# Control Systems/Nervous System

The nervous system functions by the almost instantaneous transmission of electrochemical signals. Highly specialized cells called neurons control the means of transmission. They are also the functional unit of the nervous system. The neuron is an elongated cell with three parts dendrites, cell body, and axon.
The typical neurone contains many dendrites which appear to look like thin branches that extend from the long branch of the cell body. The axon is a single long projection that extends from the cell body. It usually ends in a few small branches called axon terminals. These neurons are usually connected in chains and networks. They are physically close together, yet never actually come in contact with one another. The gap that separates the axon terminals of one neuron from the dendrites of another neuron is called the synse.
When an electrical impulse moves through the neuron, it starts at the dendrites. From there, it passes through the cell body along the axon. Impulses always follow the same path from the dendrite to the cell body, and then the axon. When the electrical impulse reaches the synapse at the end of the axon, special chemicals called neurotransmitters are released. The neurotransmitters will carry a signal across the synapse to the dendrites of the next neuron to restart the process in the next cell.
When there is no impulse traveling through a neuron, the cell is said to be at its resting potential. The inside of the cell contains a negative charge in relation to the outside. The cell requires energy to maintain this negative charge. The cell membrane of the neuron contains a protein called Na+/K+ ATPase that uses the energy provided by one molecule of ATP to pump 3 positively charged sodium molecules out of the cell. It also simultaneously takes into the cell 2 positively charged potassium ions. Thus, there is a high concentration of sodium ions outside the cell and an excess of potassium ions inside the cell. A sodium leak channel allows some of the potassium ions to flow out of the cell. Nonetheless, the difference in concentrations creates a net potential difference across the cell membrane of about -70mV, which is the value of the resting potential. action potential The action potential is the electrochemical impulse that can travel along the neuron. The neuron membrane also contains voltage-gated proteins. These proteins will respond to changes in the membrane potential by opening and allowing certain ions to cross that would normally not be allowed to do so. The neuron has both voltage-gated sodium channels and voltage-gated potassium channels. Each one will open under different circumstances. The action potential will being when another neuron sends chemical signals to depolarize, or make less negative, the potential of the cell membrane in one localized area of the cell membrane- usually in dendrites. When the neuron is stimulated so that the cell membrane potential reaches -50mV, the voltage-gated sodium channels in that region will open up. The voltage at which these channels open up is called the threshold potential. (in this case, the threshold potential is -50). When the voltage-gated channels open, the sodium ions outside the cell will follow the concentration gradient and rush into the cell. The flood of sodium ions will cause the cell to depolarize and eventually the membrane potential will increase to +35mV. At this point, the voltage-gated sodium channel will close and the voltage-gated potassium channel will reach their threshold and open up. The positive potassium ions concentrated in the cell will now rush out of the neuron to repolarize the cell membrane to its negative resting potential. The membrane potential will now drop to -90mV and the voltage-gated potassium channels will close. After this occurs, the potassium leak channels will restore the membrane back to its original state with a potential of -70mV. This whole process will take place in about one millisecond.

## VERTEBRATE NERVOUS SYSTEM

The vertebrate nervous system can be divided into 2 main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system acts as a central command that receives sensory input from all regions of the body and integrates the information toe create a response. It controls most of the basic functions that are needed for survival, such as breathing, digestion, and consciousness. On the other hand, the peripheral nervous system refers to pathways through which the central nervous system communicates with the rest of the organism. In highly evolved systems, such as the human nervous system, there are 3 types of neural binding blocks: sensory, motor, and interneurons. sensory neurons send information to the central nervous system after the organ's sense organs receive a stimulus from the environment. Another name for these neurons is afferent neurons. motor neurons carry information away from the central nervous system to an organ or muscle as a response to a stimulus or a voluntary action. Another name for these neurons is efferent neurons. interneurons provide connection between sensory neurons and motor neurons. CENTRAL NERVOUS SYSTEM It consists of the brain and the spinal chord. The spinal cord a long cylinder cord that extends along the vertebral column back bone from the head to the lower back. The brain is made of almost entirely interneurons. The cerebrum is the largest portion of the brain and controls consciousness. It controls all voluntary movement, perception, sensory perception, speech, memory, and creative thought. The cerebellum helps to fine-tune voluntary movement, but does not initiate it. It makes sure that movements are coordinated and balanced. The brainstem is responsible for the control of involuntary functions, such as breathing, cardiovascular regulation, and swallow. It is a portion of the medulla oblongata, which is essential for life and processes a great deal of information. The medulla also helps maintain alertness. The hypothalamus is responsible for maintenance of homeostasis. It regulates temperature, controls hunger, manages water balance and helps to generate emotion. The spinal cord contains 3 types of neurons: axons, interneurons, and glial cells. Axons of motor neurons extend from the spinal column into the peripheral nervous system. Interneurons link motor and sensory neurons. Glial cells provide physical and metabolic support for neurons. It serves as a link between the body and the brain so that it can regulate simple reflexes THE PERIPHERAL NERVOUS SYSTEM It is a sensory system that carries information from the senses into the central nervous system from the body and motor system that branches out from the CN to organs or muscles. The motor system can be divided into 2 parts: somatic system and the autonomic system The somatic nervous system is responsible for voluntary or conscious movement. Neurons will only target skeletal muscles needed for bodily movement. All of the neurons in the somatic system release acetylcholine, an excitatory neurotransmitter that causes skeletal muscles to contract. The autonomic nervous system controls tissues other than skeletal muscles. It controls processes that an animal that does not have voluntary control over, such as heart beat, movement of the digestive tract, and contraction of the bladder. It can be subdivided into sympathetic division and parasympathetic division. Sympathetic division works to prepare the body for emergency situations. It increases heart rate, dilates pupils, and increases breathing rate. It also stimulates the medulla of the adrenal glands to release epinephrine and norepinephrine into he bloodstream. Together, it creates the "fight of flight" response. The parasympathetic division is most active when the boy is at rest. It slows the heart rate, increases digestion, and slows breathing. it creates the "rest and digest" response.