Nerve Impulse Transmission Notes

Nerve Impulse Transmission Notes

📚 Table of Contents

🔬What is Nerve Impulse Transmission?

Nerve impulse transmission is the process by which electrical signals (action potentials) travel along neurons and communicate between neurons via synapses. This is the fundamental mechanism behind all nervous system functions—from reflex actions to complex thought processes.

For GPAT and university exams, understanding nerve impulse transmission is critical because it forms the basis of:

  • Pharmacology of CNS and ANS drugs
  • Mechanism of action of anesthetics, antiepileptics, and neuromuscular blockers
  • Pathophysiology of neurological disorders

This topic is high-yield for both theory and MCQ-based exams.

📜 Structure of a Neuron (Quick Recap)

A neuron consists of:

  • Cell body (Soma): Contains nucleus and organelles
  • Dendrites: Receive signals from other neurons
  • Axon: Conducts impulses away from the cell body
  • Axon terminals (Synaptic knobs): Release neurotransmitters
  • Myelin sheath: Insulates axon; formed by Schwann cells (PNS) or oligodendrocytes (CNS)
  • Nodes of Ranvier: Gaps in myelin where action potentials are regenerated

To understand how the nervous system communicates, we must first look at its functional unit: the neuron.

Labeled neuron structure diagram showing cell body dendrites axon myelin sheath and nodes of Ranvier
Fig. Anatomy of a typical neuron. Arrows indicate the unidirectional flow of nerve impulses from dendrites to axon terminals.

Each component listed above plays a specific role in ensuring rapid signal transmission.

⚡ Resting Membrane Potential

When a neuron is not transmitting a signal, it is in a resting state with a membrane potential of approximately -70 mV (inside negative relative to outside).

How is Resting Potential Maintained?

  • Na⁺/K⁺-ATPase pump: Actively pumps 3 Na⁺ out and 2 K⁺ in (uses ATP)
  • High K⁺ inside, high Na⁺ outside
  • Membrane is more permeable to K⁺ than Na⁺ at rest
  • Negatively charged proteins inside the cell contribute to negative charge

📌 Mnemonic: "3 Sodium OUT, 2 Potassium IN" → Remember 3-2-1 Pump!

Understanding resting potential is essential for grasping how neurons generate action potentials

Resting membrane potential diagram showing sodium potassium pump -70mV neuron at rest
Fig.: Resting membrane potential maintained by the Na⁺/K⁺-ATPase pump. The pump actively transports 3 Na⁺ out and 2 K⁺ in, creating a -70 mV charge inside the neuron.

This electrochemical gradient is the foundation for all nerve signal transmission.

🚀Action Potential: The Nerve Impulse

An action potential is a rapid, temporary reversal of membrane potential that travels along the axon. It is an all-or-none response.

Phases of Action Potential

1. Resting State (-70 mV)

  • Neuron is polarized
  • Voltage-gated Na⁺ and K⁺ channels are closed

2. Depolarization (Threshold to +30 mV)

  • Stimulus opens voltage-gated Na⁺ channels
  • Na⁺ rushes into the cell
  • Membrane potential becomes positive (+30 mV)

📌 Mnemonic: "Sodium IN = Spike UP" → Depolarization!

3. Repolarization (+30 mV back to -70 mV)

  • Na⁺ channels close
  • Voltage-gated K⁺ channels open
  • K⁺ rushes out of the cell
  • Membrane potential returns to negative

Mnemonic: "Potassium OUT = Potential DOWN" → Repolarization!

4. Hyperpolarization (Undershoot to -90 mV)

  • K⁺ channels remain open slightly longer
  • Membrane becomes more negative than resting potential
  • This is the refractory period

5. Return to Resting State

  • Na⁺/K⁺ pump restores ion balance
  • Membrane returns to -70 mV

The action potential is the electrical signal that allows neurons to communicate rapidly across long distances

Action potential graph showing phases resting depolarization repolarization hyperpolarization membrane potential
Fig.: Action potential phases graph. Shows the characteristic spike from -70 mV to +30 mV and return to baseline, essential for nerve impulse transmission


This all-or-nothing response ensures reliable signal transmission throughout the nervous system.

Refractory Periods

Type Definition Significance
Absolute Refractory Period No new action potential can be generated (Na⁺ channels inactivated) Ensures one-way conduction of impulse
Relative Refractory Period A stronger-than-normal stimulus can generate action potential Limits frequency of impulses

🏃Propagation of Nerve Impulse

Once generated, the action potential travels along the axon. There are two types of conduction:

1. Continuous Conduction (Unmyelinated Axons)

  • Action potential moves step-by-step along the entire axon
  • Slower (0.5–2 m/s)
  • Found in: C fibers (pain, temperature)

2. Saltatory Conduction (Myelinated Axons)

  • Action potential "jumps" from one Node of Ranvier to the next
  • Faster (up to 120 m/s)
  • More energy-efficient
  • Found in: A fibers (motor neurons, touch, pressure)

Mnemonic: "Saltatory = Salta (jump in Latin) → Jumps at Nodes!"

