SSLC NOTES (chapter 1)

               SENSATIONS AND RESPONSES

Stimuli : The senses that evoke responses in organisms. There are specialized cells in the sense organs and other parts of the body to receive stimuli. These cells are known as receptors. They receive stimuli and generate suitable impulses.

Neuron 

•  Axons of most of the neurons are repeatedly encircled by myelin, a membrane containing lipid. This is called myelin sheath.
•  Nerve is a group of axons. Myelin sheath in the nerves is formed of Schwann cells.
•  Myelin sheath in the brain and the spinal cord is formed of specialized cells called oligodendrocytes.
•  The myelin sheath has a shiny white colour.
• The part of the brain and the spinal cord where myelinated nerve cells are present in abundance is called white matter and the part where non-myelinated nerves cells are present is called grey matter.

Functions of the myelin sheath

1.  Provide nutrients and oxygen to the axon
2. Accelerate impulses
3. Act as an electric insulator and protect the axon from external shocks.

Generation and Transmission of Impulses

1. The outer surface of the plasma membrane of the neuron is positively charged and the inner surface        is negatively charged. This is due to the difference in the distribution of certain ions
2.  When stimulated, the distribution of ions in that particular part changes and hence the inner surface        becomes positively charged and the outer surface becomes negatively charged
3. When stimulated, the distribution of ions in that particular part changes and hence the inner surface        becomes positively charged and the outer surface becomes negatively charged

Synapse

• Synapse is the junction between two neurons or a neuron and a muscle cell or a neuron and a      glandular cell.
• When electric impulses from the axon reach the synaptic knob, certain chemical substances are    secreted from there to the synaptic cleft. These chemical substances are called neurotransmitters.
• They stimulate the adjacent dendrite or cell and new electric impulses are generated.

Example: Acetylcholine and dopamine
Function : Regulate the speed and direction of impulses.

Nerves

• Nerves are group of axons or nerve fibres

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