Human Nervous System

Overview of Human Nervous System

The human nervous system is a centralized, complex, and highly organized system which controls and coordinates the whole body. The human nervous system is divided into the central and peripheral nervous systems. The central nervous system consists of the brain and spinal cord.

The brain controls physical functions, speech, thoughts, feelings, memory, and movements. Some reflex actions are controlled by the spinal cord. Both the brain and spinal cord are highly protected. The brain is divided into three parts i.e., forebrain, midbrain, and hindbrain.

The forebrain is the thinking part and associated with feelings, memory, and thoughts. The cerebrum is the largest part of the brain accountable for voluntary movements and thinking. Midbrain is reduced in humans and includes an auditory relay center. It is a relay center linking the forebrain and hindbrain. The hindbrain is the control center for instinctive or innate or involuntary actions. It controls heartbeat, breathing, blood pressure, sleep, and so on.

It consists of three parts medulla, pons, and cerebellum. The spinal cord is the reflex center for many actions. It provides a pathway for the conduction of impulses by neurons to and from different parts of the body and brain. On the other hand, the peripheral nervous system consists of cranial nerves emerging out from the brain and spinal nerves coming from the spinal cord. It is responsible for the somatic and autonomic nervous system.


The human nervous system is a type of centralized nervous system. It is further classified into subdivisions. The nervous system of humans consists of the Central Nervous System and the Peripheral Nervous System.


Central Nervous System

The central nervous system includes the brain and spinal cord. It is described as “central” due to the fact that it integrates information from the entire body and coordinates activity in the entire organism.

The brain plays a main function in the control of most physical functions, consisting of awareness, movements, feelings, thoughts, speech, and memory. Some reflex motions can occur through spinal cord paths without the participation of brain structures. The spinal cord is linked to a section of the brain called the brainstem and runs through the spinal canal.

The brain and spinal cord are protected in three ways. The cranium, which is a part of the skull, protects the brain, and neural arches, of the vertebrae of the vertebral column, provide protection to the spinal cord. The brain and spinal cord are also safeguarded by triple layers of meninges.

The cerebrospinal fluid (CSF), similar in composition to blood plasma, bathes the nerve cells of the brain and spine and it cushions against the bumps and shocks. Both the brain and spinal cord are hollow. The spine has a central canal and the brain has numerous cavities (ventricles) filled by CSF, which is also present in between the meninges.

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The brain is the most intricate part of the body. This delicate and complex organ is the place of intelligence, interpreter of the senses, initiator of body language, and controller of behavior. The human brain is the largest brain of all vertebrates relative to body size. It weighs about 3.3 pounds. (1.5 kilograms). The typical male has a brain volume of 1,274 cubic centimeters. The typical female brain has a volume of 1,131 cm3.

The brain makes up about 2 percent of a human’s body weight. The cerebrum comprises 85 percent of the brain’s weight. It contains about 86 billion neurons the “gray matter”. It contains billions of nerve fibers (axons and dendrites) — the “white matter”. The human brain includes about 85 percent of water in brain cells.


The brain can be divided into the:

  • forebrain
  • midbrain
  • and hindbrain.

It is the main thinking part of the brain and controls voluntary actions. The forebrain processes sensory information that is gathered from the various sense organs such as ears, eyes, nose, tongue, skin. It is because of the presence of the forebrain; humans are positioned at the highest level in the animal chain.

The forebrain includes the Cerebrum, Thalamus, and Hypothalamus.


Thalamus carries sensory details to the limbic system and cerebrum.

The details include sensory input from auditory and visual paths, from the skin and from within the body.

Limbic System

The limbic system is located in an arc between the thalamus and cerebrum.


The limbic system interacts to produce our most standard and primitive emotions, drives, and behaviors, consisting of fear, rage, tranquility, cravings, thirst, satisfaction, and sexual responses.

Part of the limbic system is also important in the development of memories.

The limbic system consists of:

  1. Hypothalamus
  2. Amygdala
  3. Hippocampus


  1. Hypothalamus

The hypothalamus through its hormonal production and neural connections functions as a major coordinating center controlling body temperature, appetite, the menstruation, water balance, the sleep-wake cycle, etc.

  1. Amygdala

In the amygdala, clusters of nerve cells produce the sensation of satisfaction, pleasure, punishment, or sexual arousal when stimulated. Plus, it is involved in the feelings of fear and rage.

  1. Hippocampus

The hippocampus plays an essential function in the formation of long-term memory, and thus is required for learning.


The cerebrum is the largest part of the brain and is divided into two halves, called cerebral hemispheres. These halves interact with each other by means of a large band of axons, called the corpus callosum.

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Tens of billions of neurons are packed into this part. The external area, the cortex, forms folds called convolutions, which significantly increase its surface area. This part receives sensory information, processes it, stores some in memory for future usage, directs voluntary movements, and is accountable for the poorly understood process that we call thinking.

The cortex is highly wrinkled and makes the brain very efficient. Now the cerebral cortex again divides into 4 parts called lobes. They are:

Frontal lobe— Is related to parts of speech, reasoning, problem-solving, preparation, movement, and emotions.

Parietal lobe— connected with recognition, orientation, and understanding of stimuli.

Occipital lobe— Is accountable for visual processing.

Temporal lobe— Is related to memory, speech understanding, and detection of auditory stimuli.

Functions of Cerebral Cortex:

The cortex consists of primary sensory areas where signals originating in sensory organs such as eyes and ears are received and converted into subjective impressions, such as light and noise. This area is also associated with speech and receives and analyzes sensations of touch from all parts of the body.

This area is also a center for sending out impulses to voluntary muscles, controlling movements. This is also involved in intelligence, reasoning, and judgment.

The left cerebral hemisphere controls the right side of the body, and the right cerebral hemisphere controls the left side of the body.


Midbrain is reduced in human beings, and it consists of an auditory relay center and that manages the relax motions of the eyes. Midbrain includes reticular formation, which is a relay center linking the hindbrain with the forebrain. The reticular formation is very important in evaluating the input information, prior to they reach higher brain centers.


It is the control center for visceral function. As a result, this part of the brain plays a role in controlling the heart rate, breathing, blood pressure, sleep and getting up functions, and so on. The hindbrain has 3 parts, namely– medulla oblongata, pons, and cerebellum.


The medulla controls several automated functions, such as breathing, Heart rate, high blood pressure, and swallowing.


Specific neurons in the pons, situated above the medulla, appear to influence transitions in between sleep and wakefulness, and the rate and pattern of breathing.


The cerebellum is essential in coordinating motions of the body. The cerebellum guides, smooth and accurate motions and preserves body position. The cerebellum is also involved in the recognition and memory storage for behaviors. It is best developed in bird, which is involved in the complex activity of light.

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Spinal Cord:

Medulla oblongata narrow down into an oval-shaped hollow cylinder, the spine, going through the vertebral column.

  • Spine is comprised of a large variety of nerve cells, the cell-fibers, and bodies of which are arranged in a definite pattern.
  • In cross section, the spinal cord reveals an inner butterfly shaped grey matter, consisting of the main canal and the outer portion made up of white matter.
  • Gray matter, as in other parts of the nervous system consists of cell bodies and non-myelinated nerve fibers or tracts.
  • White matter is made up of myelinated nerve fibers or systems.


Functions of Spinal cord:

The spinal cord is the center for many reflexes and it acts as a pathway for the conduction of impulses to and from different parts of the body and brain.

The spinal cord is the primary path for information connecting the brain and peripheral nervous system.