Nerve impulses travel differently depending on axon type, affecting speed and efficiency

Propagation of nerve impulse diagram showing continuous conduction in unmyelinated axon and saltatory conduction in myelinated axon
Fig. : Nerve impulse propagation. Continuous conduction moves stepwise along unmyelinated axons; saltatory conduction jumps between Nodes of Ranvier in myelinated axons

Saltatory conduction allows rapid and energy-efficient signal transmission in myelinated neurons.

🔗 Synaptic Transmission

A synapse is the junction between two neurons (or a neuron and an effector). Transmission can be electrical or chemical.

Steps in Chemical Synaptic Transmission

  1. Action potential arrives at the axon terminal (presynaptic neuron)
  2. Voltage-gated Ca²⁺ channels open → Ca²⁺ enters the terminal
  3. Synaptic vesicles fuse with the presynaptic membrane
  4. Neurotransmitter is released into the synaptic cleft (exocytosis)
  5. Neurotransmitter binds to receptors on the postsynaptic membrane
  6. Ion channels open → Postsynaptic potential generated (EPSP or IPSP)
  7. Neurotransmitter is removed (reuptake, enzymatic degradation, or diffusion)

📌 Mnemonic: "Calcium Comes, Vesicles Release Neurotransmitters Rapidly" → C-V-R-N-R

Once an action potential reaches the axon terminal, it must be converted into a chemical signal to cross the synaptic gap.

Chemical synaptic transmission diagram showing neurotransmitter release binding and removal at synapse
Fig.: Steps in chemical synaptic transmission. Calcium influx triggers neurotransmitter release from synaptic vesicles into the synaptic cleft.

This process ensures precise communication between neurons and target cells.

Postsynaptic Potentials

  • EPSP (Excitatory Postsynaptic Potential): Depolarizes postsynaptic membrane → increases likelihood of action potential (e.g., acetylcholine at nicotinic receptors)
  • IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes postsynaptic membrane → decreases likelihood of action potential (e.g., GABA, glycine)

Major Neurotransmitters

Neurotransmitter Type Function Example Location
Acetylcholine (ACh) Excitatory Muscle contraction, memory, learning Neuromuscular junction, parasympathetic NS
Dopamine Excitatory/Inhibitory Reward, movement, motivation Basal ganglia, limbic system
Norepinephrine Excitatory Alertness, arousal, fight-or-flight Sympathetic NS, locus coeruleus
Serotonin (5-HT) Inhibitory Mood, sleep, appetite Raphe nuclei, gut
GABA Inhibitory Reduces neuronal excitability CNS (widespread)
Glutamate Excitatory Learning, memory, synaptic plasticity CNS (most abundant excitatory NT)
Glycine Inhibitory Motor control, sensory processing Spinal cord, brainstem

📌 Mnemonic for Major Neurotransmitters: "ACh Drives Nice Serotonin, GABA Gives Glycine" → ACh, Dopamine, Norepinephrine, Serotonin, GABA, Glutamate, Glycine

Comparison: Electrical vs Chemical Synapse

Feature Electrical Synapse Chemical Synapse
Structure Gap junctions (connexons) Synaptic cleft (~20-40 nm)
Transmission Speed Very fast (instantaneous) Slower (0.5–5 ms delay)
Direction Bidirectional Unidirectional
Neurotransmitter Not required Required
Modulation Limited Highly modifiable (drugs, plasticity)
Example Cardiac muscle, some neurons in CNS Most synapses in the nervous system

💊 Pharmacy/Clinical Angle (Why This Matters)

Understanding nerve impulse transmission is essential for pharmacology because most CNS and ANS drugs act by modulating neurotransmission.

Drug Classes Acting on Nerve Impulse Transmission

1. Local Anesthetics (e.g., Lignocaine, Bupivacaine)

  • Mechanism: Block voltage-gated Na⁺ channels → prevent depolarization → no action potential
  • Clinical use: Dental procedures, minor surgeries

2. Antiepileptics (e.g., Phenytoin, Carbamazepine)

  • Mechanism: Stabilize Na⁺ channels in inactivated state → reduce neuronal excitability
  • Clinical use: Seizure control

3. Neuromuscular Blockers (e.g., Succinylcholine, Rocuronium)

  • Mechanism: Block nicotinic ACh receptors at neuromuscular junction → muscle paralysis
  • Clinical use: Surgical muscle relaxation

4. Cholinesterase Inhibitors (e.g., Neostigmine, Donepezil)

  • Mechanism: Inhibit acetylcholinesterase → increase ACh in synaptic cleft
  • Clinical use: Myasthenia gravis, Alzheimer's disease

5. SSRIs (e.g., Fluoxetine, Sertraline)

  • Mechanism: Block serotonin reuptake → increase serotonin in synapse
  • Clinical use: Depression, anxiety

6. Benzodiazepines (e.g., Diazepam, Lorazepam)

  • Mechanism: Enhance GABAA receptor activity → increase inhibitory transmission
  • Clinical use: Anxiety, seizures, sedation

Exam Pearl: Most psychotropic drugs work by modulating neurotransmitter levels (reuptake inhibition, receptor agonism/antagonism, or enzymatic inhibition).

