Neuron as Structural and Functional Unit of Neural System Questions in English

(D) The correct answer is $D$. The $Myelin$ $sheath$ is a lipid-rich,strong insulating layer that surrounds the axons of many neurons.
It acts as an electrical insulator,which prevents almost all flow of ions through the axonic surface.
By preventing ion leakage,it ensures that the nerve impulse is conducted efficiently and rapidly along the nerve fibre.

(B) The transmission of a nerve impulse involves the movement of ions ($Na^+$ and $K^+$) across the axonal membrane against their concentration gradients. This process is mediated by the $Na^+-K^+$ pump,which is an active transport mechanism. This pump requires $ATP$ (Adenosine Triphosphate) to function. The $ATP$ is generated through cellular respiration occurring within the mitochondria present in the nerve fibre (axon). Therefore,the nerve fibre itself provides the necessary energy for impulse transmission.

(D) The neuron consists of three main parts: cell body (cyton),dendrites,and axon.
$1.$ Dendrites are short,branched processes that receive stimuli and conduct them towards the cell body.
$2.$ The axon is a long,single process that conducts nerve impulses away from the cell body towards a synapse or a neuromuscular junction.
Therefore,the axon is highly modified for the conduction of impulses away from the cell body of the neuron.

(A) The $Nissl's$ granules are characteristic granular bodies found in the cell body $(cyton)$ and dendrites of neurons.
They are composed of rough endoplasmic reticulum $(RER)$ with ribosomes.
Their primary function is the synthesis of proteins required for the maintenance and functioning of the nerve cell.

(C) Polio is caused by the poliovirus,which enters the body through the mouth and multiplies in the intestine. From there,it enters the bloodstream and reaches the central nervous system. The virus specifically targets and destroys the motor neurons located in the anterior horn cells of the spinal cord. Since these motor neurons are responsible for transmitting signals from the brain to the muscles,their destruction leads to the loss of muscle control,resulting in paralysis and the eventual wasting away of the affected limbs.

(A) The ventricles of the brain and the central canal of the spinal cord are lined by a specialized type of glial cells called $Ependymal$ cells. These cells are ciliated epithelial cells that play a crucial role in the production and circulation of the cerebrospinal fluid $(CSF)$.

(C) Regeneration is the ability of cells to divide and replace damaged or dead tissues.
$1$. Neurons $(C)$ are highly specialized cells that have lost the ability to divide after reaching maturity.
$2$. Osteocytes $(A)$ are bone cells that retain some capacity for repair.
$3$. Hepatocytes $(B)$ have a high regenerative capacity,allowing the liver to regrow even after significant damage.
$4$. The Malpighian layer of the skin $(D)$ consists of actively dividing cells that constantly renew the epidermis.
Therefore,neurons show the least amount of regeneration because they do not divide in adults.

(C) The cranial cavity,which contains the brain,is lined by a specialized type of glial cell known as $Ependymal$ cells. These cells form the epithelial-like lining of the ventricles of the brain and the central canal of the spinal cord. They are involved in the production and circulation of cerebrospinal fluid $(CSF)$.

(A) Dendrites are short,branched extensions of a neuron that receive signals from other neurons or sensory receptors.
These structures are a fundamental component of the nervous system,which is responsible for transmitting electrical impulses throughout the body.
Therefore,dendrites are directly associated with the nervous system.

(B) In the central nervous system,the nervous tissue is divided into grey matter and white matter.
Grey matter primarily consists of neuronal cell bodies,dendrites,and unmyelinated axons.
White matter primarily consists of myelinated axons.
The characteristic 'white' color of white matter is due to the presence of the myelin sheath,which is a lipid-rich insulating layer.
Conversely,grey matter appears grey because it lacks this myelin sheath,making the absence of the myelin sheath the distinguishing factor.

(B) The nerve fibers (axons) are often covered by a fatty insulating layer called the $Myelin$ $sheath$. This sheath is formed by $Schwann$ cells in the peripheral nervous system and oligodendrocytes in the central nervous system. It helps in increasing the speed of nerve impulse conduction.

(B) In a myelinated nerve fiber,the axon is covered by a myelin sheath,which is formed by Schwann cells. The myelin sheath is interrupted at intervals known as the Nodes of Ranvier. In contrast,an unmyelinated nerve fiber is enclosed by a Schwann cell that does not form a myelin sheath around the axon,and consequently,it lacks the Nodes of Ranvier.

