Nervous System: Complete Notes

Glial Cells • Action Potential • Conduction of Nerve Impulse • Neurotransmitter Release

CSIR-NET • GATE • DBT • ICMR • MSc Biotechnology

Study Tip: Understand nervous-system physiology as a continuous sequence: stimulus → receptor → membrane potential → action potential → conduction → synaptic transmission → neurotransmitter release → response. For competitive examinations, focus especially on resting membrane potential, threshold, voltage-gated Na⁺ and K⁺ channels, refractory periods, myelination, saltatory conduction and Ca²⁺-dependent neurotransmitter release.

📚 Table of Contents / Index

Introduction to the Nervous System
Organization of the Nervous System
Neuron: Structural and Functional Unit
Glial Cells
Resting Membrane Potential
Action Potential
Ion Channels During Action Potential
Refractory Period
Conduction of Nerve Impulse
Myelination and Saltatory Conduction
Synapse and Synaptic Transmission
Release of Neurotransmitters
Neurotransmitter Receptors
Excitatory and Inhibitory Synaptic Potentials
Important Comparisons
Important Applied Concepts
Quick Revision Notes
10 MCQs with Hidden Answers
Final Exam-Oriented Summary

1. Introduction to the Nervous System

The nervous system is one of the major regulatory systems of the human body. It receives information from the internal and external environment, processes that information and generates appropriate responses. The nervous system allows an organism to detect changes, integrate sensory information, coordinate movement, regulate physiological functions and participate in higher functions such as learning, memory, emotion and cognition.

Nervous communication is generally rapid because neurons communicate through electrical signals and chemical signals. Electrical changes in the plasma membrane allow information to travel along neurons, while chemical neurotransmitters transmit information across most synapses.

Major functions of the nervous system

Sensory function: Detects changes in the external and internal environment.
Integration: Processes and interprets incoming information.
Motor function: Produces responses through muscles and glands.
Coordination: Coordinates activities of different organs.
Homeostasis: Helps maintain stable internal conditions.
Higher functions: Participates in learning, memory, thought, emotion and consciousness.
Reflex activity: Produces rapid responses to specific stimuli.

Key Point

The basic functional principle of the nervous system is: receive information → process information → produce an appropriate response.

2. Organization of the Nervous System

Anatomically and functionally, the nervous system can be divided into several interconnected components. The two major anatomical divisions are the central nervous system and peripheral nervous system.

Central Nervous System (CNS)

Consists mainly of the brain and spinal cord. It acts as a major site for integration and processing of information.

Peripheral Nervous System (PNS)

Includes nerves and ganglia outside the brain and spinal cord. It connects the CNS with receptors, muscles and glands.

Somatic Nervous System

Primarily involved in sensory information and voluntary control of skeletal muscles.

Autonomic Nervous System

Regulates many involuntary functions involving smooth muscle, cardiac muscle and glands.

Functional direction of information

Afferent pathway: Carries sensory information toward the CNS.
Efferent pathway: Carries motor commands away from the CNS.
Sensory neuron: Transmits information from sensory receptors.
Motor neuron: Transmits signals to effectors.
Interneuron: Connects neurons within the CNS and participates in integration.

Division	Main components	Major role
CNS	Brain and spinal cord	Integration and information processing
PNS	Nerves and ganglia	Communication between CNS and body
Somatic system	Sensory and skeletal motor pathways	Voluntary movement and sensory function
Autonomic system	Sympathetic, parasympathetic and enteric components	Regulation of visceral functions

3. Neuron: Structural and Functional Unit

The neuron is the principal electrically excitable cell of the nervous system. Neurons are specialized for receiving, integrating and transmitting information. Although neurons differ greatly in size and shape, most contain a cell body and processes specialized for receiving or transmitting signals.

Major parts of a neuron

Cell body (soma): Contains the nucleus and most metabolic machinery.
Dendrites: Usually receive synaptic or sensory information.
Axon: Conducts electrical signals away from the cell body.
Axon hillock/initial segment: Important region for action-potential initiation in many neurons.
Axon terminals: Specialized endings involved in communication with another cell.
Myelin sheath: Insulating structure surrounding many axons.
Nodes of Ranvier: Gaps between myelinated segments where voltage-gated channels are concentrated.

