human nervous system

What is the Nervous System? Definition, Types (CNS and PNS)

What is a Nervous System?

The nervous system is the most important system in our body. It consists of the brain and nerves. It directs the movements of the muscles and receives messages from the sensory organs. Then it sends for further action through the spinal cord and the neurons. It controls all our actions like thinking, hearing, speaking, seeing, and learning, and so on.

The nervous system can be divided into two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). And, further CNS can be divided into a brain and spinal cord. Similarly, PNS into the autonomic nervous system and somatic nervous system. See:

division of nervous system

Central Nervous System (CNS)

The CNS has further two parts, called brain and spinal cord. Describing them,

Brain

The brain is the central and most complex part of our body. It is also called the old brain because its evolutionary aspect can be traced more than 500 million years ago. It contains three important divisions: the hindbrain, midbrain, and forebrain.

human brain structure

Hindbrain

The information received by the spinal cord is passed through the hindbrain. It is the lowest part of the brain regulating the routine “housekeeping” function for daily activities. Medulla oblongata, pons, and cerebellum are the three major components of the hindbrain.

  • Medulla Oblongata – It regulates the automatic function of the brain. For example, breathing, blood circulation, swallowing, digestion, heartbeat, and so on.
  • Pons – It is the bridge that connects two halves of the brain at the hindbrain. Both medulla and pons are located just above the point where the spinal cord enters the brain. Pons control sleep and arousal.
  • Cerebellum – It is located behind the medulla and pons. It controls motor activities. Damage to this part of the brain creates poor muscle functioning such as in walking, running, and standing.

Midbrain

After the hindbrain, information travel to the midbrain on their way to higher brain centers. Midbrain is a small area that serves as a center for several postural reflexes, associated with the senses. For example, the movement of the eye to be fixed at an object and ears to a sound are mediated through the midbrain.

The midbrain consists of the brain stem and reticular formation. The hindbrain and midbrain together form the brain stem. It is so-called because brain parts rest on it. Another section of the midbrain is a reticular formation which is a network of fibers. It is responsible to activate other parts of the brain to produce general body arousal and alertness.

Forebrain

The information after the hindbrain and midbrain travels to the forebrain. The forebrain consists of two main areas. One area covers the thalamus, hypothalamus, and limbic system while in the other lies the cerebral cortex.

  • Thalamus – It lies between two cerebral hemispheric. It is hidden within the forebrain. It is like a relay station. It receives input from all incoming fibers except olfaction (smell). It helps to mediate and link between upper and lower centers by sending incoming stimuli from the sense organ to the appropriate part of the brain.
  • Hypothalamus – It is situated below the thalamus, which is why it is called the hypothalamus. Though small it is very important in regulating internal bodily functions like blood chemistry, temperature, metabolism, emotions, etc. It also plays important role in eating, sleeping, drinking, sexual motivation, and so on.
  • Limbic System – It is a group of structures. The largest of these structures is the hippocampus. The hippocampus plays important role in emotional reactivity and the storage of memories. Another part of the limbic system is the amygdala, which is related closely to the hypothalamus. It plays an important role in activating emotions of fear, violence, and rage.
  • Cerebral Cortex: The New Brain – The cerebral cortex also known as the cerebrum contains billions of neurons connected with each other. It is often referred to as “gray matter” because of its brownish-gray color. It is also referred to as the “new brain” because of its recent evolution. The human cortex has a very wrinkled and ups-down appearance that resembles a walnut. It contains about 80 percent of the brain’s total structure and is one-eighth of an inch thick. Cortex is responsible for our reasoning, planning, remembering, and imagining abilities.

Spinal Cord

The spinal cord is a rope-like segment about the thickness of a pencil that connects nerve tissues extending down the back. It works as an information highway. It has basically two important functions:

  • It connects higher brain centers to and from and regulates reflexes e.g. hiccoughing, yawning, knee jerk, etc.
  • It exchanges several messages between the brain and the body.

The structure of the spinal cord is less complex than the brain. The spinal cord is protected by the vertebral column. The human has 24 bones caller vertebrae. The sensory nerves enter and the motor nerves come out of the spinal cord with the help of vertebrae in a smooth manner. Information received from the outside is sent to the brain by ascending pathway. Processed information received is sent to the descending pathways.

spinal cord

Reflex action is an important function carried out by the spinal cord. It is quick, automatic, and involuntary. For example, when a hot stimulus touches our hand we jerk, the neural activity excited by the heat travels via sensory nerves to inter-neurons in the spinal cord. It takes a shorter route, it does not travel to the brain to give a response. The sensory nerve connected by an interneuron in the spinal cord to a motor nerve gives knowledge to that sensation and we withdraw our hand.

Peripheral Nervous System (PNS)

The peripheral nervous system connects the outer portions or (periphery) of the body with the central nervous system. It is like the keyboard, monitor, and a printer that passes information in and out on a computer.

The PNS is divided into two broad groups: the somatic nervous system (SNS) and the autonomic nervous system (ANS).

Somatic Nervous System (SNS)

The activities basically related to the muscles that control the movement of the body are regulated by the somatic nervous system. It carries information that receptors received from the environment to the spinal cord, brain, and nerves by afferent nerves and passes back the information from the brain and spinal cord to the muscles through efferent nerves. It is involved both in sending and responding to the information. Usually, the information we receive is planned, scanned, and organized on our wish so SNS called a “voluntary system”.

For example, when I write, my eyes scan the words and pick up the materials. Afferent nerves help to convey information to the CNS to process and choose the words and sentences I formulate. Efferent nerves then convey this information from the CNS to the muscles, joints, and tendons to make a response. Then somatic nervous system retrieves and interprets the information.

Autonomic Nervous System (ANS)

The autonomic nervous system derives its name because of its self-control nature. It is self-regulating or autonomous. This division handles the bodily functioning of the heart, blood vessels, glands, lungs, and other organs that functions involuntarily, without our awareness.

It has two types: the sympathetic and parasympathetic nervous system:

The Sympathetic Nervous System

The sympathetic nervous system constitutes the fiber coming from the spinal cord in the thoracic and lumbar regions. It is active during aroused state and prepares the body to handle the emergency situation through “fight or flight”. The sympathetic system does it by increasing heart rate, blood pressure, sugar, levels, and hormones in the blood by engaging all the organism’s resources to respond to that stressful emergency situation.

The Parasympathetic System

The parasympathetic system falls into two parts: the cranial and the sacral region that is above and below the sympathetic division. The parasympathetic division works in an opposite manner to the sympathetic system. It works to store the body’s energy. It slows the processes accelerated by the sympathetic system. It tends to be active when we are calm and relaxed.

When the parasympathetic system is active it decreases heart rate, reduces blood pressure, and diverts blood to the digestive system. It helps to promote the general function of nutrition and reproduction and daily routine work to a more normal or balanced state of functioning maintaining homeostasis processes.

It is seen that the autonomic nervous system plays a major role in emotion and motivation, transmitting information to and from the CNS. Thus its action is basically controlled by the central nervous system.

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