The Nervous System
The nervous system plays a major part in the body’s function of moving, responding to stimuli, and maintaining homeostasis or a stable environment. It is a very general term and is broken down into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The PNS is further broken down into the Autonomic and Somatic Nervous Systems, and the Autonomic system is divided into Sympathetic and Parasympathetic functions. All of these systems function differently but are interdependent to keep the body working properly. A commonality is the use of neurons to carry messages between parts of the system, providing the cell communication that makes the system so efficient. When all these factors work together, the body is in full communication with itself.
Central Nervous System
The central nervous system consists of the brain, the spinal cord, and the nerves that connect them. The structures are isolated by a blood-brain barrier, blood vessels that have small gaps to let only a few materials through, as protection. The brain is the control center, where all stimulus information goes to and all instructions for movement or functions come from. The spinal cord runs down the body and primarily collects messages from the neurons in the body to send them up to the brain. Its other use is the reflex arc, in which a stimulus that is labeled as dangerous, such as touching a hot stove, goes to the spinal cord. Without going up to the brain to save time and prevent further damage, the spinal cord sends a response back, causing a path to go from muscle to spinal cord to muscle.
Peripheral Nervous System
The Peripheral Nervous System is all of the nerves in the body not found in the spinal cord or brain. Most are found in the muscles or organs so that messages can be delivered to maintain function or stimulate movement. For example, the legs can get messages via the PNS to start walking or the heart can be stimulated to beat faster when running. The PNS is connected to the CNS by the spinal cord, so impulses are carried down and away from the brain and are picked up by nerve cells of the PNS. From here, the PNS is further divided into the Autonomic and Somatic Nervous Systems.
Somatic vs Autonomic vs Enteric
The PNS is further divided into the somatic and autonomic nervous systems. The autonomic nervous system includes all of the subconscious actions and functions. For example, since you continuously breathe without telling your body to inhale to bring in oxygen to go to your muscles and then exhale waste carbon dioxide, breathing is an autonomic function. Somatic functions are the opposite, meaning that they require a message from the brain to tell them to happen. For example, if you want to walk across the room, you control this movement with the direction from your brain, telling your muscles to move your legs and carry you across the room. The enteric nervous system consists of all parts of the gastrointestinal system, working to break down food into usable energy. It works independently of the other nervous systems, but it both influences and is influenced by the somatic and autonomic nervous systems.
Sympathetic and Parasympathetic
Part of the function of the autonomic system is to respond to stimuli that require action by the body. Specifically, the sympathetic system responds when there is a danger, sending the body into fight, flight, or freeze mode. Triggering the release of epinephrine (adrenaline) from the adrenal glands, the sympathetic system increases heart rate and breathing rate, contracts blood vessels, and stops digestion to put all available energy into the fight or flight response. This feeling is the rush of adrenaline you feel when someone jumps out from behind you or a car speeds past where you’re standing, almost hitting you. The body is preparing itself to fight a predator or run away as a method of survival. This response, however, takes a great deal of energy that the body cannot sustain for long amounts of time, so the parasympathetic system functions to release norepinephrine (noradrenaline) to return body levels to normal. Therefore, in the release of this hormone results in slowed breathing and heart rate, dilation of blood vessels, and the resumption of digestion.
The Neuron
All of the structures of the brain and nervous system are made up of neurons.The neuron is the type of cell that makes up the structures of the neurological system. With the telltale shape, parts of the neuron include the dendrites, the cell body, the axon, and the synapse. The cell body is found at the center and contains the nucleus and organelles that control the cell. Protruding from one side is the network of dendrites, long strings that connect each neuron with up to 7,000 others and make the brain into one large circuit. Coming out of the other side of the cell body is the axon, which is a long strand covered in myelin that conducts impulses toward the next neuron. The synapse is found at the end of the axon and is the site where neurotransmitters are released and impulses leave the neuron. Three general types of neurons make up the system: motor neurons, sensory neurons, and interneurons. Sensory neurons travel from muscles to the brain via the spinal cord to deliver stimuli or other messages. Motor neurons send instructions from the brain to muscles and organs to coordinate movement and functions. However, motor and sensory neurons cannot interact, so interneurons have the job of delivering messages between the two.
Electrical Stimulation and Impulse Travel
Neurons at resting potential, meaning that there is no impulse travelling across them, are negatively charged due to the number of positive protons outside the neuron. The difference between the negative inside and positive outside is what enables impulses to be carried down the neuron. When an impulse comes down the axon, causing the neuron to fire, the cell is depolarized as a result of the exchange of positive sodium ions and negative potassium ions. As a result, the neuron turns positive and is in the stage known as action potential. The depolarization of the neuron makes it more permeable, and more sodium ions are allowed in as most of the potassium ions are repelled. The purpose of a neuron firing is to either carry the impulse and relay it to a connected neuron or to release neurotransmitter chemicals to cause a response. This way, impulses can quickly be carried along the circuits in the brain, enabling fast spread of information, and neurotransmitters can cause reactions to stimuli sent to the brain.
Once the neuron is depolarized, it must be polarized so that it can fire again. In order to do this, sodium potassium pumps in the cell membrane of the neurons start working to pump three sodium ions out of the cell for every two potassium ions pumped in. As a result, the neuron can return to its negative resting potential and can fire again when the next impulse comes.