The nervous system has two main parts:
- The central nervous system is made up of the brain and spinal cord.
- The peripheral nervous system is made up of nerves that branch off from the spinal cord and extend to all parts of the body.
The nervous system transmits signals between the brain and the rest of the body, including internal organs. In this way, the nervous system’s activity controls the ability to move, breathe, see, think, and more.1
The basic unit of the nervous system is a nerve cell, or neuron. The human brain contains about 100 billion neurons. A neuron has a cell body, which includes the cell nucleus, and special extensions called axons [pronounced AK-sonz] and dendrites [pronounced DEN-drahytz]. Bundles of axons, called nerves, are found throughout the body. Axons and dendrites allow neurons to communicate, even across long distances.
Different types of neurons control or perform different activities. For instance, motor neurons transmit messages from the brain to the muscles to generate movement. Sensory neurons detect light, sound, odor, taste, pressure, and heat and send messages about those things to the brain. Other parts of the nervous system control involuntary processes. These include keeping a regular heartbeat, releasing hormones like adrenaline, opening the pupil in response to light, and regulating the digestive system.
When a neuron sends a message to another neuron, it sends an electrical signal down the length of its axon. At the end of the axon, the electrical signal changes to a chemical signal. The axon then releases the chemical signal with chemical messengers called neurotransmitters [pronounced noor-oh-TRANS-mit-erz] into the synapse [pronounced SIN-aps]—the space between the end of an axon and the tip of a dendrite from another neuron. The neurotransmitters move the signal through the synapse to the neighboring dendrite, which converts the chemical signal back into an electrical signal. The electrical signal then travels through the neuron and goes through the same conversion processes as it moves to neighboring neurons.
The nervous system also includes non-neuron cells, called glia [pronounced GLEE-uh]. Glia perform many important functions that keep the nervous system working properly. For example, glia:
- Help support and hold neurons in place
- Protect neurons
- Create insulation called myelin, which helps move nerve impulses
- Repair neurons and help restore neuron function
- Trim out dead neurons
- Regulate neurotransmitters
The brain is made up of many networks of communicating neurons and glia. These networks allow different parts of the brain to “talk” to each other and work together to control body functions, emotions, thinking, behavior, and other activities.1,2,3
Citations
Open Citations
- MedlinePlus. [2016]. Neurosciences. Retrieved June 16, 2017, from //medlineplus.gov/ency/article/007456.htm
- Society for Neuroscience. [2012]. Brain facts. Washington, DC. Retrieved January 17, 2018, from //www.brainfacts.org/The-Brain-Facts-Book
- National Institute of Neurological Disorders and Stroke. [2018]. Brain basics: Know your brain. Retrieved August 9, 2018, from //www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Know-Your-Brain
The CNS is the processing centre of the body and consists of the brain and the spinal cord. Both of these are protected by three layers of membranes known as meninges. For further protection, the brain is encased within the hard bones of the skull, while the spinal cord is protected with the bony vertebrae of our backbones. A third form of protection is cerebrospinal fluid, which provides a buffer that limits impact between the brain and skull or between spinal cord and vertebrae.
Grey and white matter
In terms of tissue, the CNS is divided into grey matter and white matter. Grey matter comprises neuron cell bodies and their dendrites, glial cells, and capillaries. Because of the abundant blood supply of this tissue, it’s actually more pink-coloured than grey.
In the brain, grey matter is mainly found in the outer layers, while in the spinal cord it forms the core ‘butterfly’ shape.
White matter refers to the areas of the CNS which host the majority of axons, the long cords that extend from neurons. Most axons are coated in myelin - a white, fatty insulating cover that helps nerve signals travel quickly and reliably. In the brain, white matter is buried under the grey surface, carrying signals across different parts of the brain. In the spinal cord, white matter is the external layer surrounding the grey core.
The brain
Image: QBI/Levent Efe
If the CNS is the processing centre of the human body, the brain is its headquarters. It is broadly organised into three main regions - the forebrain, the midbrain, and the hindbrain. The largest of these three is the forebrain [derived from the prosencephalon in the developing brain]. It contains the large outermost layer of the brain, the wrinkly cerebral cortex, and smaller structures towards its centre, such as the thalamus, hypothalamus, and the pineal gland.
The midbrain [derived from the mesencephalon in the developing brain] serves as the vital connection point between the forebrain and the hindbrain. It’s the top part of the brainstem, which connects the brain to the spinal cord.
The hindbrain [derived from the rhombencephalon in the developing brain] is the lowest back portion of the brain, containing the rest of the brainstem made up of medulla oblongata and the pons, and also the cerebellum - a small ball of dense brain tissue nestled right against the back of the brainstem.
Parts of the brain
The brain’s cerebral cortex is the outermost layer that gives the brain its characteristic wrinkly appearance. The cerebral cortex is divided lengthways into two cerebral hemispheres, each of which traditionally have been divided into four lobes: frontal, parietal, temporal and occipital. Read more.