The brain is the master organ of the body. The brain takes in all information relating to the body’s internal and external environments, and it produces the appropriate responses.
In humans, the nervous system is divided into the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which contains all the nerves that run everywhere in the body.
The spinal cord, which is attached to the brain, runs down the center of your body, so equate that with the CNS. All the nerves that branch off the spinal cord, including the cranial nerves and spinal nerves, and reach to the periphery of your body make up the PNS.
The structures of the brain
Inside the skull, the meninges cover the cerebrum (the large, gray, bumpy part of the brain). The meninges are strong membranes that cover the brain and spinal cord. Cerebrovascular fluid flows between the membranes. An infection here is called meningitis for inflammation of the meninges.
The cerebrum is the largest part of the brain and is the part responsible for consciousness. The cerebrum is divided into left and right halves, which are called cerebral hemispheres. Each cerebral hemisphere has four lobes named for the bones of the skull that cover them: frontal, parietal, temporal, and occipital.
Specific areas of the lobes are responsible for certain functions, such as concentration, understanding speech, recognizing objects, memory, and so on.
At the center of the brain are the thalamus and hypothalamus, which form the structure called the diencephalon. The hypothalamus generates many neurosecretions, which are carried to the pituitary gland at the base of the hypothalamus. The hypothalamus controls homeostasis by regulating hunger, thirst, sleep, body temperature, water balance, and blood pressure.
The pituitary gland is called the master gland because, along with the hypothalamus, it helps to maintain homeostasis by secreting many important hormones.
At the base of the brain are the cerebellum and the brain stem. The cerebellum coordinates muscle functions such as maintaining normal muscle tone and maintaining posture. The brain stem is formed by three structures: the midbrain, the pons, and the medulla oblongata. The spinal cord is a continuation of the brain stem that runs down through the vertebrae of the spine.
How reflex arcs work
Reflex arcs are connections between sensory neurons, the spinal cord, and motor neurons. They are good examples of how the nervous system protects you by making you get out of danger almost before you realize you are in danger.
Here’s an example: You are cooking dinner, and you accidentally grab the lid of a pot without using a hot pad. You just want to check on the vegetables. Your nervous system has other ideas:
When you grab that hot lid, the endings of the sensory nerves in your skin detect the heat and send an impulse up through the axon of a sensory neuron to the nerve cell body of the sensory neuron.
The impulse continues through sensory neurons until it reaches an interneuron in the spinal cord.
The interneuron determines the appropriate response — which, in this case, would be stimulating the muscles to pull your hand away.
The excitatory impulse is transferred to the cell body of a motor neuron and travels down the axon of the motor neuron until it reaches muscle tissue.
The muscle responds by contracting to pull your hand away from the hot lid.
With all these words describing what happens, it makes it seem like this process takes quite a while. But think about when you’ve touched something hot by mistake. You pulled your hand away immediately thanks to a quick-reacting reflex arc. Without the reflex arc protecting you, you might just unknowingly hold that hot lid in your hand until real damage is done!
