NCERT Class 11-Biology: Chapter –21 Neural Control and Coordination Part 4

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Question 6:

Label the following parts in the given diagram using arrow.

The external layer of eye ball

The External Layer of Eye Ball

The external layer of eye ball

a. Aqueous chamber

b. Cornea

c. Lens

d. Retina

e. Vitreous chamber

f. Blind spot

Answer:

  • The anterior portion of sclera (the external layer of eye ball) is called cornea.The eye ball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body

  • The inner layer of the human eye is the retina.

  • The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humor.

  • The space between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called vitreous humor.

  • Blind spot is the small region in the eye where photoreceptor cells are absent. Therefore, there is no photo detection in this area.

Blind spot

Blind Spot

Blind spot

Long Answer Type Questions

Question 1:

Explain the process of the transport and release of a neurotransmitter with the help of a labelled diagram showing a complete neuron, axon terminal and synapse.

Answer:

  • A neuron can be divided mainly into three parts –

    • Axon

    • Cell body

    • Dendrites

  • Any nerve impulse (bioelectric/electrochemical disturbance that passes along neuron during conduction of an excitation) is passed from one neuron to another neuron via axon.

  • Transport and release of a neurotransmitter occurs within a synapse.

  • At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft.

  • Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses.

  • The axon terminals contain vesicles filled with these neurotransmitters.

  • When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane, where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft.

  • The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane.

  • This binding opens ion channels allowing the entry of ions which can generate a new action potential in the post-synaptic neuron.

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