(A) neuron is the structural and functional unit of the neural system. It consists of three major parts:
$1$. Cell body (Cyton/Soma)
$2$. Dendrites (represented as $a$)
$3$. Axon (represented as $b$)
Options $c$ (sarcolemma) and $d$ (neurilemma) are not parts of the neuron structure itself. Sarcolemma is the plasma membrane of a muscle fiber,and neurilemma is the outermost nucleated cytoplasmic layer of Schwann cells.
Therefore,the neuron consists of dendrites and an axon. The correct option is $A$.

(B) Nissl granules (also known as Nissl bodies) are large granular bodies found in neurons.
They are composed of rough endoplasmic reticulum $(RER)$ with rosettes of free ribosomes.
These structures are the sites of protein synthesis within the neuron,which is essential for the maintenance and repair of the nerve cell.

(A) In a myelinated nerve fiber, the myelin sheath is not continuous.
It is interrupted at regular intervals by gaps known as the $Node of Ranvier$.
These nodes allow for saltatory conduction, which significantly increases the speed of nerve impulse transmission along the axon.

(A) $Nissl$ granules are irregular masses of rough endoplasmic reticulum $(RER)$ with numerous free ribosomes and are the sites of protein synthesis. They are found in the cell body $(cyton)$ and dendrites of neurons. However,they are characteristically absent in the axon and the axon hillock.

(C) Nissl's granules (or Nissl bodies) are large granular bodies found in neurons.
These structures are sites of protein synthesis.
Electron microscopy has revealed that Nissl's granules are composed of rough endoplasmic reticulum $(RER)$ and free ribosomes.
Therefore,they are currently identified as clusters of ribosomes.

(C) The "Nodes of Ranvier" are periodic gaps in the insulating myelin sheath on the axon of certain neurons.
These gaps facilitate the rapid conduction of nerve impulses through saltatory conduction.
Therefore, they are specifically associated with the axon of a neuron.

(A) Nissl granules are granular bodies found in the cell body $(cyton)$ and dendrites of a neuron.
They are composed of rough endoplasmic reticulum $(RER)$ and free ribosomes, which are involved in protein synthesis.
These granules are characteristically absent in the axon and the axon hillock, which is the region where the axon originates from the cell body.

(B) Afferent nerve fibers (also known as sensory nerve fibers) are responsible for transmitting nerve impulses from the sensory receptors (which detect stimuli) to the Central Nervous System $(C.N.S.)$.
In contrast,efferent nerve fibers transmit impulses from the $C.N.S.$ to the peripheral effector organs (such as muscles or glands) to initiate a response.
Therefore,the correct pathway for afferent fibers is from the receptor to the $C.N.S.$

(A) Nissl granules are irregular masses of rough endoplasmic reticulum $(RER)$ with ribosomes found in the cell body $(cyton)$ and dendrites of neurons.
Their primary function is the synthesis of proteins,which are essential for the maintenance,repair,and metabolic activities of the nerve cell.

(A) The $Neuron$ (nerve cell) is the longest cell in the human body. Some neurons, such as those extending from the spinal cord to the feet, can reach lengths of over $1$ meter ($3.28$ feet), which is significantly longer than one foot.

(B) Neurons are highly specialized cells that form the structural and functional units of the nervous system. Once they reach maturity after birth,they lose the ability to divide through mitosis. This is why damage to the central nervous system is often permanent,as these cells cannot be replaced by cell division.

(C) Neuroglial cells,commonly known as glial cells or glia,are non-neuronal cells in the central nervous system $(CNS)$ and peripheral nervous system $(PNS)$.
They provide support,protection,and nutrition to neurons.
Since the brain is the primary component of the central nervous system,neuroglial cells are essentially associated with the brain and spinal cord.
Therefore,the correct option is $C$.

(B) Neurons are the structural and functional units of the neural system. $A$ typical neuron consists of three major parts: the cell body (cyton),dendrites,and an axon.
$1.$ Dendrites are short fibers that branch repeatedly and project out of the cell body.
$2.$ The axon is a long fiber,the distal end of which is branched.
$3.$ Myofibrils are specialized contractile structures found in muscle cells,not in neurons.
$4.$ Neurilemma is the outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of the neuron.
Therefore,neurons do not contain myofibrils.

(D) Medullated or myelinated nerve fibers are characterized by the presence of a myelin sheath around the axon.
In vertebrates,myelinated fibers are generally present in the central and peripheral nervous systems.
However,in $Cyclostomes$ (such as lampreys and hagfish),the nerve fibers are typically non-myelinated or unmyelinated.
Therefore,$Cyclostomes$ are the group among the given options where medullated nerve fibers are not found.

(B) neuron consists of three main parts: the cell body (cyton),dendrites,and the axon.
$1$. Dendrites are short,highly branched projections that receive signals from other neurons.
$2$. The axon is a long,single fiber that carries impulses away from the cell body.
$3$. The terminal end of the axon is branched into synaptic knobs,but the dendrites are characterized by being highly branched and distributed around the cell body to increase the surface area for signal reception.
Therefore,dendrites are the highly branched structures.

