Biology Study Notes: Nervous System

Biology Study Notes: Nervous System

The Central Nervous System (CNS) includes the brain and spinal cord.
The Peripheral Nervous System (PNS) connects the CNS to other parts of the body and is            composed of nerves (bundles of neurons)

The Neuron
Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit
nerve messages. Glial cells are in direct contact with neurons and often surround them.
The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone!

Functions of the three parts of a neuron:
  • Axon: It conducts messages away from the cell body.
  • Dendrite: It receives information from the axon of another cell and conducts the messages towards the cell body.
  • Cell body: It contains the nucleus, mitochondria, and other organelles. It is mainly concerned with the maintenance and growth.

The junction between a nerve cell and another cell is called a synapse.
The space between two cells is known as the synaptic cleft.
The function between two neurons is called a ‘ganglion’.

The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina.
The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
The eye lens forms an inverted real image of the object on the retina.

RETINA - The retina is a delicate membrane having an enormous number of light-sensitive cells.

CORNEA - Light enters the eye through a thin membrane called the cornea. It is the eye’s outermost layer. It is the clear, dome­shaped surface that covers the front of the eye. It plays an important role in focusing your vision.

PUPIL - The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly or absorbed after diffuse reflections within the eye. The pupil regulates and controls the amount of light entering the eye.

IRIS - It is a dark muscular diaphragm that controls the size of the pupil and thus the amount of light reaching the retina.

CILIARY MUSCLE - The ciliary muscle is a ring of smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humor into Schlemm's canal. It changes the shape of the lens within the eye, not the size of the pupil. 
The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.

Note: When the light is very bright, the iris contracts the pupil to allow less light to enter the eye. However, in dim light, the iris expands the pupil to allow more light to enter the eye. Thus, the pupil opens completely through the relaxation of the iris.
A human being has a horizontal field of view of about 150° with one eye and of about 180° with two eyes. 


The brain is the most complex part of the human body. This three-pound organ is the seat of intelligence, interpreter of the senses, initiator of body movement, and controller of behavior.

The brain can be divided into three basic units:
The forebrain,
The midbrain, and
The hindbrain

The forebrain is the largest and main thinking part of the brain. It has regions which receive sensory impulses from various receptors. Separate areas of the fore-brain are specialized for hearing, smell, sight and so on.
The Midbrain connects the forebrain to the hindbrain. 
The hindbrain controls the body’s vital functions such as respiration and heart rate.

CEREBRUM - the Largest part of the human brain

  • It is at the topmost part of the brain. 
  • It is the source of intellectual activities. 
  • It holds your memories, allows you to plan, enables you to imagine and think. 
  • It allows you to recognize friends, read books, and play games.
  • It controls the voluntary motor actions.
  • It is the seat of learning and memory.
  • It is the site of sensory perceptions; like tactile and auditory perceptions.
  • It is divided into two hemispheres; called cerebral hemispheres.


  • It lies at the base of the cerebrum. 
  • It controls sleep and wake cycle (circadian rhythm) of the body. 
  • It also controls the urges for eating and drinking.
  • It gets the adrenaline flowing during a test or job interview. 

  • It lies below the cerebrum and at the back of the whole structure.
  • It coordinates the motor functions. 
  • It is responsible for a precision of voluntary actions and maintaining the posture and balance of the body.
  • Example: When you are riding your bicycle; the perfect coordination between your pedaling and steering control is achieved by the cerebellum.


  • It forms the brain stem; along with the pons. 
  • It lies at the base of the brain and continues into the spinal cord. 
  • It controls various involuntary functions 
  • Example: heartbeat, respiration, size of the pupil, blood pressure, salivation, and vomiting etc.


  • A major clearinghouse for information going to and from the spinal cord and the cerebrum.
  • Cerebrospinal fluid (CSF) is a watery fluid that circulates through the brain's ventricles (cavities or hollow spaces) and around the surface of the brain and spinal cord. 

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