The nervous system is the major system of communication within the body. Our thoughts, emotions, and actions are all left up to the signalling done by this system. In tandem with the endocrine system, the nervous system helps regulate and control internal conditions to maintain homeostasis. Most all of the glands discussed in the endocrine system are signaled by nerves to secrete their hormones. However, the nervous system also responds to external stimuli like light and temperature. Every response our body has to any stimulus, whether internal or external, is controlled by the nervous system.
Divisions of the Nervous System
The nervous system has several divisions, all branching from the central nervous system (CNS). The CNS houses the brain and spinal cord that act as the central command for all actions of the body. Nerves in the brain and nerves that extend from the spinal cord to the various regions of the body create the peripheral nervous system (PNS). The PNS is responsible for linking the body to the CNS so that signals created by the CNS are able to reach their targets. Within the PNS are the somatic nervous system and the autonomic nervous system. The somatic nervous system controls voluntary body movements, like muscular contractions, and the autonomic nervous system controls involuntary movements, like the dilation of your pupils or rhythm of your heartbeat. Finally, within the autonomic nervous system are the sympathetic (SNS) and parasympathetic (PSNS) systems. The SNS preps the body for action, creating the flight or fight response to stimuli. In contrast, the PSNS relaxes the body, bringing it back to normal after an exciting stimulus.
Structure of a Neuron
Despite how intricate this system may seem, nervous tissue comes from just two types of cells: glial cells and neurons. Glial cells, also known as neuroglia, are found in both the CNS and PNS. These cells protect and support nerve cells called neurons. Neurons are the basic functional unit of the nervous system, transmitting messages around the body. Their unique structure allows them to be very fast, efficient communicators. Neurons have a cell body that holds a nucleus, which acts as the “brain” of the cell. Surrounding the cell body are dendrites, the regions that receive signals. Dendrites send the signal through the axon until it reaches the axon terminals. The signal travels through the axon with the help of Schwann cells that wrap around the axon and act as an insulator. Between these cells are nodes of Ranvier. A typical neuron is shown in this diagram:
Sending a Signal
The process of sending a signal begins with a stimulus. The dendrites of the first neuron in the signal chain receives the stimulus and transmit the signal through the axon. Once through the axon the signal travels down each axon terminal. When the signal reaches the terminal it causes the release of synaptic vesicles that carry a neurotransmitter (chemical messengers). The vesicle fuses with the membrane of the axon terminal, releasing the neurotransmitter into the synaptic cleft, or synapse. The neurotransmitter then travels across the synapse until it reaches another neuron’s dendrites or the target cell/tissue. For example, the mechanism of signaling muscular contraction begins when the neuron that is part of the motor unit releases a neurotransmitter called acetylcholine (Ach) into the synaptic cleft. Ach travels across the cleft until it reaches the Ach receptors in the membrane of the muscle fiber. Once there, channels open to allow an influx of charged particles that signal the release of calcium, starting the contraction process. A diagram of a typical neuromuscular junction is shown below:
The Autonomic Nervous System
As mentioned above, the autonomic nervous system is divided into the parasympathetic and sympathetic systems. A key role of the PSNS is stimulating saliva production in the mouth and stimulating the stomach and intestines to complete digestion. The PSNS maintains a calm state of arousal, with its function often simplified to “rest and digest”. Conversely, the SNS creates alertness in the body. When you are scared, you may notice an increased heart rate, faster breathing, and an energized feeling. This is due to the SNS readying your body to face a potential threat. When the threat is gone, the PSNS will return the body to its normal, calmer state. The diagram below details the effects of the PSNS and SNS on the body and also distinguishes the CNS and PNS from one another:
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