(C) In a resting state,the axonal membrane is significantly more permeable to potassium ions $(K^+)$ and nearly impermeable to sodium ions $(Na^+)$.
Additionally,the membrane is impermeable to the negatively charged proteins present in the axoplasm.
These conditions maintain the resting membrane potential,where the interior of the axon is negatively charged relative to the exterior.
Therefore,both statements $(A)$ and $(B)$ are correct.

(A) The transmission of a nerve impulse follows a specific pathway through a neuron and across a synapse.
$1$. The impulse is first received by the $D$ (Dendrite).
$2$. It then travels to the $CB$ (Cell Body).
$3$. From the cell body,it moves along the $A$ (Axon).
$4$. Finally,it reaches the $S$ (Synapse),where it is transmitted to the next neuron.
$5$. Therefore,the sequence for a single neuron transmission is $D \rightarrow CB \rightarrow A \rightarrow S$. When considering the path across multiple neurons,the sequence repeats: $D - CB - A - S - D - CB - A$.

(A) Neurons (nerve cells) are structurally distinct from other cells in the body due to the presence of $Nissl's$ granules.
$Nissl's$ granules are irregular masses of rough endoplasmic reticulum $(RER)$ with attached ribosomes,which are involved in protein synthesis.
These granules are found in the cell body $(cyton)$ and dendrites of neurons but are absent in the axon.
While mitochondria and other organelles are present in most cells,$Nissl's$ granules are a characteristic feature unique to neurons.

(C) Neurons are highly specialized cells that have lost the ability to divide. The process of cell division requires the presence of centrioles to form the spindle apparatus. Since mature neurons do not undergo mitosis,they lack centrioles,which prevents them from dividing.

(B) Schwann cells are specialized glial cells that wrap around the axons of neurons in the peripheral nervous system $(PNS)$.
They are responsible for the formation of the myelin sheath,which insulates the axon and increases the speed of nerve impulse conduction.
Therefore,Schwann cells are specifically associated with the axon structure of a neuron.

(D) $Schwann$ cells are the primary glial cells of the peripheral nervous system $(PNS)$.
They are responsible for the formation of the myelin sheath around the axons of neurons.
In myelinated nerve fibers,$Schwann$ cells wrap around the axon multiple times to form the myelin sheath.
In unmyelinated nerve fibers,$Schwann$ cells still surround the axons but do not form a thick myelin sheath.
Therefore,$Schwann$ cells are associated with both myelinated and unmyelinated nerve fibers.

(B) The myelin sheath is an insulating layer that forms around nerves,including those in the brain and spinal cord.
In the peripheral nervous system $(PNS)$,the myelin sheath is formed by $Schwann$ cells,which wrap their plasma membranes around the axon multiple times.
In the central nervous system $(CNS)$,the myelin sheath is formed by oligodendrocytes.
Both $Schwann$ cells and oligodendrocytes are types of neuroglial cells.
However,since $Schwann$ cells are specifically responsible for the formation of the myelin sheath in the peripheral nerves,they are the most direct answer among the given options.

(A) The nervous system is composed of specialized cells called neurons (nerve cells).
Neurons are the structural and functional units of the nervous system.
They are responsible for receiving,processing,and transmitting information through electrical and chemical signals.
Therefore,the correct option is $A$.

(D) Neurons are specialized cells that possess unique structures not found in typical animal cells.
$1$. $Perikaryon$ (also known as the cell body or soma) contains $Nissl$ granules,which are specialized rough endoplasmic reticulum involved in protein synthesis.
$2$. Dendrites are specialized cytoplasmic extensions that receive signals from other neurons.
$3$. While other cells have nuclei and mitochondria,the presence of $Perikaryon$ with $Nissl$ granules and specialized processes like dendrites and axons is unique to neurons.

(D) The neuron consists of three main parts: the cell body (soma),dendrites,and axon.
$1$. The cell body (soma) contains cytoplasm with typical cell organelles and Nissl's granules.
$2$. These organelles are responsible for the synthesis of proteins and other cellular components,which is the basic cellular function.
$3$. Dendrites receive signals,and the axon transmits signals away from the cell body.
Therefore,the cell body is the site of synthesis.

(B) The myelin sheath is an insulating layer or sheath that forms around nerves,including those in the brain and spinal cord. It is composed of protein and fatty substances. This myelin sheath covers the axon of a nerve fiber,which allows electrical impulses to transmit quickly and efficiently along the nerve cells.

