The nervous system is the part of the human body that coordinates its voluntary and involuntary actions and functions transmitting signals to and from different parts of the body.

The nervous system consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body.

Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent nerves. Most nerves serve both functions and are called mixed nerves.

The Peripheral Nervous System (PNS)

The peripheral nervous system can be divided into two parts. The somatic nervous system which controls voluntary functions and the autonomic nervous system which controls autonomous functions such as heart rate.

Somatic Nervous System (SoNS)

The somatic nervous system (SoNS) mediate voluntary movement such as moving limbs and is associated with skeletal muscle voluntary control of body movements. The SoNS consists of afferent nerves and efferent nerves. Afferent nerves are responsible for relaying sensation from the body to the central nervous system (CNS); efferent nerves are responsible for sending out commands from the CNS to the body, stimulating muscle contraction; they include all the non-sensory neurons connected with skeletal muscles and skin.

Autonomic nervous system (ANS)

The autonomic nervous system (ANS) is a control system that acts largely unconsciously and regulates bodily functions such as the heart rate, digestion, respiratory rate, urination, and sexual arousal.  Certain reflex actions such as coughing, sneezing, swallowing or vomiting are also linked to the autonomic nervous system.

This system is also the primary mechanism in control of the fight-or-flight response and the freeze-and-dissociate response.

In general, the autonomic nervous system functions can be divided into sensory (afferent) and motor (efferent) subsystems. Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control.

The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems.

Sympathetic Nervous System

The sympathetic nervous system is activated in cases of emergencies to mobilize energy. The sympathetic nervous system activates what is often termed the fight or flight response.

Like other parts of the nervous system, the sympathetic nervous system operates through a series of interconnected neurons.

Sympathetic neurons are frequently considered part of the peripheral nervous system (PNS), although there are many that lie within the central nervous system (CNS).

Sympathetic neurons of the spinal cord (which is part of the CNS) communicate with peripheral sympathetic neurons via a series of sympathetic ganglia.

Parasympathetic Nervous System

The parasympathetic nervous system is one of three divisions of the autonomic nervous system. Sometimes called the rest and digest system, the parasympathetic system conserves energy as it slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.

The Central Nervous System (CNS)

The central nervous system (CNS) controls most functions of the body and mind. It consists of two parts: the brain and the spinal cord.

The brain is the center of our thoughts, the interpreter of our external environment, and the origin of control over body movement. Like a central computer, it interprets information from our eyes (sight), ears (sound), nose (smell), tongue (taste), and skin (touch), as well as from internal organs such as the stomach.

The spinal cord is the highway for communication between the body and the brain. When the spinal cord is injured, the exchange of information between the brain and other parts of the body is disrupted.

Most systems and organs of the body control just one function, but the central nervous system does many jobs at the same time. It controls all voluntary movement, such as speech and walking, and involuntary movements, such as blinking and breathing. It is also the core of our thoughts, perceptions, and emotions.

In multiple sclerosis (MS) a person’s immune system attacks nerve cells within the central nervous system. Damage to the nerves ability to communicate or death of nerve cells results in the symptoms associated with MS.

Nerve Cells

At the cellular level, the nervous system is defined by the presence of a special type of cell, called the neuron, also known as a “nerve cell”. Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical waves traveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. The connections between neurons can form neural circuits and also neural networks that generate an organism’s perception of the world and determine its behavior.

The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants.