Structure of a Neuron MCQ - Practice Questions with Answers

Neuron As Structural And Functional Unit Of Neural System - A Revision of Neuron

• A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon. Approximately 100 billion neurons are found in the human brain
• The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules.
• Short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites. 
• These fibres transmit impulses towards the cell body. 
• The axon is a long fibre, the distal end of which is branched.
• Each branch terminates as a bulb-like structure called synaptic knob which possesses synaptic vesicles containing chemicals called neurotransmitters. 
• The axons transmit nerve impulses away from the cell body to a synapse or to a neuromuscular junction

Neurons are classified in number of ways 

      A) Based on the number of axon and dendrites, the neurons are divided into four types:

1. Unipolar (cell body with one axon only; usually found in the embryonic stage)
2. Bipolar (with one axon and one dendrite, found in the retina of the eye) 
3. Pseudounipolar (one axon that splits into two branches)
4. Multipolar (with one axon and two or more dendrites; found in the cerebral cortex)

       B) Based on absence and presence of Myelin sheath, the neurons are of two types :

1. Unmyelinated Neurons : Axon is not covered by myelin sheath
2. Myelinated Neurons : Axon shows a covering known as Myelin Sheath that acts as thermal insulator

      C) Based on direction of impulses 

1. Sensory : Also known as Afferent Neurons. These transmit information from sensory receptors in the skin, or the internal organs toward the CNS for processing
2. Motor : Also known as Efferent Neurons. These transmit information away from the CNS toward some type of effector. 
3. Mixed : These transmit impulses in both the directions

NOTE : NERVOUS TISSUE CONSISTS OF NEURONS AND NEUROGLIAL CELLS. NEUROGLIAL CELLS ARE NON-CONDUCTING CELLS THAT ARE SUPPORTING CELLS. ASTROCYTES, EPENDYMAL CELLS, SCHWANN CELLS AND OLIODENDROCYTES ARE GLIAL CELLS

Terminology

• Nerve Termination: Nerve fibre can terminate in the following ways:

                     - At a synapse when the axon of one neuron ends at the dendrites of the other neuron.
                     - At a neuromotor junction when the axon of a neuron ends on the muscle fibre.
                     - At a neuroglandular junction when the axon of a neuron terminates on a gland.
                     - Sensory nerve endings are formed when the nerve fibre divides into fine branches.

• Nerve: Bundle of axons in the peripheral nervous system
• Tract: Bundle of axons in the central nervous system
• Ganglion: Bundle of cell bodies in the peripheral nervous system
• Nucleus: Bundle of cell bodies in the central nervous system
• A stimulus is a sudden change in the environment (can be internal or external). It is capable of causing neuron excitation. If the stimulus is capable of exciting the neural tissue, it is called the threshold stimulus. 
• Excitability is the ability of neurons to get into the active state in response to a stimulus. 
• Conductivity is the ability of neurons to transmit the excitation in a particular direction.
   

Saltatory Conduction & All-or-None Principle of Nerve Impulse

• If an axon is unmyelinated, an action potential at one locale stimulates an adjacent part of the axon membrane to produce an action potential. 
• In myelinated fibres, an action potential at one node of Ranvier causes an action potential at the next node. 
• This is called saltation. 
• This type of conduction, called saltatory conduction, is much faster than otherwise. 
• In thin, unmyelinated axons, the action potential travels about 1.0 m/sec, and in thick, myelinated fibres, the rate is more than 100 m/sec. 
• The conduction of a nerve impulse (action potential) is an all-or-none event; that is, either an axon conducts a nerve impulse or it does not. 
• The intensity of a message is determined by how many nerve impulses are generated within a given time span.