Exam Point: The axon initial segment is often the site where action potentials are initiated because it has a high density of voltage-gated ion channels.

4. Glial Cells

Glial cells, or neuroglia, are non-neuronal cells that provide structural, metabolic, electrical and protective support to neurons. Glia are not simply passive supporting cells. They actively participate in maintaining the neural environment, myelination, immune defense and regulation of neuronal communication.

Major glial cells of the CNS

Glial cell	Location	Major functions
Astrocytes	CNS	Support neurons, regulate extracellular ions and neurotransmitters, participate in the blood-brain barrier and metabolic support.
Oligodendrocytes	CNS	Form myelin around axons in the CNS.
Microglia	CNS	Immune surveillance and phagocytic functions in nervous tissue.
Ependymal cells	CNS	Line ventricular and central canal surfaces and participate in cerebrospinal-fluid-related functions.

Major glial cells of the PNS

Schwann cells: Form myelin around axons in the peripheral nervous system.
Satellite cells: Surround neuronal cell bodies in peripheral ganglia and help regulate the local environment.

High-yield comparison:
Oligodendrocyte → CNS myelin
Schwann cell → PNS myelin
Microglia → immune/phagocytic cells of CNS
Astrocytes → structural, metabolic and homeostatic support

Functions of glial cells

Provide physical support to neurons.
Maintain extracellular ionic composition.
Provide metabolic support.
Produce myelin.
Participate in repair and immune responses.
Regulate neurotransmitter concentrations.
Contribute to the blood-brain barrier.
Maintain an appropriate environment for neuronal signaling.

5. Resting Membrane Potential

Neurons maintain an electrical potential difference across their plasma membrane. When a neuron is not generating an action potential, its membrane is said to have a resting membrane potential. In many neurons, the resting membrane potential is approximately -70 mV, although the exact value varies among cells.

Why is the inside of the neuron negative?

The resting membrane potential results from unequal distribution of ions across the membrane and selective permeability of the membrane to those ions. Potassium ions (K⁺) are particularly important because many resting neuronal membranes are relatively permeable to K⁺.

K⁺ concentration is relatively high inside the cell.
Na⁺ concentration is relatively high outside the cell.
Large negatively charged molecules are present inside the cell.
The membrane has selective permeability to different ions.
Ion gradients are maintained partly by the Na⁺/K⁺-ATPase.

Na⁺/K⁺-ATPase

The sodium-potassium pump uses ATP to transport ions against their electrochemical gradients. A typical cycle moves three Na⁺ ions out of the cell and two K⁺ ions into the cell.

Remember:
3 Na⁺ out
2 K⁺ in
Uses ATP

The pump is essential for maintaining ion gradients over time, while selective ion permeability makes a major contribution to the immediate resting membrane potential.

6. Action Potential

An action potential is a rapid, transient and regenerative change in membrane potential that allows neurons to transmit information over considerable distances. It is generated when membrane depolarization reaches a critical threshold and activates voltage-gated ion channels.

Major stages of an action potential

Resting state: Membrane remains near its resting potential.
Threshold: Sufficient depolarization activates voltage-gated Na⁺ channels.
Depolarization: Rapid Na⁺ influx makes the membrane potential less negative and then positive.
Peak: Na⁺ channels become inactivated.
Repolarization: K⁺ exits through voltage-gated K⁺ channels.
Hyperpolarization: K⁺ conductance remains elevated temporarily.
Return to resting state: Ion-channel activity and membrane transport restore the resting condition.

::contentReference[oaicite:0]{index=0}

All-or-none principle

Once threshold is reached, an action potential is generated with a characteristic amplitude for that neuron under a given set of conditions. Increasing stimulus strength above threshold does not normally produce a larger action potential. Instead, stronger stimuli can increase the frequency of action potentials.

Exam Point: Action potentials are generally described as all-or-none events. Stimulus intensity can be encoded by changes in action-potential frequency and recruitment of neurons rather than by simply increasing the amplitude of each action potential.

