Neuromuscular Junction


This is one of the most important concepts in physiology, so let's go through it step by step.

Think of acetylcholine (ACh) as a key that unlocks the muscle.

Step 1: The nerve carries an electrical signal

Your brain decides to move your arm.

An electrical impulse (action potential) travels down the motor neuron until it reaches the end of the nerve.

Brain
   │
   ▼
Motor neuron ─────────────► Nerve ending

Step 2: Calcium enters the nerve

When the electrical impulse reaches the nerve ending, it opens voltage-gated calcium (Ca²⁺) channels.

Calcium rushes into the nerve terminal.

Why?
Calcium is the signal that tells the nerve:

"Release the acetylcholine!"

Step 3: Acetylcholine is released

Inside the nerve ending are tiny sacs called synaptic vesicles.

These vesicles are filled with acetylcholine.

The calcium causes the vesicles to fuse with the nerve membrane and release acetylcholine into the tiny gap between the nerve and muscle.

This gap is called the synaptic cleft.

Nerve ending
     │
     │  ACh ACh ACh
     ▼ ▼ ▼ ▼ ▼
-------------------   ← synaptic cleft

Step 4: Acetylcholine binds to receptors

On the muscle membrane are acetylcholine receptors.

Think of them like locks.

Acetylcholine molecules are the keys.

ACh (key)
      ▼
[ Receptor ]  ← lock

When acetylcholine binds, the receptor changes shape.

Step 5: Sodium rushes into the muscle

The receptor is actually an ion channel.

When acetylcholine binds, the channel opens.

This allows sodium (Na⁺) to rush into the muscle.

Outside muscle
Na⁺ Na⁺ Na⁺ Na⁺
      ↓↓↓↓

====================
Muscle membrane

Inside muscle

Since sodium carries a positive charge, the inside of the muscle becomes less negative.

This is called depolarization.

Step 6: The muscle generates its own action potential

If enough sodium enters, the muscle reaches its threshold.

Now the muscle creates its own electrical impulse.

That electrical signal spreads across the muscle membrane and dives deep into the muscle through structures called T-tubules.

Step 7: Calcium is released inside the muscle

The electrical signal tells the sarcoplasmic reticulum (SR)—the muscle's calcium storage site—to release Ca²⁺ into the muscle fiber.

Now calcium floods the muscle cell.

Step 8: Calcium allows contraction

Inside muscle fibers are two proteins:

  • Actin

  • Myosin

Normally, another protein called tropomyosin blocks myosin from attaching to actin.

Calcium binds to troponin, which moves tropomyosin out of the way.

Now myosin can grab actin.

Without calcium

Actin ----XXXX---- Myosin
          Blocked

With calcium

Actin ----------- Myosin
           ✔ Attached

The myosin heads pull on the actin filaments, shortening the muscle and causing it to contract.

Step 9: The signal stops

The muscle shouldn't stay contracted forever.

An enzyme called acetylcholinesterase (AChE) quickly breaks down acetylcholine in the synaptic cleft.

Without acetylcholine:

  • the receptors close,

  • sodium stops entering,

  • the electrical signal ends.

Step 10: The muscle relaxes

Calcium is pumped back into the sarcoplasmic reticulum.

Without calcium:

  • troponin returns to its original shape,

  • tropomyosin covers the binding sites again,

  • myosin can no longer pull on actin,

  • the muscle relaxes.

The entire process in one flow

Brain
   ↓
Motor neuron fires
   ↓
Calcium enters nerve ending
   ↓
Acetylcholine released
   ↓
ACh binds receptors on muscle
   ↓
Sodium enters muscle
   ↓
Muscle depolarizes
   ↓
Muscle action potential
   ↓
Calcium released from sarcoplasmic reticulum
   ↓
Actin + Myosin interact
   ↓
Muscle contracts
   ↓
Acetylcholinesterase destroys ACh
   ↓
Calcium pumped back into SR
   ↓
Muscle relaxes

Memory trick

Think of acetylcholine as a doorbell:

  • 🧠 The brain decides someone should answer the door.

  • ⚡ The nerve carries the electrical signal to the house.

  • 🔔 Acetylcholine rings the doorbell (binds to receptors).

  • 🚪 The door opens (sodium channels open).

  • 💪 The muscle wakes up and starts working (contracts).

  • 🧹 Acetylcholinesterase removes the finger from the doorbell, so it stops ringing and the muscle can relax.

The key idea to remember is that acetylcholine itself does not directly make the muscle shorten. Instead, it starts a chain reaction:
ACh → sodium enters → muscle action potential → calcium release → actin and myosin slide → contraction.


This is one of the most important words in muscle physiology.

A muscle contraction means that the muscle generates force by its fibers shortening or attempting to shorten.

The important part is that contraction does not always mean the whole muscle gets shorter. It means the muscle is actively producing force.

What happens inside the muscle?

Inside every muscle fiber are tiny protein filaments:

  • Actin (thin filament)

  • Myosin (thick filament)

When calcium is released, the myosin heads grab onto actin and pull it inward.

Before contraction

Actin  =========>
               <=========  Actin

        Myosin

During contraction, the actin filaments slide toward the center:

After contraction

    Actin ====><==== Actin
          Myosin

The filaments themselves do not get shorter. Instead, they slide past each other, making the sarcomere (the basic contractile unit of muscle) shorter. This is called the sliding filament theory.

Example: Bending your arm

Suppose you bend your elbow.

  1. Your brain sends a signal to the biceps.

  2. Acetylcholine is released.

  3. The muscle fiber depolarizes.

  4. Calcium is released inside the muscle.

  5. Myosin pulls on actin.

  6. The biceps contracts.

  7. The forearm moves upward.

Brain
   ↓
Motor neuron
   ↓
Acetylcholine
   ↓
Calcium released
   ↓
Actin + Myosin slide
   ↓
Muscle contracts
   ↓
Arm bends

Contraction doesn't always mean movement

For example, if you push against a wall:

  • Your muscles are contracting.

  • They're producing force.

  • But the wall doesn't move.

This is called an isometric contraction.

Easy way to remember

Think of a game of tug-of-war:

  • 🪢 The rope is actin.

  • 💪 The people pulling are myosin.

The people don't shrink, and the rope doesn't shrink. The people pull the rope toward themselves, bringing the ends closer together. That's exactly what myosin does to actin inside a muscle fiber.

Exam tip

A simple definition to memorize is:

Muscle contraction = the generation of force when myosin pulls actin, causing the muscle fibers to shorten or attempt to shorten.

So when you read, "acetylcholine causes muscle contraction," you can mentally translate it to:

Acetylcholine starts the process that lets myosin pull on actin, producing force and often shortening the muscle.