📌 GPAT / Exam Pearls

  • Resting membrane potential = -70 mV
  • Action potential peak = +30 mV
  • Na⁺/K⁺-ATPase pumps 3 Na⁺ out, 2 K⁺ in
  • Depolarization = Na⁺ influx; Repolarization = K⁺ efflux
  • Saltatory conduction occurs in myelinated axons (faster)
  • Synaptic transmission requires Ca²⁺ influx
  • GABA and Glycine are inhibitory; Glutamate is excitatory
  • Local anesthetics block Na⁺ channels
  • Absolute refractory period ensures unidirectional impulse propagation
  • Acetylcholinesterase breaks down ACh in the synaptic cleft

📜 Quick Revision Box

Resting Potential: -70 mV (Na⁺/K⁺ pump: 3 out, 2 in)

Action Potential Phases: Resting → Depolarization (Na⁺ in) → Repolarization (K⁺ out) → Hyperpolarization → Resting

Conduction Types: Continuous (slow, unmyelinated) vs Saltatory (fast, myelinated)

Synaptic Transmission: Ca²⁺ → Vesicle fusion → NT release → Receptor binding → EPSP/IPSP

Key Neurotransmitters: ACh (excitatory), GABA (inhibitory), Glutamate (excitatory), Dopamine, Serotonin

Drug Targets: Na⁺ channels (local anesthetics), ACh receptors (neuromuscular blockers), GABA receptors (benzodiazepines)

❓ Frequently Asked Exam Questions

Q1. What is the role of the Na⁺/K⁺-ATPase pump?

A: Maintains resting membrane potential by pumping 3 Na⁺ out and 2 K⁺ in, using ATP.

Q2. Why is saltatory conduction faster?

A: Action potential jumps between Nodes of Ranvier in myelinated axons, skipping myelinated segments.

Q3. What is the function of Ca²⁺ in synaptic transmission?

A: Ca²⁺ influx triggers synaptic vesicle fusion and neurotransmitter release.

Q4. How do local anesthetics work?

A: They block voltage-gated Na⁺ channels, preventing depolarization and action potential generation.

Q5. What is the difference between EPSP and IPSP?

A: EPSP depolarizes the postsynaptic membrane (excitatory), while IPSP hyperpolarizes it (inhibitory).

Q6. Name two inhibitory neurotransmitters.

A: GABA and Glycine.

Q7. What happens during the absolute refractory period?

A: No new action potential can be generated because Na⁺ channels are inactivated.

📣 Call to Action

🎯 Bookmark this page for quick revision before your university exams!

💡 Share with your classmates preparing for pharmacy and nursing exams

🔖 Save the mnemonics and diagrams for last-minute prep

📝 Practice drawing the action potential graph and synaptic transmission diagram

💬 Comment below if you need clarification on any concept!

💡 Study Hacks:

  • Draw the action potential graph from memory daily
  • Create flashcards for neurotransmitters and their functions
  • Practice MCQs on ion movements and drug mechanisms

😂 Meme: "Na⁺ rushing in during depolarization" with a crowd rushing into a store on Black Friday!

Save this post, share it with your study group, and bookmark it for last-minute revision before your exams. You've got this! 💪🎓



Comments

Post a Comment

Popular posts from this blog

Digestive System

Image
The digestive system is a group of organs that break down food (physically and chemically) so that it can be used by the body cells. Fig. 1: The digestive system The organs of the digestive system can be broadly divided into two: Gastrointestinal (GI) tract or alimentary canal  Accessory digestive organs
Read more

Laboratory Animals

Image
Laboratory Animals Laboratory animals are used in a variety of research studies, including drug discovery. They are used to test the safety and efficacy of new drugs, to understand the mechanisms of disease, and to develop new treatments. There are a number of reasons why laboratory animals are important in drug discovery. First, they provide a controlled environment in which to test new drugs. This is important because it allows researchers to control for factors such as diet, exercise, and stress, which can affect the way a drug works. Second, laboratory animals are similar to humans in many ways. They have similar biological systems, and they can develop many of the same diseases. This makes them useful models for studying human diseases and for testing new drugs. Third, laboratory animals are relatively easy to obtain and care for. This makes them a cost-effective option for drug discovery. While laboratory animals are an important part of drug discovery, it is important to not...
Read more

Hemopoiesis

Image
Hemopoiesis: How Your Body Makes Blood Cells Why do we need constant blood cell production? Most blood cells live fast and die young. Many white blood cells survive only hours to days; platelets last about a week; red blood cells last around 120 days. To keep you healthy, the body must continually replace these “formed elements.” Negative feedback systems keep RBCs and platelets relatively steady, while white blood cell numbers swing up or down depending on infections, inflammation, and other immune challenges. Where does hemopoiesis happen across life? Hemopoiesis (also called hematopoiesis) is the process of forming blood cells. Early embryo: yolk sac Fetus: liver, spleen, thymus, and lymph nodes Last trimester onward and throughout life: red bone marrow becomes the primary site In newborns, almost all marrow is red and active. With growth, much of the marrow in long bones turns into yellow marrow (fatty and inactive). During severe blood loss, yellow marrow can revert to red an...
Read more