Nervous System
Question 1. Define the nervous system and list its functions.
Answer.
The nervous system is the most complicated, widely investigated, and least understood system of the body. It controls all the other systems of the body. Hence, it is also called the master system of the body.
Nervous System Functions
- Provides information on the internal and external environment.
- Integrates and processes sensory information.
- Controls motor activities of the body.
- Assimilates experience.
- Responsible for memory, learning, and intelligence.
Question 2. What is Neuron? Describe the structure of a typical neuron.
Answer.
The nerve cell along with its processes is called a neuron.
Structure of a typical neuron
The typical neuron consists of a cell body (perikaryon) and two types of processes: short and long. The typical neurons have many short processes called dendrites, and a single long process called an axon.
- Cell body (perikaryon): It is a mass of cytoplasm bounded by a plasma membrane. The cytoplasm contains a large nucleus with prominent nucleolus and basophilic Nissl bodies (Nissl substance). The Nissl substance forms the characteristic feature of nerve cells. It consists of large aggregations of rough endoplasmic reticulum made of ribonucleic acid and is concerned with protein synthesis.
- Dendrites: These are many small processes that conduct impulses towards the cell body. They branch to form a dendritic tree.
- Axon: It is a single long process, which conducts impulses away from the cell body. It begins at the axon hillock (a cone-shaped portion of the cell body) and terminates by dividing into axon terminals (telocentric).
Question 3. What are the three main Morphological Types of Neurons?
Answer.
The three main morphological types of neurons are bipolar, multipolar, and pseudounipolar.
Bipolar neurons: They have two processes arising one at each pole of the cell body. Such types of neurons are found in the retina, olfactory neuroepithelium, and vestibular and cochlear ganglia.
Read And Learn More: Selective Anatomy Notes And Question And Answers
Multipolar neurons: They have several processes arising from the cell body (i.e. many dendrites and a single axon), e.g. lower motor neurons (LMNs) of the brain and spinal cord.
Pseudounipolar neurons: They have a single process arising from the cell body, which after a short course bifurcates at T-junction into peripheral and central processes, e.g. neurons of dorsal root ganglia and sensory ganglia of cranial nerves.
Question 4. Briefly describe a typical spinal nerve. What is its distribution and applied anatomy?
Answer.
There are 31 pairs of spinal nerves. However, only T3 to T6 spinal nerves represent the typical spinal nerves.
The typical spinal nerve arises from the spinal cord by two roots: anterior and posterior. The anterior root is motor while the posterior root is sensory and possesses a ganglion called dorsal/posterior root ganglion.
The two roots unite to form the nerve trunk, which divides into two rami: dorsal and ventral. The small dorsal ramus passes backward to the muscles on the back of the vertebral column. Here, it divides into medial and lateral branches which supply the muscles. Thereafter, one of them divides into medial and lateral cutaneous branches to supply the overlying skin of the back of the body.
- Soon after its formation, the large ventral ramus is connected to the sympathetic ganglion by two rami communicantes – grey and white.
- Grey ramus communicans: It consists of nonmyelinated nerve fibers, which arise from the sympathetic ganglion. These fibers (sympathetic fibers) are distributed to the blood vessels, hair, and sweat glands through the branches of both the ventral and dorsal rami of the spinal nerve.
- White ramus communicans: It consists of myelinated nerve fibers, which arise from the lateral grey horn of the spinal cord and leave the ventral ramus to enter into the ganglion of the sympathetic trunk for its distribution.
- The large ventral ramus runs laterally into the intercostal space and supplies the intercostal muscles of this space. It also gives rise to lateral and anterior cutaneous branches.
- The cutaneous branches of the dorsal and ventral rami of the spinal nerve together supply a strip of skin from the posterior median line to the anterior median line called the dermatome.
Applied anatomy
- Spinal neuralgia: It is a sharp burning sensation in the area of skin supplied by the single spinal nerve (i.e. dermatome). The most important cause of this neuralgia is Herpes Zoster, a viral infection of the spinal ganglia.
