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

(D) The nervous system is primarily composed of two main categories of cells:
$(i)$ Neurons: These are the structural and functional units of the nervous system responsible for the transmission of nerve impulses.
$(ii)$ Glial cells (Neuroglia): These are non-neuronal cells that provide support,protection,and nutrition to the neurons. They are much more numerous than neurons.
While other specialized cells like ependymal cells and neurosecretory cells exist,the fundamental classification of nervous tissue cells is into neurons and glia.

(B) Bipolar neurons are neurons that possess one axon and one dendrite extending from opposite poles of the cell body. These neurons are specialized for sensory transmission and are found in specific locations such as the retina of the eye,the olfactory epithelium,and the inner ear.

(A) Myelinated nerve fibres are those that are enveloped with Schwann cells,which form a myelin sheath around the axon.
In contrast,non-myelinated nerve fibres are enclosed by a Schwann cell that does not form a myelin sheath around the axon.

(A) Dendrites are the short,branched processes of a neuron that receive signals from other neurons or sensory receptors.
These impulses are then transmitted towards the cell body (also known as the soma or cyton) of the neuron.
In contrast,the axon transmits impulses away from the cell body.

(D) The cell body (soma) of a neuron contains cytoplasm,which includes typical cell organelles such as mitochondria,Golgi apparatus,and endoplasmic reticulum. Additionally,it contains characteristic granular bodies known as Nissl's granules,which are clusters of rough endoplasmic reticulum and ribosomes.

(D) Neurons are highly specialized cells that form the structural and functional unit of the nervous system.
$1$. They possess the ability to detect stimuli through their receptors.
$2$. They receive these stimuli as electrical or chemical signals.
$3$. They transmit these signals to other neurons,muscles,or glands.
Therefore,all the given functions are performed by neurons.

(D) All statements $(I)$,$(II)$,and $(III)$ are correct.
Each neuron consists of three main parts: a cell body (cyton),an axon,and one or more dendrites.
The cell body contains the nucleus and cytoplasm.
The axon is a long,slender projection that conducts nerve impulses away from the cell body,and each neuron typically possesses a single axon.
Neurons are the structural and functional units of the nervous system,responsible for the generation and transmission of nerve impulses.

(A) Ependymal cells are specialized epithelial cells that line the ventricles of the brain and the central canal of the spinal cord. They are ciliated and play a crucial role in the production and circulation of cerebrospinal fluid $(CSF)$,providing both structural support and nutritional maintenance for the central nervous system $(CNS)$.
Microglia are small glial cells that act as the primary immune defense in the $CNS$,functioning as phagocytes to remove cellular debris and pathogens.
Astrocytes are star-shaped cells that provide structural and metabolic support to neurons,regulate the chemical environment,and contribute to the formation of the blood-brain barrier.
Oligodendrocytes are responsible for the formation of the myelin sheath around axons in the $CNS$.

(B) The myelin sheath is a fatty insulating layer that surrounds the axon of many neurons. However,this sheath is not continuous along the entire length of the axon. It is interrupted at regular intervals by gaps known as the nodes of Ranvier. These nodes allow for saltatory conduction of nerve impulses,which significantly increases the speed of signal transmission.

(A) neuron consists of a cell body,an axon,and dendrites.
Statement $I$ is incorrect because dendrites are short fibres,not long.
Statement $II$ is incorrect because axons are long fibres,not short.
Statement $III$ is correct as the cell body (cyton) contains cytoplasm,a nucleus,cell organelles,and characteristic Nissl's granules.
Statement $IV$ is incorrect because dendrites transmit nerve impulses towards the cell body,whereas the axon transmits impulses away from the cell body.
Therefore,only statement $III$ is correct.

(C) In myelinated nerve fibres, the myelin sheath is not continuous and remains interrupted at some places. These gaps are known as nodes of $Ranvier$.
These nodes help in the saltatory conduction of nerve impulses.
Non-myelinated nerve fibres do not possess nodes of $Ranvier$ because they lack the myelin sheath interruptions.
Therefore, the statement that non-myelinated nerve fibres do not possess nodes of $Ranvier$ is correct.

