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peripheral nervous system | reflex action | receptors in humans | colour blindness

Peripheral Nervous System

It is composed of nerves along with their ganglia. In humans, there are  total 12 pairs of cranial nerves and Thirty one pairs of spinal nerves. Some cranial nerves are sensory, Some are Motor And some are found. All the  nerves  of the spinal cord  found. It includes all of the motor neurons that are connected to skeletal muscles.

Autonomic Nervous System:

It consists of motor neurons that are connected to cardiac muscles, smooth muscle and glands. It works without conscious control.


The automatically  Nervous system consists of the sympathetic and parasympathetic system.

The Sympathetic  system  torment system prepares the body to deal with emergencies. This is often called the "fight or flight" response, This system dilates pupils, speeds up heartbeat, increases breathing rate and inhibits digestion.

 When there is little or no stress, parasympathetic nervous system normalizes all activities of body. It is called "rest and digest" response, It causes pupils to contract, promotes digestion, and normalizes the rate of heartbeat and breathing.

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Reflex Action

Quick involuntary responses are called reflex actions. Brain is not involved in carrying out reflex actions. Reflex action is controlled by a reflex arc. A reflex arc is the pathway of neurons over which the nerve impulses travel in a reflex action.


             We pull our hand away from a hot flam without thinking about it. It is a reflex action and happens in the following way;

  1. Skin receptors detect the stimulus of heat. Nerve stimulation occurrs in sensory neuron.

  1. Sensory neuron sends nerve impulse to the inter-neuron of spinal cord.

  1. The inter-neuron transmits the nerve impulse to motor neuron

  1. motor neurons carry nerve impulse to the affected area (arm muscles)

  1. The effector produces a response by contracting. So our hand is pulled.

Figure 12.9 reflex arc in w reflex action

12.3-Receptors in Humans

Receptors are the organs or parts of the body which detect stimuli and send messages to the CNS. The Main human are eyes, ears, nose, taste buds, touch receptors, heat and cold etc.


The wall of the eye is composed of three layers The external layer is composed of a dense connective tissue and is called the sclera.The anterior portion of sclera is transparent and is called cornea.

The middle layer of the wall of eye is choroid. It contains many blood vessels and gives darker colour to the inner eye. The choroid layer is thick at the Back part and forms a ciliary body. The ciliary body Progress  to form a lubricating structure called the iris.

 The iris is the visible coloured portion of the eye. The eye contains a transparent lens which is held in place by ligaments attached to the ciliary body in front of the lens, the aperture surrounded by the iris is called the pupil.

 The diameter of the Slender is regulated by the muscles of iris. In bright light, the iris's circular muscles contract and the pupil.Narrows.Similarly, in dim light the radial muscles of iris contract and so pupil dilates.

The inner layer of the wall of the eye is retina. Jit contains photoreceptor cells and associated neurons.There are two types of photoreceptor cells i.e. rods and cones.

The rods are sensitive to dim light. They contain a purplish-red pigment called the rhodopsin. Body synthesizes rhodopsin from vitamin A.if there is deficiency of vitamin A, the body cannot prepare rhodopsin in required amounts. So eyes cannot see clearly in dim light. This problem is called night blindness,

Cones are sensitive to bright light and colours. The cones contain a lubricant are called iodopsin.There are three types of cones. Each type contains specific iodopsin for red, green or blue light.

 The sensations of different colours are produced by different types of cones. When all cones are moving evenly, a sensation of white light is created.

The optic nerve leaves the eye at the posterior side of the eye. This place has no photoreceptor cells so it is called a blind spot.

 At the posterior side of the eye, opposite to the lens, there is a yellowish spot on the retina. It is called fovea It is a thin portion of the retina where only the cones are densely packed. Here, the visual acuity is greater than at other spots.

It contains a thin watery fluid called aqueous humor.The space between the lens and the retina is called the vitreous is filled with a transparent gel called vitreous humor.

Mechanism of Vision

The light rays coming through the cornea and lens are focused on the retina. This light is received by rods and cones. These cells produce nerve impulses in the neurons present in retina. The axes of these neurons are part of the optic nerve. The optic nerve carries nerve impulses from retina to the occipital lobe of cerebrum. Here, the impulse are processed to produce a meaningful sensation of shape and color.

figure 12.10 structure of human eyes

Colour blindness

Eyes can distinguish among different colours due to the presence of photoreceptor cells called cones. Each of the three types of cones contains any one of the three iodopsins (red, blue and green).

 Some people cannot make any one or two types of cones, due to genetic problems. Such people cannot sense the colours for which the cones are absent/So they are not able to distinguish colours. This problem is called colour blindness

Colour vision is essential at many professions, for example for drivers, surgeons, textile designers etc. Aircraft pilots have to recognize runway lights, runway signals, airport beacons, and chart symbols, especially at night. A pilot must be able to distinguish different colour for the safe performance of his/her duties..

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