(D) neuron is the structural and functional unit of the neural system. It consists of three major parts: the cell body (cyton),dendrites,and axon.
$1$. Dendrites are short fibers that branch repeatedly and project out of the cell body; they transmit impulses towards the cell body.
$2$. The axon is a long fiber,the distal end of which is branched into synaptic knobs called telodendria; it transmits nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.
$3$. Nissl's granules are present in the cell body and dendrites but are absent in the axon.
$4$. Based on the number of axon and dendrites,neurons are divided into multipolar,bipolar,and unipolar,not apolar.

(B) Neurons are classified based on the number of axons and dendrites extending from the cell body.
$1$. $Unipolar$ neurons have only one axon and are typically found in embryonic stages.
$2$. $Bipolar$ neurons have one axon and one dendrite, found in the retina of the eye.
$3$. $Multipolar$ neurons have one axon and two or more dendrites. These are the most common type of neurons in the human body, especially in the cerebral cortex.
$4$. $Pseudounipolar$ neurons have a single process that splits into two branches, commonly found in the dorsal root ganglia.
Therefore, $Multipolar$ neurons are the most abundant.

(A) Nissl's granules are irregular masses of rough endoplasmic reticulum $(RER)$ and free ribosomes found in the cell body $(cyton)$ and dendrites of neurons.
These granules are primarily composed of ribosomal $RNA$ $(rRNA)$ and proteins,which are essential for the synthesis of proteins required for neuronal function and repair.

(B) In a normal myelinated nerve fiber,the myelin sheath is interrupted by gaps known as the $Nodes$ $of$ $Ranvier$.
These nodes allow for saltatory conduction,where the nerve impulse 'jumps' from one node to the next,significantly increasing the speed of transmission.
If the myelin sheath were continuous,the saltatory conduction mechanism would be impossible.
The impulse would have to travel along the entire length of the axon membrane without jumping,which would drastically reduce the speed of nerve conduction.

(B) The $Nodes$ of $Ranvier$ are small gaps or interruptions found in the myelin sheath of myelinated axons.
These gaps allow for the saltatory conduction of nerve impulses,which significantly increases the speed of transmission.
While the myelin sheath is discontinuous at these points,the neurilemma (the outermost layer of the Schwann cell) remains continuous.
Therefore,the $Nodes$ of $Ranvier$ are characterized by the discontinuity of the myelin sheath.

(B) $Nissl$ granules are irregular masses of rough endoplasmic reticulum $(RER)$ with numerous free ribosomes and polyribosomes.
They are the site of protein synthesis within the cell.
These granules are characteristically found in the cell body $(cyton$ or $soma)$ and dendrites of neurons.
They are absent in the axon and axon hillock.

(B) Multiple sclerosis $(MS)$ is an autoimmune disorder where the immune system mistakenly attacks the protective covering of nerve fibers,known as the myelin sheath.
This damage disrupts the communication between the brain and the rest of the body.
As the myelin sheath is destroyed,nerve impulses are slowed down or blocked,leading to various neurological symptoms.
Therefore,the correct answer is an antibody attack on the myelin sheath of neurons.

(D) Multiple Sclerosis is an autoimmune disorder of the central nervous system.
In this condition,the immune system mistakenly attacks the protective covering of nerve fibers,known as the myelin sheath.
This damage disrupts the communication between the brain and the rest of the body,leading to various neurological symptoms.
Therefore,the correct characteristic is the attack of antibodies on the myelin sheath of nerve fibers.

(C) The nerve fiber (axon) is often surrounded by a fatty,lipid-rich insulating layer known as the myelin sheath. This layer is formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. Its primary function is to increase the speed of nerve impulse conduction along the axon.

(A) The myelin sheath is an insulating layer or sheath that forms around nerves,including those in the brain and spinal cord. It is composed of protein and fatty substances (lipids). Specifically,the myelin sheath is primarily made of lipids (about $70-80\%$) and proteins (about $20-30\%$). Among the given options,lipids are the primary structural component that provides the insulating property to the nerve fiber.

(B) Myelinated nerve fibers are covered by a myelin sheath.
This myelin sheath is primarily composed of lipids,specifically sphingomyelin,along with some proteins.
Among the given options,lipids are the most characteristic organic substance found in the myelin sheath of myelinated nerve fibers.

(A) The myelin sheath is a lipid-rich layer that surrounds the axons of many neurons.
It acts as an electrical insulator.
By insulating the axon,the myelin sheath prevents the nerve impulse (action potential) from leaking out to adjacent nerve fibers,thereby ensuring that the signal is transmitted efficiently and rapidly along the axon.
Thus,both the statement and the reason are correct,and the reason provides the correct explanation for the function of the lipid-rich myelin sheath.