- Root pain: Compression of nerve roots during their exit from intervertebral foramina in cases of spondylitis and disc prolapse causes pain along the distribution of that nerve.
Question 5. What are the various types of fibers present in a typical spinal nerve?
Answer.
The following four types of fibers are present in a typical spinal nerve:
- Somatic of event fibers, which supply the skeletal muscles.
- Somatic of event fibers, which carry general and proprioceptive sensations from the skin, joints, and muscles.
- Visceral of event fibers, which supply the smooth muscles of viscera, blood vessels, and glands.
- Visceral of event fibers, which carry proprioceptive sensations from viscera, blood vessels, and glands.
Question 6. Write a short note on dermatome.
Answer.
The area of skin supplied by a single spinal nerve, and therefore a single spinal segment is termed dermatome. With the exception of the C1 spinal nerve, all the spinal nerves are associated with a specific dermatome.
Question 7. Write a short note on a synapse.
Answer.
- The neurons form long chains along which impulses travel from one neuron to the other and so on for long distances in different directions.
- The site of contact between two neurons where nerve impulses pass from one neuron to another is called a synapse. It is important to know that at the synapse, the contact between the neurons is by contiguity and not by continuity.
Synapse Structure Each synapse consists of
- The presynaptic membrane of one neuron,
- Synaptic cleft and
- The postsynaptic membrane of another neuron.
The pre-and postsynaptic membranes are separated by a small gap (20 nm wide) called synaptic clef.
Thus, the following are the three essential components of a synapse:
- Presynaptic knob/membrane
- Synaptic cleft
- Postsynaptic membrane
Mechanism of transmission of the impulse
- The arrival of nerve impulses at a presynaptic knob (terminal) causes the release of a neurotransmitter from presynaptic vesicles into the synaptic cleft, which binds with the receptors on the postsynaptic membrane. This binding produces a response in the postsynaptic membrane in the form of depolarization or hyperpolarization.
- The excitatory nerve impulse causes depolarization of the postsynaptic membrane, while an inhibitory impulse causes its hyperpolarization. When the depolarization reaches a threshold, an action potential is produced in the postsynaptic neuron. In this way, action potentials are transferred from one neuron to another neuron.
Question 8. Enumerate the different types of synapses.
Answer.
Anatomically, the synapses are of the following types:
- Axodendritic synapses (commonest)
- Axosomatic synapses
- Dendrosomatic synapses
- Dendroaxonic synapses
- Dendrodendritic synapses
- Axoaxonic synapses
Question 9. What are neuroglia? Enumerate the different types of neuroglia in central and peripheral nervous systems. Name the neuralgia responsible for the myelination of nerve fibers in the peripheral and central nervous systems.
Answer.
- The neuroglia are interstitial cells of the nervous system that provide structural and functional support to the neurons.
- The different types of neuroglia are present in the central nervous system (CNS) and peripheral nervous system (PNS).
- The Schwann cells (neurolemmocytes) form a myelin sheath around the peripheral nerve fibers. They also take part in the formation of the blood-brain barrier.
- The oligodendrocytes form a myelin sheath around the fibers of the central nervous system.
Different Types of Neuroglia Present in CNS and PNS
Question 10. What is peripheral nerve? List its connective tissue sheaths.
Answer.
The peripheral nerve is a bundle of nerve fibers outside the central nervous system. Examples of peripheral nerves are 12 pairs of cranial nerves and 31 pairs of spinal nerves.
Connective tissue sheaths of peripheral nerve They are three in number as follows:
- Endoneurium, which encloses a single nerve fiber.
- Perineurium, which encloses a bundle (group) of nerve fibers.
- Epineurium, which encloses the entire nerve.
Question 11. Write a short note on myelin sheath and the process of myelination of a peripheral nerve fiber.
Answer.
The thick axons of peripheral nerves are surrounded by myelin sheath, which is made up of concentric layers of plasma membrane of Schwann cells and lipids (fatty substance) between these layers. It is formed by Schwann cells by a process called myelination.
Formation of myelin sheath (myelination)
The Schwann cells are first arranged in a linear fashion (end to end) along the nerve fiber and surround it.
