Chapter 1 biology

 SENSATIONS AND RESPONSES

A. Stimulus : 

The sense that evokes responses in  organisms 

 External stimuli :-Sound, touch, heat,pressure, cold 

 Internal stimuli :- Hunger, thirst, infection …

 The Nervous system generates and coordinates 

 responses through impulses.

B. Neurons (nerve cells):-

 The structural and functional units of the nervous system.

          Function 

Dendrite        :         Receives impulses

Dendron    :        Carries impulses from dendrite to 

                          the cell body 

Cyton / 

Cellbody      :    Passes impulses to the axon.

                             (controlling centre)

Axon           :      Carries impulses from the cell 

                           body to outside.

Myelin sheath :    Protects the axon and increases the 

                                 speed of impulse

Axonite         :     Carries impulses to the synaptic 

                             knob

Synaptic knob:     Secretes neurotransmitter

       The Schwann cells of myelin sheath is formed of   

 specialized cells called oligodendrocytes.

Myelin sheath : Provide nutrients and oxygen to the 

 axon, accelerate impulse, act as an electric insulator.

 protects the axon from external shocks and gives 

 white appearance ('white matter') to the neural parts.

The part of nerve, where myelinated neurons are 

present in abundance,is called as the white matter and the part of nerve where myelinated neurons are absent, is called as  the grey matter.

C. Synapse : 

The junction between neurons or 

 between neurons and muscles or glands.

 It regulates the speed and direction of impulses.

The impulses are transmitted across the synaptic 

cleft only through a chemical (neurotransmitter), 

 secreted from the synaptic knobs.

Acetyl choline and Dopamine are neurotransmitters.

D. Impulse generation : 

 When stimulated, this ionic equilibrium (polarity) changes there and the outer surface becomes egatively charged and inner  become positively charged. As a result, impulse  generated. Impulses are electrical messages conducted through nerves.

Transmission of impulse :

 Impulse due to stimulus – dendrites – dendrons – 

 cyton – axon – axonites – synaptic knob – secretion 

 of neurotransmitter to the synaptic cleft –stimulation

 in the adjacent dendrites – impulse forms.

E. Types of neurons :

Sensory : Carry impulses from sense organs to the 

 brain and spinal cord.

Motor : Carry impulses from brain and spinal cord to

 different organs.

Mixed : Carry impulses from brain and spinal cord to 

 different organs and vise versa