(D) Nissl's granules are granular bodies found in the cell body (cyton) of neurons.
They are composed of irregular masses of ribosomes and rough endoplasmic reticulum $(ER)$.
These structures are primarily responsible for the synthesis of proteins within the neuron.
Therefore,statements $II, III,$ and $IV$ are correct.

(A) $I -$ True: Neuroglial cells are indeed the supporting and packing cells of the central nervous system.
$II -$ True: Oligodendrocytes are responsible for forming the myelin sheath around axons in the central nervous system.
$III -$ False: Microglia are primarily phagocytic cells that act as the immune defense of the central nervous system,not mechanical support.
$IV -$ False: Astrocytes communicate with one another primarily through gap junctions,which allow the passage of ions and small molecules,rather than just potassium channels.

(B) The neuron consists of three main parts: the cell body (cyton),dendrites,and the axon.
The cell body (cyton) contains cytoplasm with typical cell organelles and characteristic granular bodies called $Nissl's$ granules.
These $Nissl's$ granules are composed of rough endoplasmic reticulum and ribosomes,which are involved in protein synthesis.
$Nissl's$ granules are absent in the axon and the axon hillock.

(C) Nissl's granules are irregular masses of rough endoplasmic reticulum $(RER)$ with ribosomes,which are involved in protein synthesis.
They are found in the cell body (cyton) and the dendrites of a neuron.
They are characteristically absent in the axon and the axon hillock.

(D) Multipolar neurons are characterized by one axon and two or more dendrites. They are commonly found in the cerebral cortex of the brain.
Bipolar neurons,which have one axon and one dendrite,are found in the retina of the eye.
Unipolar neurons,which have a cell body with one axon only,are typically found in the embryonic stage.

(A) Excitable cells are those that can generate and conduct electrical impulses (action potentials) in response to stimuli.
$1$. Neuroglia: These are non-excitable supporting cells of the nervous system.
$2$. Muscle fiber: These are excitable cells capable of contraction upon stimulation.
$3$. Fibroblast: These are connective tissue cells involved in matrix production; they are non-excitable.
$4$. Mast cell: These are immune cells involved in inflammatory responses; they are non-excitable.
$5$. Neuron: These are the primary excitable cells of the nervous system.
$6$. Macrophage: These are phagocytic immune cells; they are non-excitable.
Among the given list,only Muscle fibers and Neurons are excitable cells.
Therefore,the total count is $2$.

(B) The nervous system is composed of two main types of cells: neurons and neuroglia.
Neurons (nerve cells) are the structural and functional units of the nervous system,responsible for the reception,conduction,and transmission of nerve impulses.
Neuroglia are supporting cells that protect and support neurons but do not conduct nerve impulses.
Therefore,the correct answer is $B$.

(A) In the provided image of a neuron:
$P$ points to the branched processes extending from the cell body,which are known as dendrites.
$Q$ points to the central part of the neuron containing the nucleus and cytoplasm,known as the cell body (or cyton/soma).
$R$ (labeled as $S$ in the image,but referring to the long projection) points to the axon,which is the long,slender projection that conducts nerve impulses away from the cell body.
Therefore,the correct sequence is $P$ = Dendrite,$Q$ = Cell body,$R$ = Axon.
Thus,the correct option is $A$.

(D) The neural system is composed of highly specialized cells called neurons. $A$ neuron (or nerve cell) is the structural and functional unit of the neural system. It is responsible for the reception,integration,and transmission of nerve impulses throughout the body.

(D) neuron is the structural and functional unit of the nervous system. Its primary functions include:
$1$. Detecting stimuli through sensory receptors.
$2$. Receiving these stimuli as electrical signals.
$3$. Conducting these signals (nerve impulses) to other neurons,muscles,or glands.
Therefore,all the listed options are correct functions of a neuron.

(B) neuron is a microscopic structure composed of $3$ major parts: the cell body (cyton/soma),dendrites,and the axon.
$1$. Cell body: Contains cytoplasm with typical cell organelles and Nissl's granules.
$2$. Dendrites: Short fibers which branch repeatedly and project out of the cell body.
$3$. Axon: $A$ long fiber,the distal end of which is branched.