7. Ion Channels During Action Potential

The action potential depends primarily on voltage-gated Na⁺ and K⁺ channels. These channels respond to changes in membrane voltage and undergo characteristic conformational changes.

Voltage-gated Na⁺ channels

Open rapidly when membrane depolarization reaches threshold.
Allow Na⁺ to move into the neuron.
Cause the rapid rising phase of the action potential.
Become inactivated shortly after opening.
Recovery from inactivation is important for subsequent action potentials.

Voltage-gated K⁺ channels

Open more slowly than voltage-gated Na⁺ channels.
Allow K⁺ to move out of the cell.
Contribute strongly to repolarization.
May remain open long enough to produce after-hyperpolarization.

Phase	Main channel event	Major ion movement
Resting	Voltage-gated channels mostly closed	Dominant resting K⁺ permeability
Depolarization	Voltage-gated Na⁺ channels open	Na⁺ enters
Peak	Na⁺ channels become inactivated	Na⁺ influx decreases
Repolarization	Voltage-gated K⁺ channels are open	K⁺ exits
Hyperpolarization	K⁺ conductance remains elevated temporarily	Continued K⁺ efflux

8. Refractory Period

Following an action potential, the membrane enters a period during which generation of another action potential is either impossible or requires a stronger-than-normal stimulus. This is called the refractory period.

Absolute refractory period

During the absolute refractory period, another action potential cannot normally be initiated regardless of the strength of the stimulus. This is primarily related to the inactivated state of voltage-gated Na⁺ channels.

Relative refractory period

During the relative refractory period, another action potential can be generated, but a stronger stimulus is required. Persistent K⁺ conductance and membrane hyperpolarization contribute to this period.

Feature	Absolute refractory period	Relative refractory period
Action potential generation	Not normally possible	Possible with stronger stimulus
Major mechanism	Na⁺ channel inactivation	Persistent K⁺ conductance and membrane hyperpolarization
Functional significance	Limits maximum firing frequency	Influences firing threshold after an action potential

9. Conduction of Nerve Impulse

Once an action potential is generated, the electrical disturbance can propagate along the axon. Local current generated by an active membrane region depolarizes adjacent membrane to threshold, leading to sequential activation of voltage-gated channels.

Important characteristics of impulse conduction

The action potential is regenerated along the axon.
Propagation occurs because one active membrane region influences the next region.
The amplitude of an individual action potential is not progressively reduced along a healthy axon.
The refractory period helps prevent backward re-excitation of recently activated membrane.
Myelination greatly increases conduction velocity.

Factors affecting conduction velocity

Myelination: Myelinated axons generally conduct faster than unmyelinated axons of similar diameter.
Axon diameter: Larger axons generally have lower internal resistance and conduct impulses more rapidly.
Temperature: Within physiological limits, temperature can influence channel kinetics and conduction velocity.

High-yield concept: Larger diameter + myelination generally results in faster nerve impulse conduction.

10. Myelination and Saltatory Conduction

Myelin is a lipid-rich insulating structure surrounding many axons. It reduces current leakage across the membrane and allows rapid propagation of action potentials.

Myelin-producing cells

Cell	Location	Function
Oligodendrocyte	CNS	Myelinates axons in the central nervous system.
Schwann cell	PNS	Myelinates axons in the peripheral nervous system.

Saltatory conduction

In myelinated axons, voltage-gated ion channels are concentrated at the nodes of Ranvier. The electrical signal effectively travels rapidly between successive nodes, a process called saltatory conduction.

Nodes of Ranvier are exposed regions between myelinated segments.
Voltage-gated Na⁺ channels are concentrated at nodes.
Myelin increases membrane resistance.
Myelin decreases membrane capacitance.
Action-potential regeneration occurs at the nodes.
Saltatory conduction is much faster than continuous conduction in comparable unmyelinated axons.

Exam Point: Saltatory conduction does not mean that the action potential physically disappears between nodes. Rather, local electrical current spreads rapidly beneath the myelin and depolarizes the next node to threshold, where the action potential is regenerated.

11. Synapse and Synaptic Transmission

A synapse is a specialized junction through which a neuron communicates with another neuron or an effector cell. Synapses can be electrical or chemical, although chemical synapses are extremely common in the nervous system.

