The nervous system is a complex collection of nerves and specialized cells known as neurons that transmit signals between different parts of the body. It is essentially the body's electrical wiring.
Structurally, the nervous system has two components: the central nervous system and the peripheral nervous system. According to the National Institutes of Health, the central nervous system is made up of the brain, spinal cord and nerves. The peripheral nervous system consists of sensory neurons, ganglia (clusters of neurons) and nerves that connect to one another and to the central nervous system.
Central nervous system
The central nervous system consists of the brain and spinal cord. It is referred to as "central" because it combines information from the entire body and coordinates activity across the whole organism.
The brain is protected by the skull (the cranial cavity) and the spinal cord travels from the back of the brain, down the center of the spine, stopping in the lumbar region of the lower back.
The brain and spinal cord are both housed within a protective triple-layered membrane called the meninges.
The central nervous system has been thoroughly studied by anatomists and physiologists, but it still holds many secrets; it controls our thoughts, movements, emotions, and desires. It also controls our breathing, heart rate, the release of some hormones, body temperature, and much more.
The brain is the most complex organ in the human body; the cerebral cortex (the outermost part of the brain and the largest part by volume) contains an estimated 15–33 billion neurons, each of which is connected to thousands of other neurons.
total, around 100 billion neurons and 1,000 billion glial (support) cells make up the human brain. Our brain uses around 20 percent of our body's total energy. The brain is the central control module of the body and coordinates activity. From physical motion to the secretion of hormones, the creation of memories, and the sensation of emotion.
To carry out these functions, some sections of the brain have dedicated roles. However, many higher functions — reasoning, problem-solving, creativity — involve different areas working together in networks.
The brain is roughly split into four lobes:
Temporal lobe (green): important for processing sensory input and assigning it emotional meaning. It is also involved in laying down long-term memories. Some aspects of language perception are also housed here.
Occipital lobe (purple): visual processing region of the brain, housing the visual cortex.
Parietal lobe (yellow): the parietal lobe integrates sensory information including touch, spatial awareness, and navigation.
Frontal lobe (pink): positioned at the front of the brain, the frontal lobe contains the majority of dopamine-sensitive neurons and is involved in attention, reward, short-term memory, motivation, and planning.
Spinal cord
The spinal cord, running almost the full length of the back, carries information between the brain and body, but also carries out other tasks. From the brainstem, where the spinal cord meets the brain, 31 spinal nerves enter the cord. Along its length, it connects with the nerves of the peripheral nervous system (PNS) that run in from the skin, muscles, and joints.
Motor commands from the brain travel from the spine to the muscles and sensory information travels from the sensory tissues — such as the skin — toward the spinal cord and finally up to the brain. The spinal cord contains circuits that control certain reflexive responses, such as the involuntary movement your arm might make if your finger was to touch a flame.
The circuits within the spine can also generate more complex movements such as walking. Even without input from the brain, the spinal nerves can coordinate all of the muscles necessary to walk. For instance, if the brain of a cat is separated from its spine so that its brain has no contact with its body, it will start spontaneously walking when placed on a treadmill. The brain is only required to stop and start the process, or make changes if, for instance, an object appears in your path.
Peripheral nervous system
The peripheral nervous system (PNS) consists of all neurons that exist outside the brain and spinal cord. This includes long nerve fibers containing bundles of axons as well as ganglia made of neural cell bodies. The peripheral nervous system connects the central nervous system (CNS) made of the brain and spinal cord to various parts of the body and receives input from the external environment as well.
Functionally, the PNS is divided into sensory (afferent) and motor (efferent) nerves, depending on whether they bring information to the CNS from sensory receptors or carry instructions towards muscles, organs or other effectors. Motor nerves can be further classified as somatic or autonomic nerves, depending on whether the motor activity is under voluntary conscious control.
Anatomically, the PNS can be divided into spinal and cranial nerves, depending on whether they emerge from the spinal cord or the brain and brainstem. Both cranial and spinal nerves can have sensory, motor or mixed functions. The enteric nervous system, surrounding the gastrointestinal tract is another important part of the peripheral nervous system. While it receives signals from the autonomic nervous system, it can function independently as well and contains nearly five times as many neurons as the spinal cord.
The primary function of the peripheral nervous system is to connect the brain and spinal cord to the rest of the body and the external environment. This is accomplished through nerves that carry information from sensory receptors in the eyes, ears, skin, nose and tongue, as well as stretch receptors and nociceptors in muscles, glands and other internal organs. When the CNS integrates these varied signals, and formulates a response, motor nerves of the PNS innervate effector organs and mediate the contraction or relaxation of skeletal, smooth or cardiac muscle.
Thus, the PNS regulates internal homeostasis through the autonomic nervous system, modulating respiration, heart rate, blood pressure, digestion reproduction and immune responses. It can increase or decrease the strength of muscle contractility across the body, whether it is sphincters in the digestive and excretory systems, cardiac muscles in the heart or skeletal muscles for movement. It is necessary for all voluntary action, balance and maintenance of posture and for the release of secretions from most exocrine glands. The PNS innervates the muscles surrounding sense organs, so it is involved in chewing, swallowing, biting and speaking. At the same time, it mediates the response of the body to noxious stimuli, quickly removing the body from the injurious stimulus, whether it is extremes in temperature, pH, or pressure, as well as stretching and compressing forces.
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