If the human body were a building, the nervous system would be the electric wiring. It consists of two main parts: the central and peripheral nervous systems. These parts work in tandem to send signals from cell to cell and from body part to body part. At a basic level, these signals are responsible for coordinating organ systems and maintaining many other bodily functions. They also give humans the capacity for language and the understanding of abstract concepts that other organisms lack.
Because the nervous system is one of the most complex and intricate systems in nature, it can be difficult to understand without first understanding its components. Each section of the nervous system contains neurons that receive, process, and transmit electrical and chemical signals through connections called synapses. These signals carry the information necessary for the body to operate. Each neuron possesses a specific purpose and responds to stimuli only suited for that purpose. Some neurons are responsible for sensory inputs, while others assist with muscle contraction, for instance.
A neuron has three main components: a cell body, dendrites, and an axon. Dendrites are thin extensions of the cell body that act as the receivers for signals. The neuron then sends these signals out through the axon. Like wires in a cable, many axons may form a bundle called a fascicle. In the peripheral nervous system, these are nerves. In the central nervous system, they are tracts. The nerves act as pathways for the signals to reach peripheral organs and other parts of the body.
The central nervous system contains the brain and spinal cord. Despite being studied by researchers for years, this part still contains many secrets and mysteries. The brain alone has around 100 billion neurons and multiple lobes that work together to perform deceptively simple actions such as physical motion and advanced functions like problem-solving. The spinal cord is one of the main highways for the brain’s signals and can help automate many of its processes. For example, when walking, the brain is only needed to change direction and avoid obstacles. The spinal cord can perform the actual movement without thought.
Nerves and collections of neurons called ganglia make up the peripheral nervous system. The main purpose of the peripheral system is to act as the routes along which the central system sends signals. The peripheral system divides its actions into two subsystems, one for voluntary actions (the somatic nervous system) and one for automatic and self-regulating actions (the autonomic nervous system).
A portion of the peripheral nervous system connects the brain and spinal cord with the muscles controlled through conscious effort. This somatic nervous system also connects the sensory receptors in the skin to the central nervous system. These receptors collect useful information from inside and at the surface of the body. Twelve cranial nerves and 31 pairs of spinal nerves act as pathways for this information. These 43 segments of nerves then connect to thousands of associated nerves that assist in the process.
The autonomic nervous system is responsible for the function of internal organs. Bodily functions like heart rate, digestion, urination, and breathing are all the responsibility of the autonomic nervous system. Though the majority of these functions occur automatically, the autonomic nervous system can work alongside the somatic nervous system. This is why humans can choose to hold their breath. Researchers describe the autonomic nervous system as having two branches: sympathetic and parasympathetic. Some refer to a third branch, the enteric system.
The three branches of the autonomic system work hand-in-hand with each other. Usually, one branch inhibits an internal organ’s action, while the others activate it. Experts will sometimes describe the sympathetic branch’s function as fight or flight. It prepares the body for stressful or dangerous situations. The parasympathetic branch is the opposite. It maintains normal body behavior during situations that aren’t dangerous or stressful. Researchers refer to the enteric branch as a second brain due to its complexity and number of neurons. It controls autonomic functions such as the body’s reflexes.
The 12 cranial nerves of the somatic system connect the brain to the eyes, ears, nose, and throat, as well as parts of the head and neck. Each of the nerves has a name based on its purpose. For example, the nerve responsible for smelling is the olfactory nerve. Each nerve also has a number based on its proximity to the front of the brain. The closer to the front, the smaller the number. The olfactory nerve is the first, and the hypoglossal nerve is the twelfth.
Experts refer to the spinal nerves as pairs because each nerve emerges from the spaces between the vertebrae as two branches. One branch emerges from the front of the spinal cord. This is the anterior nerve root and carries commands to the muscles. The other branch emerges from the back and is the posterior nerve root. It carries sensory information from the body to the brain. Some spinal nerves combine to form networks of nerves called plexuses. Each plexus travels to specific areas of the body and acts as a singular nerve.
Because of its complexity and many individual parts, damage to the nervous system from diseases and disorders can be debilitating. Many of these issues interrupt motor and sensory signals. A membrane layer protects the brain and spinal cord from most damage, but conditions such as Huntington’s disease can cause the neurons in the brain to degenerate. The peripheral nervous system lacks defensive features aside from a thin lining. Multiple sclerosis causes the body to attack its own nerves’ defensive lining, resulting in serious motor and sensory issues.
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