(C) The $Nissl$ granules are granular bodies found in the cell body ($cyton$ or $soma$) and the dendrites of a neuron. They are composed of rough endoplasmic reticulum and ribosomes,which are responsible for protein synthesis. They are notably absent in the axon.

(A) The classification of neurons based on the number of axons and dendrites is as follows:
$1$. Unipolar neurons: These have a cell body with only one axon. They are typically found in the embryonic stage.
$2$. Bipolar neurons: These have one axon and one dendrite. They are found in the retina of the eye.
$3$. Multipolar neurons: These have one axon and two or more dendrites. They are commonly found in the cerebral cortex.
Therefore, the correct matching is: $P-I, Q-III, R-II$.

(B) The myelin sheath is an insulating layer that surrounds the axons of many neurons.
It is not continuous along the entire length of the axon.
The gaps or interruptions between adjacent myelin sheaths are known as the $Node of Ranvier$.
These nodes allow for the rapid conduction of nerve impulses through saltatory conduction.

(C) Dendrites are short,highly branched fibers that receive nerve impulses and transmit them towards the cell body $(I, IV, V)$.
Axons are typically long,unbranched fibers that transmit nerve impulses away from the cell body $(II, III, VI)$.

(D) The axon of a neuron terminates at a specialized junction called a synapse.
This synapse can be formed between the axon terminal of one neuron and the dendrites or cell body of another neuron.
Additionally,axons can terminate at neuromuscular junctions (connecting to muscles) or neuroglandular junctions (connecting to glands).
Therefore,the axon can end in another neuron,muscles,or glands.
Thus,the correct option is $D$.

(D) Ion channels are specialized transmembrane proteins that allow specific ions to pass through the cell membrane.
In neurons,these ion channels are embedded within the neural membrane (axolemma) to facilitate the movement of ions like $Na^+$ and $K^+$ during the generation and conduction of a nerve impulse.
Therefore,$P$ is Proteins and $Q$ is Neural membrane.

(D) The correct matching is as follows:
$1$. Multipolar neurons have one axon and two or more dendrites and are found in the cerebral cortex $(a - ii)$.
$2$. Bipolar neurons have one axon and one dendrite and are found in the retina of the eye $(b - iii)$.
$3$. Myelinated nerve fibres are found in spinal and cranial nerves $(c - iv)$.
$4$. Unmyelinated nerve fibres are commonly found in the autonomous and the somatic neural systems $(d - i)$.
Therefore,the correct sequence is $(a - ii, b - iii, c - iv, d - i)$.

(C) Grey matter in the central nervous system is primarily composed of neuronal cell bodies,dendrites,and unmyelinated axons.
Myelinated axons are primarily found in the white matter,as the myelin sheath gives the tissue a white appearance.
Therefore,the structures included in the grey matter from the given list are:
$1$. Cell body
$2$. Unmyelinated axon
$3$. Dendron
Thus,there are $3$ structures included in the grey matter.

(D) The statement 'Astrocytes mediate immune response in the $CNS$' is incorrect.
Microglial cells are the specialized immune cells of the $CNS$ that mediate immune responses,such as phagocytosis of debris and pathogens.
Astrocytes primarily function to provide structural and metabolic support to neurons,maintain the blood-brain barrier $(BBB)$,regulate the chemical environment (including neurotransmitter uptake),and assist in the repair of neural tissue.

(C) The figure shows Ependymal cells,which are ciliated epithelial cells lining the ventricles of the brain and the central canal of the spinal cord.
These cells are responsible for the production and circulation of Cerebrospinal Fluid $(CSF)$.
Therefore,the correct match is: Glial cell - Ependymal cell; Function - Production of $CSF$.

(C) Neuroglia (or glial cells) are non-neuronal cells in the nervous system that provide support and protection for neurons.
They perform functions such as providing nourishment (e.g.,astrocytes),phagocytosis (e.g.,microglia),and they retain the ability to divide (regeneration).
However,unlike neurons,neuroglia cells do not possess the property of excitability,which is the ability to generate and conduct nerve impulses (action potentials).
Therefore,excitability is the correct answer.