Nervous system

The nervous system is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events.  In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of 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. Spinal nerves serve both functions and are called mixed nerves. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves. 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 pathways, neural circuits, and larger networks that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. Everything your body does is connected in some way to your nervous system. It tells your heart to beat. It tells your lungs to breathe. It controls the way you move, the words you say, and how you think and learn. It also controls your senses and memories. How Does It Work? The messages traveling in your nerves are sent through billions of nerve cells called neurons. The spaces between these cells are called synapses. The cells are linked to one another through chemicals called neurotransmitters that move across the synapses to the next neuron. Dopamine and serotonin are types of neurotransmitters. This process continues until the message gets to the right place. Some messages move faster than 200 miles per hour. This is also how messages get from your body back to your brain and spinal cord. For example, if you step on something sharp, the nerves in your foot send a message from neuron to neuron to your central nervous system that says, Hey, this hurts. Your brain and spinal cord respond with a message to your foot: Pull away now. What Conditions Can Affect Your Nervous System? Your nervous system has lots of protection. Your brain is guarded by your skull, and your spinal cord is shielded by small bones in your spine (vertebrae) and thin coverings (membranes). They’re both cushioned by a clear fluid called cerebrospinal fluid. Still, things can go wrong with your nervous system just like any other part of your body. When a disorder damages it, that affects the communication between your brain, your spinal cord, and your body. Examples of these disorders include: Infections like meningitis, encephalitis, or polio Physical problems like an injury, Bell’s palsy, or carpal tunnel syndrome Conditions like Parkinson’s disease, multiple sclerosis, or Alzheimer’s disease Issues with your blood vessels, like strokes, transient ischemic attacks (TIAs), or subdural hematoma (when blood collects outside your brain, typically after a serious head injury) Stress also can affect your nervous system.