ICSE Class 10 Biology Chapter 09 The Nervous System

The Nervous System

Syllabus: Nervous system - Structure of Neuron; central, autonomous and peripheral nervous system (in brief); brain and spinal cord; reflex action and how it differs from voluntary reflex. Sense organs - Eye and ear - Eye defects and corrective measures (myopia, hypermetropia, presbyopia, astigmatism and cataract).

Scope of syllabus: Various parts of the external structure of the brain and its parts (Medulla Oblongata, Cerebrum, Cerebellum, Thalamus, Hypothalamus) and their functions; reference should be made to the distribution of white and gray matter internally. Diagrammatic explanation of the reflex arc, showing the pathway from receptor to effector, differences between natural and acquired reflex should be taught. Structure and function of the Eye and Ear and their various parts. The external and V.S. of the eye must be taught with a brief idea of stereoscopic vision. The course of perception of sound in human ear. Role of ear in maintaining balance.

Every organism must somehow become aware of what is going on around it and accordingly perform actions for its survival. Apart from actions which provide adjustments to the external environment, there are so many activities going on inside our body of which we are unaware. All such actions have to be properly timed and coordinated. Such coordination occurs by two agencies - the nervous system and the hormonal system. The nervous system is dealt here in chapter 9 and the hormonal system in the next chapter 10. In section 9 A of this chapter, you will read about the central and peripheral nervous system and in section 9 B, about the sense organs.

9 A. The Brain, The Spinal Cord And The Peripheral Nervous System

9.1 Need Of Nervous System

The nervous system in our body performs the following major functions:

1. Keeps us informed about the outside world through the sense organs.

2. Enables us to remember, to think and to reason out.

3. Controls and harmonises all voluntary muscular activities, e.g., running or even, holding this book in your hand while you are reading it.

4. Regulates involuntary activities such as breathing or the beating of the heart, without our thinking about them.

9.2 Neuron (or Nerve Cell) - The Unit Of The Nervous System

Our nervous system consists of brain, spinal cord, sense receptors and a whole lot of nerves. The brain and spinal cord are made up of neurons or nerve cells. So, let us first learn about these cells, the neurons.

9.2.1 Structure Of The Neuron (Fig. 9.1 A)

The three main parts of a neuron are the cell body, dendrites and axon:

(i) The cell body (Perikaryon or Cyton) (peri: surrounding, karyon: nucleus)

It contains a well-defined nucleus, surrounded by granular cytoplasm.

It has all the cell organelles like other cells, only centrisome is absent because nerve cells have lost the ability to divide.

(ii) Dendrites (dendron: tree-branch)

These are branched cytoplasmic projections of the cell body. They conduct nerve impulses to the cyton.

(iii) Axon

It is a long process from the cell body.

It varies in size from a few millimetres to even more than one metre in length.

In most neurons, the axon is surrounded by a white insulating sheath known as myelin (or medullary) sheath which is covered by an outermost thin sheath called (neurolemma).

The myelin sheath shows gaps throughout the length, which are called Nodes of Ranvier.

Some axons may have side branches called collaterals.

The end portions of the axons have swollen ends like "bulbs", which store certain chemicals called neurotransmitters.

Axon terminals are closely placed near the dendrites of another one or more neurons but are not connected (Fig. 9.1 B). Such gaps in between are called synaptic clefts (syn: together, apse: gap/window).

Some basic terms in nervous activity:

Stimulus: an agent or a sudden change of the external or internal environment that results in a change in an organism or any of its body parts.

Response: the change in an organism resulting due to stimulus.

Impulse: a wave of irritability (electrical disturbance that sweeps over the nerve cell)

Receptors: the specialised epithelial cells which, on receiving the stimulus, set up waves of impulses towards the central nervous system.

Effectors: muscles or glands which, on receiving the impulse from the brain or spinal cord, contract or secrete substances.

The generalised structure of a neuron is shown in Fig. 9.1 A.

Transmission Of The Nerve Impulse

In the normal (resting) condition, the outer side of the nerve fibre carries positive (+) charge. This is called polarised state. This polarisation is due to more Na+ ions outside the axon membrane (Fig. 9.2 A).

On stimulation (mechanical, vertical, chemical or heat, etc.), the axon membrane at that spot becomes more permeable to Na+ ions which move inwards and cause loss of polarisation (depolarisation). This is known as the excited region.

The point of depolarisation becomes a stimulus for the next neighbouring area of the membrane which in turn becomes depolarised (Fig. 9.2 C).

Meanwhile, the previous area becomes repolarised due to active transport of Na+ ions again to the outside (Fig. 9.2 C). This transport is achieved by what is called "sodium pump" using energy through ATP.

Conduction of nerve impulse is a wave of depolarisation followed by repolarisation.

Do not compare conduction of nerve impulse in a nerve fibre with the flow of electricity through an electric wire. In the latter, the electrons actually do move along the wire whereas neither any substance nor any electrons or the ions move along the nerve fibre.

Speed of nerve impulse and speed of electricity:

Electricity is conducted through a wire at a speed of about 150,000 km per second, but the nerve impulse travels at a maximum speed of about 100 metres per second only.

9.2.2 Synapse

Synapse is the point of contact between the terminal branches of the axon of a neuron with the dendrites of another neuron separated by a fine gap (Fig. 9.1B). Here, the nerve impulse "jumps" into the next neuron. This is a chemical process. As the impulse reaches the terminal end of an axon, a chemical acetylcholine is released. This chemical sets a new impulse in the dendrites of the adjacent (next) neuron. The chemical is soon broken down by an enzyme to make the synapse ready for the next transmission.

9.2.3 Types Of Neurons (Fig. 9.1 B)

1. Sensory neurons convey the impulse from the receptor (sense organ) to the main nervous system (the brain or spinal cord).

2. Motor neurons carry the impulse from the main nervous system to an effector (muscle or gland).

3. Association (connecting) neurons are located in the brain and spinal cord, which interconnect the sensory and motor neurons.

9.3 Nerves

Nerve is a bundle of nerve fibres (axons) of separate neurons, enclosed in a tubular sheath.

Nerves are the thread-like white structures which emerge from the brain and spinal cord and branch out to almost all parts of the body. A nerve (Fig. 9.3) may be compared to an underground electric cable containing numerous conducting wires, each insulated from the other. The myelin sheath of the axon acts like an insulation and prevents mixing of impulses in the adjacent axons.

9.3.1 Three Kinds Of Nerves Are As Follows:

1. Sensory nerves contain only sensory fibres bringing impulses from the receptors (sense organs) to the brain or spinal cord.

Example: Optic nerve arising from the eye and ending in the brain.

2. Motor nerves contain only motor fibres carrying impulses from the brain or spinal cord to effector organs (muscles or glands) to bring them into action.

Example: a nerve arising from the brain and supplying the muscles of the eyeball for rotating the eye.

3. Mixed nerves are those, that contain both sensory and motor fibres.

Example: a spinal nerve.

Ganglia (sing. ganglion) are the aggregates of the nerve cells (cell bodies) from which the nerve fibres may arise or enter into.

Teacher's Note

The nervous system allows you to react instantly to changes around you - like pulling your hand away from a hot surface. This quick response is possible because of the efficient structure and organization of neurons and nerves working together.

This is a preview of the first 3 pages. To get the complete book, click below.