Chemical synapse

A typical chemical synapse consists of a presynaptic terminal, a synaptic cleft and a postsynaptic membrane containing receptors. Neurotransmitters are released from the presynaptic terminal and bind receptors on the postsynaptic cell.

Basic sequence of chemical synaptic transmission

Action potential reaches the presynaptic terminal.
Presynaptic membrane depolarizes.
Voltage-gated Ca²⁺ channels open.
Ca²⁺ enters the presynaptic terminal.
Ca²⁺ promotes synaptic-vesicle fusion with the membrane.
Neurotransmitter is released into the synaptic cleft.
Neurotransmitter binds receptors on the postsynaptic membrane.
Postsynaptic ion channels or signaling pathways are activated.
The postsynaptic membrane undergoes a change in electrical state.
Neurotransmitter action is terminated by degradation, reuptake, diffusion or other clearance mechanisms.

12. Release of Neurotransmitters

Neurotransmitter release at a typical chemical synapse is a Ca²⁺-dependent process. When an action potential arrives at the presynaptic terminal, voltage-gated Ca²⁺ channels open. Because extracellular Ca²⁺ concentration is normally higher than intracellular free Ca²⁺ concentration, Ca²⁺ enters the terminal.

Role of calcium

Ca²⁺ acts as the critical intracellular signal for rapid vesicle fusion.
Calcium binds specialized proteins associated with synaptic vesicles.
Vesicles fuse with the presynaptic membrane.
Neurotransmitter is released by exocytosis.
The amount of transmitter released is influenced by presynaptic Ca²⁺ entry.

Most important sequence:
Action potential → Ca²⁺ channel opening → Ca²⁺ influx → vesicle fusion → exocytosis → neurotransmitter release.

Termination of neurotransmitter action

Neurotransmitter signaling must be terminated to allow accurate and controlled communication. Different neurotransmitters are cleared by different mechanisms.

Reuptake: Transport proteins remove neurotransmitters from the synaptic cleft.
Enzymatic degradation: Enzymes chemically modify or break down neurotransmitters.
Diffusion: Some transmitter molecules diffuse away from the synaptic cleft.
Glial uptake: Glial cells can participate in neurotransmitter clearance.

13. Neurotransmitter Receptors

Neurotransmitters produce their effects by binding to receptors on postsynaptic or presynaptic membranes. Receptors can be broadly divided into ionotropic and metabotropic types.

Ionotropic receptors

Ionotropic receptors are ligand-gated ion channels. Binding of a neurotransmitter directly changes ion-channel activity and can produce rapid postsynaptic responses.

Directly associated with an ion channel.
Usually produce rapid responses.
Often mediate fast synaptic transmission.

Metabotropic receptors

Metabotropic receptors are not themselves ion channels. They generally activate intracellular signaling pathways, often through G proteins. Their effects are usually slower in onset but can be longer-lasting and more modulatory.

Indirectly influence ion channels or cellular processes.
Often involve G proteins and second-messenger pathways.
Responses are generally slower than ionotropic signaling.
Can amplify and modulate cellular responses.

Feature	Ionotropic receptor	Metabotropic receptor
Structure	Ligand-gated ion channel	Usually receptor linked to intracellular signaling pathways
Speed	Fast	Usually slower
Mechanism	Direct ion-channel opening/closing	Indirect modulation through signaling pathways
Duration	Often relatively brief	Can be longer-lasting

14. Excitatory and Inhibitory Synaptic Potentials

Neurotransmitters can either increase or decrease the probability that the postsynaptic neuron will generate an action potential. The effect depends not simply on the name of the neurotransmitter but on the receptor type, ion permeability and cellular context.

Excitatory postsynaptic potential (EPSP)

An EPSP is a postsynaptic depolarization that generally makes the membrane potential closer to the threshold for action-potential generation.

Generally depolarizes the postsynaptic membrane.
Increases the probability of action-potential generation.
Often involves inward movement of positive ions or reduction in outward positive current.

Inhibitory postsynaptic potential (IPSP)

An IPSP generally reduces the probability of action-potential generation. This may occur through hyperpolarization or by increasing membrane conductance in a way that opposes depolarization.

