Class 10, Science, Chapter-6, Lecture-1, Nervous System (Notes)

NERVOUS SYSTEM:

An organised network of nerve cells (neurons) which is specialised for conducting information via electrical impulses from one part of body to another is termed as nervous system.

NEURON:

A cell which is specialised for conducting information via electrical impulses from one part of body to another is called a neuron. 
A neuron is the structural and functional unit of nervous system.

Structure Of A Neuron:

A neuron consists of three distinct regions.

• Dendrites – the tip acquires information and transmits it as electrical impulse to the cyton
• Cell body or cyton – receives electrical impulse from dendrites.
• Axon ­– transmits electrical impulse away from the cyton to the nerve ending.

The axons communicate with other neurons through Synapses.

DIFFERENCES BETWEEN:

TYPES OF FUNCTIONAL NEURONS:

Sensory neurons:
carry nerve impulses from receptor to central nervous system (CNS).
Receptors $$ \to $$ CNS

Motor neurons:
take nerve impulses(messages) from CNS to effector.
CNS $$ \to $$ Effectors

Interneurons: (association neurons)
found completely within the CNS and make links between sensory and motor neurons.

MECHANISM OF TRANSFER OF IMPULSE:

Steps involved–

1. The receptors present in various sense organs detect information from the environment.
2. The dendritic tip of the neuron present in the receptors causes a chemical reaction.
3. The chemical reaction creates an electrical impulse.
4. The dendrite sends the impulse towards the cell body and the axon sends it away from the cyton to the nerve ending.
5.  At the nerve ending the electrical impulse causes release of some chemicals into the synapse.
6.  The chemical passes through the synapse and creates an electrical impulse in the dendrite of the next neuron.
7. This process is repeated till the impulse reaches its target.

Drawbacks of electrical impulse: 

1. (i) Electrical impulse cannot reach to the cells which are not connected by nervous tissue. 
2. (ii) Cells cannot continually create and transmit electrical impulse as a cell takes time to reset for
   generation of new 
   impulse.
   The body overcomes this by creating chemical impulse in which the stimulated cells release a chemical that diffuses all around the cell and reaches to the cells which are not connected by nerve cells.

REFLEX ACTION:

An unconscious and involuntary response of the body to a stimulus is termed as a reflex action.

REFLEX ARC:

The nerve pathway involved in an unconscious and involuntary response of the body to a stimulus is called a reflex arc.

EXAMPLE OF REFLEX ACTION:

Knee-jerk reflex:
While sitting with freely hanging legs, a strike below the knee cap gives a spontaneous jerk and brings the legs forward.
It involves sensory neurons, spinal cord and motor neurons.

Role of human brain in reflex action: 

Brain is not directly involved in reflex action, but the impulse also reaches brain. This helps the brain to record this event and remember it for future use. Sometimes brain may use previous information to stop reflex action.

HUMAN NERVOUS SYSTEM:

Human nervous system consists of CNS (Central Nervous System) and PNS (Peripheral Nervous System)
CNS includes brain and spinal cord.
PNS includes all the nerves arising from CNS and reaching all external and internal organs. 

• cranial nerves arise from brain.
• spinal nerves arise from spinal cord.
• visceral nerves arise from brain and spinal cord and reaches internal organs to form Autonomic Nervous System (involuntary peripheral nervous system).

DIFFERENCE BETWEEN: