a small space. The space between two cells is known as the synaptic cleft. For the signal to cross the synaptic cleft requires the actions of neurotransmitters. Neurotransmitters are stored in small synaptic vessicles clustered at the tip of the axon which migrate the cleft to the dendrite of the next neuron. The membrane of the axon carrying the signal to the cleft is termed the presynaptic membrane and membrane of the dendrite recieving the signal on the other side is termed the postsynaptic membrane.

Arrival of the action potential causes the axomembrane to become permeable to calcium ions. These ions interact with the microfilaments causing some of the vesicles (containing neurotransmitters) to move to the end of the axon and discharge their contents into the synaptic cleft. Released neurotransmitters diffuse across the cleft, and bind to receptors on the other cell's membrane, causing ion channels on that cell to open. Some neurotransmitters cause an action potential, others are inhibitory. Neurotransmitters tend to be small molecules, some are even hormones. The time for neurotransmitter action is between 0,5 and 1 millisecond. Neurotransmitters are either destroyed by specific enzymes in the synaptic cleft, diffuse out of the cleft, or are reabsorbed by the cell. More than 30 organic molecules are thought to act as neurotransmitters. The neurotransmitters cross the cleft, binding to receptor molecules on the next cell, prompting transmission of the message along that cell's membrane. Acetylcholine is an example of a neurotransmitter, as is norepinephrine, although each acts in different responses.

[Diseases that affect the function of signal transmission can have serious consequences. Parkinson's disease has a deficiency of the neurotransmitter dopamine. Progressive death of brain cells increases this deficit, causing tremors, rigidity and unstable posture. L-dopa is a chemical related to dopamine that eases some of the symptoms (by acting as a substitute neurotransmitter) but cannot reverse the progression of the disease.

The bacterium Clostridium tetani produces a toxin that prevents the release of GABA. GABA is important in control of skeletal muscles. Without this control chemical, regulation of muscle contraction is lost; it can be fatal when it effects the muscles used in breathing. Clostridium botulinum produces a toxin found in improperly canned foods. This toxin causes the progressive relaxation of muscles, and can be fatal. A wide range of drugs also operate in the synapses: cocaine, LSD, caffeine, and insecticides.]

Once in the cleft, neurotransmitters are active for only a short time. In some synapses the cleft contains enzymes that inactivate the neurotransmitters. For example, acetylecholinesterase breaks down acetylcholine. Inactivated neurotransmitters are taken back into the axon and recycled. In other synapses, the synaptic ending rapidly absorbs the neurotransmitter substance, possibly for repackaging in synaptic vessicles or for chemical breakdown. For example, the enzyme monoamine oxidase breaks down norepinephrione after it is absorbed. The short existence of neurotransmitters in the synapse prevents continuous stimulation or inhibition of postsynaptic membranes.

[Turn to Figure 16.11 Page 288 before continuing] The reflex arc allows an automatic, involuntary reaction to a stimulus. When the doctor taps your knee with the rubber hammer, she/he is testing your reflex (or knee-jerk). The reaction to the stimulus is involuntary, with the CNS being informed but not consciously controlling the response. Examples of reflex arcs include balance, the blinking reflex, and the stretch reflex. The reflex arc is essentially a short cut between your receptor sensing a potentially harmful situation and your muscles which can remove the receptors from that situation. It is a short cut because normally information from the receptor would travel up the spinal cord to the brain for processing and then back down the cord and to the muscle with the action decided on. In the reflex arc the sensory receptor sends a signal to the spinal cord where an interneuron connects it immediatly to the appropriate effector muscle. This saves time to spare the body of furhter potential harm. For example, If you touch a very hot object, a receptor in the skin generates nerve impulses which move along the dendrite of a sensory neuron toward the cell body in the dorsal-root ganglion just outside the cord. From the cell body the impulses travel along the axon of the sensory neuron and enter the cord by way of the ventral root of a spinal nerve. The impulses then pass to many interneurons, one of which connects to a motor neuron. The impulse will travel through the dendrite, cell body and finally axon of the motor neuron and ultimately cause the muscle fibers to contract so that your hand is pulled away from the hot object. Connections to other interneurons also cause you to look towards the object, jump back and utter appropriate exclamations.