Generally decreases neuronal excitability.
Can involve increased Cl⁻ conductance or K⁺ conductance, depending on the cell.
Contributes to integration of excitatory and inhibitory signals.

Important: The same neurotransmitter can produce different effects in different tissues because receptor subtype and ionic environment determine the physiological response.

Summation

A neuron receives signals from many synapses. Postsynaptic potentials can combine through spatial and temporal summation.

Spatial summation: Signals from different synaptic locations combine.
Temporal summation: Repeated signals from the same synapse occur close together in time and combine.
The axon initial segment integrates the resulting electrical inputs and may initiate an action potential if threshold is reached.

15. Important Comparisons for Competitive Exams

Concept	Meaning	Key Point
Neuron	Excitable cell specialized for communication	Generates and transmits electrical signals
Glial cell	Non-neuronal support cell	Support, myelination, homeostasis and defense
Resting potential	Electrical potential difference across resting membrane	Often around -70 mV in neurons
Depolarization	Membrane potential becomes less negative	Na⁺ influx is important during neuronal action potentials
Repolarization	Membrane returns toward negative values	K⁺ efflux is important
Hyperpolarization	Membrane becomes more negative than resting level	Often associated with continued K⁺ conductance
Absolute refractory period	New action potential cannot normally be generated	Na⁺ channels are inactivated
Relative refractory period	New action potential requires stronger stimulus	Often associated with elevated K⁺ conductance
Saltatory conduction	Rapid propagation in myelinated axons	Action potential regenerated at nodes
Synapse	Specialized communication junction	Can be chemical or electrical
Ionotropic receptor	Ligand-gated ion channel	Fast response
Metabotropic receptor	Receptor linked to intracellular signaling	Slower and modulatory response

16. Important Applied Concepts

Myelin disorders

Proper myelination is essential for rapid and reliable nerve conduction. Damage to myelin can slow or disrupt electrical signal propagation. Disorders affecting CNS myelin and PNS myelin have different cellular and anatomical characteristics.

CNS myelin is produced by oligodendrocytes.
PNS myelin is produced by Schwann cells.
Loss of myelin can reduce conduction velocity.
Severe disruption of myelin can result in conduction failure.

Ion-channel toxins and drugs

Neuronal excitability depends on voltage-gated ion channels. Substances that interfere with Na⁺, K⁺ or Ca²⁺ channels can substantially alter neuronal signaling.

Blocking voltage-gated Na⁺ channels can prevent action-potential propagation.
Changes in K⁺ channel function can alter repolarization and excitability.
Changes in Ca²⁺ entry can influence neurotransmitter release.
Ion-channel function is therefore central to neurophysiology and pharmacology.

Synaptic transmission and disease

Abnormalities in neurotransmitter synthesis, release, receptors, reuptake or degradation can affect nervous-system function. Because synaptic communication is highly regulated, even small alterations in signaling pathways can influence neuronal activity.

Competitive-exam concept: If a question asks what happens when Ca²⁺ entry into a presynaptic terminal is prevented, the expected answer is a marked reduction in Ca²⁺-dependent synaptic-vesicle fusion and neurotransmitter release.