- The axon invaginates the side of the Schwann cell. As a result, the plasma membrane of the Schwann cell forms a mesaxon, which suspends the axon within the Schwann cell.
- The Schwann cell now rotates around the axon again and again, i.e. repeatedly. Consequently, the axon becomes spirally wrapped by many layers of elongated mesaxon. Later, the inner layers of the plasma membrane fuse with each other so that the axon becomes surrounded by several layers of modified membrane.
- The lipids are deposited between adjacent layers of the membrane. These layers of mesaxon along with lipids constitute the myelin sheath. The nerve fibers surrounded by myelin sheath are termed myelinated fibers.
Functions of myelin sheath
Provides support to nerve fibers.
- Aids speed in the conduction of nerve impulses.
- Insulates the axons from the extracellular environment.
- Provides white color to nerve fibers.
- Helps in the regeneration of nerve fibers.
Question 12. What are the nodes of Ranvier and the incisures of Schmidt–Lanterman?
Answer.
In myelinated nerve fibers, the myelin sheath is segmented. These segments are separated at regular intervals by nodes of Ranvier.
The incisures of Schmidt–Lanterman are oblique clefts in the myelin sheath and act as conduction channels for metabolites (nutrition) into depths of the myelin sheath and axon.
Question 13. Write a short note on reflex action.
Answer.
- A reflex action is an involuntary and almost instantaneous motor response of the body to sensory stimulus/stimuli, for example, when a pin is pricked in the hand, the pain impulses reach the spinal cord by sensory neurons.
- The spinal cord, then, through its motor neurons moves the muscles of the hand to keep it away from the pin. The complete pathway involved in this act constitutes the reflex action.
The reflex action is automatic and occurs independently of the higher centers of the brain. This reflex arc involved in reflex action consists of the following components:
- Receptor, for example, sensory nerve ending
- Sensory component (sensory neuron)
- Central component (spinal cord/brain)
- Motor component (motor neuron)
- Effector, for example, muscle
Question 14. Write a short on Wallerian degeneration.
Answer.
When the peripheral nerve is injured, it undergoes a process of degeneration. The changes in the part of the axon, distal to the injury are referred to as Wallerian (anterograde) degeneration. When the myelinated nerve fiber is cut, the distal segment of the cut nerve fiber gets completely fragmented and lost.
Its sheath also degenerates; however, myelin-producing sheath cells (astrocytes) survive. The Schwann cells proliferate, enlarge in size, and form solid cellular cords within the sheath of the endoneurium. Now, the proximal segment of the nerve fiber grows into the endoneural tube.
Question 15. Write a short note on referred pain.
Answer.
The pain arising from viscera is not felt at the site of diseased viscera but refers to an area of skin supplied by the nerve arising from the same spinal segment or segments that supply the viscera. For example, pain of the appendix (due to appendicitis) refers to the skin around the umbilicus because both the appendix and the skin around the umbilicus are supplied by the T10 spinal segment. The important sites of referred pain.
Important Sites of Referred Pain
Question 16. What are the sympathetic and parasympathetic nervous systems?
Answer.
The sympathetic and parasympathetic are the two main divisions of the autonomic nervous system.
- Sympathetic nervous system: This division of the autonomic nervous system is also called ‘thoracolumbar outflow’ because its preganglionic nerve fibres arise from thoracic and upper two lumbar (T1 to L2) spinal segments.
- In general, the sympathetic nervous system mobilizes the body’s energy during flight and fight, i.e. during an emergency.
- Parasympathetic nervous system: This division of the autonomic nervous system is also called craniosacral outflow because its preganglionic fibres arise from brainstem nuclei (e.g. Edinger–Westphal nucleus, lacrimatory nucleus, and superior and inferior salivatory nuclei) and 2nd, 3rd and 4th sacral (S2, S3, and S4) spinal segments.
- In general, the parasympathetic nervous system conserves the body’s energy, during sleep.
Question 17. Enumerate the functional differences between sympathetic and parasympathetic nervous systems.
Answer.
Differences Between Sympathetic and Parasympathetic Nervous System
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