17. Quick Revision Notes

⭐ Must-Remember Points

The nervous system receives, integrates and responds to information.
The CNS consists of the brain and spinal cord.
The PNS connects the CNS with the rest of the body.
The neuron is the principal electrically excitable cell of the nervous system.
Dendrites generally receive information, whereas axons generally transmit information away from the cell body.
Glial cells support, protect and regulate the neuronal environment.
Astrocytes provide metabolic and homeostatic support in the CNS.
Oligodendrocytes form myelin in the CNS.
Schwann cells form myelin in the PNS.
Microglia provide immune and phagocytic functions in the CNS.
Resting neuronal membrane potential is often approximately -70 mV.
K⁺ permeability is an important contributor to the resting membrane potential.
The Na⁺/K⁺-ATPase uses ATP.
Each pump cycle commonly transports 3 Na⁺ out and 2 K⁺ in.
Action potentials are triggered when membrane depolarization reaches threshold.
Voltage-gated Na⁺ channels are responsible for the rapid depolarizing phase.
Voltage-gated K⁺ channels contribute strongly to repolarization.
After-hyperpolarization can occur because K⁺ conductance remains elevated temporarily.
The absolute refractory period is mainly associated with Na⁺ channel inactivation.
During the relative refractory period, a stronger stimulus is usually required.
Myelination increases the speed of nerve impulse conduction.
Saltatory conduction occurs in myelinated axons.
Nodes of Ranvier contain high concentrations of voltage-gated ion channels.
Larger axon diameter generally increases conduction velocity.
At a chemical synapse, an action potential reaches the presynaptic terminal.
Depolarization of the terminal opens voltage-gated Ca²⁺ channels.
Ca²⁺ influx triggers synaptic-vesicle fusion.
Neurotransmitter is released by exocytosis.
Neurotransmitters bind receptors on the postsynaptic cell.
Ionotropic receptors are ligand-gated ion channels.
Metabotropic receptors generally act through intracellular signaling pathways.
EPSPs generally increase the probability of action-potential generation.
IPSPs generally reduce the probability of action-potential generation.
Spatial summation combines signals from different synaptic locations.
Temporal summation combines repeated signals occurring close together in time.
Neurotransmitter action can be terminated by reuptake, enzymatic degradation and diffusion.

19. Final Exam-Oriented Summary

The nervous system is a highly organized communication network that receives sensory information, processes that information and produces appropriate responses. Neurons are specialized excitable cells that generate electrical signals, while glial cells provide essential support, protection, metabolic regulation, immune functions and myelination.

At rest, neurons maintain an electrical potential difference across their membranes. Ion concentration gradients and selective membrane permeability are fundamental to this resting state. When sufficient depolarization reaches threshold, voltage-gated Na⁺ channels produce rapid depolarization. Na⁺ channel inactivation and activation of voltage-gated K⁺ channels then contribute to repolarization. Continued K⁺ conductance can produce hyperpolarization.

Action potentials propagate along axons through sequential regeneration of electrical activity. Myelinated axons conduct impulses rapidly by saltatory conduction, in which action potentials are regenerated at the nodes of Ranvier. Oligodendrocytes produce myelin in the CNS, whereas Schwann cells produce myelin in the PNS.

At a typical chemical synapse, an action potential reaches the presynaptic terminal and opens voltage-gated Ca²⁺ channels. Calcium enters the terminal and triggers synaptic-vesicle fusion. Neurotransmitter is subsequently released into the synaptic cleft, where it binds receptors on the postsynaptic cell.

Neuron: Main electrically excitable cell.
Glial cells: Support and regulate neural tissue.
Astrocytes: CNS support and homeostasis.
Oligodendrocytes: CNS myelin.
Schwann cells: PNS myelin.
Microglia: CNS immune/phagocytic cells.
Resting potential: Often approximately -70 mV.
Depolarization: Mainly associated with Na⁺ influx during a typical neuronal action potential.
Repolarization: Mainly associated with K⁺ efflux.
Absolute refractory period: Na⁺ channel inactivation is important.
Relative refractory period: Stronger stimulus is required.
Saltatory conduction: Rapid conduction in myelinated axons.
Chemical synapse: Uses neurotransmitter-mediated communication.
Critical ion for neurotransmitter release: Ca²⁺.
Ionotropic receptor: Ligand-gated ion channel.
Metabotropic receptor: Acts through intracellular signaling pathways.
EPSP: Generally increases neuronal excitability.
IPSP: Generally decreases neuronal excitability.
Spatial summation: Integration of inputs from different locations.
Temporal summation: Integration of repeated inputs over time.

For CSIR-NET, GATE Biotechnology, DBT-BET, ICMR-JRF and postgraduate examinations, the most important strategy is to connect the concepts rather than memorize them separately: resting potential → threshold → Na⁺ influx → depolarization → Na⁺ inactivation → K⁺ efflux → repolarization → hyperpolarization → refractory period → impulse conduction → Ca²⁺-dependent neurotransmitter release.

Home

Sunday, 23 August 2026

NERVOUS SYSTEM