Somatic Nervous System unit 2 pt.1

Somatic Nervous System (SNS)

1-2

Voluntary Control

This is the part of your nervous system that lets you move your skeletal muscles on purpose .

Examples:

• Walking 🚶

• Writing ✍

• Smiling 😊

• Lifting weights 💪

Easy Breakdown 1. Impulses originate in the CNS

CNS = Brain + Spinal Cord

Your brain decides to move.

Example: i want to pick up my phone

The brain sends a message down the spinal cord.

➡ Brain → Spinal Cord → Motor Neuron

2. Somatic Motor Neurons carry the message

These neurons are like electrical wires .

They carry the movement signal from the CNS to the muscle.

➡ Brain → Motor Neuron → Muscle

Somatic Nervous System

3-5

3. Neuromuscular Junction (NMJ)

The neuron cannot directly touch the muscle.

There is a tiny gap called the:

NMJ = Neuromuscular Junction

Think of it as

📞 Nerve calls the muscle

The nerve sends a chemical message across the gap.

4. Acetylcholine (ACh) is released

The nerve releases a neurotransmitter called:

Acetylcholine (ACh)

Think:

📨 ACh = the message

The message says: MUSCLE CONTRACT NOW

5. ACh binds to Nicotinic (Nm) Receptors

On the muscle are special receptors:

Nicotinic Nm receptors

ACh attaches to these receptors like a:

🔑 Key fitting into a lock

ACh (key) + Nm receiver (lock)

↓

Muscle contracts!

💪 Muscle moves

Somatic Nervous System

Brain

↓

Spinal Cord

↓

Somatic Motor Neuron

↓

Neuromuscular Junction (NMJ)

↓

ACh Released

↓

Nm Receptor Activated

↓

Skeletal Muscle Contracts

• ACH acts on nicotinic 2/Nm receptors

  • respirations

  • contractions of diaphragm and chest muscles

  • tone for movement

  • body posture
    

Respiration (Breathing)

• tone for movement

• body posture
  
  Somatic Nervous System

Respiration (Breathing)

The somatic nervous system also controls muscles used for breathing.

Diaphragm

Main breathing muscle

Chest Muscles

Help expand the lungs

When the brain sends signals:

➡ Diaphragm contracts

➡ Chest expands

➡ Air moves into lungs

🫁 You breathe

Tone for Movement

Muscle tone = slight muscle tension all the time

Your muscles are never completely relaxed.

This helps:

• Be ready to move quickly

• Maintain stability

• Prevent floppiness

Think:

💪 Muscles stay slightly "on"

Body Posture

The somatic nervous system helps you:

• Sit upright

• Stand up straight

• Keep your balance

• Hold your head up

Without muscle tone and somatic control:

😵 You would slump over.

SOMATIC = SKELETAL = VOLUNTARY

Somatic Nervous System

Brain decides
↓
Motor neuron sends signal
↓
ACh released
↓
Nm receptor activated
↓
Skeletal muscle contracts

Remember:

🧠 → ⚡ → ACh → Nm → 💪

Brain → Signal → Acetylcholine → Nicotinic receptor → Muscle movement

Somatic Nervous System

Spinal Cord

↓

One Neuron

↓

ACh

↓

Skeletal Muscle

↓

Movement

✅ Voluntary control

✅ Controls skeletal muscles

✅ One neuron from CNS to muscle

✅ No lymph node

✅ Neurotransmitter = Acetylcholine (ACh)

✅ Causes muscle contraction

CNS → one motor neuron → ACh released → skeletal muscle contracts. ✅

Skeletal Muscle Relaxants 1

What do they do?

Skeletal muscle relaxants = drugs that make muscles relax.

Normally:

🧠 Brain → Nerve → ACh → 💪 Muscle contracts

With a muscle relaxant:

🧠 Brain → Nerve → 🚫 Signal blocked/reduced → 💪 Muscle relaxes

So the muscle does less contracting and less spasming .

MOA = How the drug works

These drugs:

✅ Interferes with neuromuscular function

✅ Reduces muscle contractions

✅ Relax tight muscles

✅ Decrease muscle spasms

Think:

💪 Muscle is squeezing too much

💊 Muscle relaxant says:

"Calm down and relax."

Result:

💪 ➡ 😌

Skeletal Muscle Relaxants

• Uses:

• painful injuries

Muscles tighten up after injury.

💥 Injury → 💪 Muscle tightens → 😖 Pain

Muscle relaxant:

💥 Injury → 💊 Drug → 😌 Muscle relaxes → Less pain

• decrease contraction or spasms from trauma=Spasm = Muscle won't stop squeezing

• , overexertion, stress, or tension

spastic diseases

such as cerebral palsy=The brain has trouble controlling muscles.

Result:

💪 Tight muscles

and multiple sclerosis= Nerve damage causes muscles to receive abnormal signals.

• pre-op, surgeries

Doctors sometimes want muscles completely relaxed during surgery.

✅ Easier procedure ✅ Prevents muscle movement

Skeletal Muscle Relaxants

• Central or Peripheral Acting

Central Acting	Peripheral Acting
Works in brain/spinal cord	Works on muscle
Decreases nerve signals	Decreases muscle contraction
CNS action	Muscle action
"Control center"	"At the muscle"

Central-Acting Muscle Relaxants Work in the CNS

(CNS = Brain and Spinal Cord)

🧠 Brain/Spinal Cord
⬇
Drug acts here
⬇
Less signals sent to muscles
⬇
💪 Muscle relaxes

Peripheral-Acting Muscle Relaxants Work at the muscle or neuromuscular junction

The drug acts closer to the muscle itself

Nerve → 🚫 Signal blocked → Muscle can't contract

Example:

• Dantrolene

Think:

Peripheral = Out by the muscle

Exam Memory Trick Skeletal Muscle Relaxants

Reduce Excessive Muscle Activity

✅ Relax muscles

✅ Reduces spasms

✅ Help with painful injuries

✅ Used in cerebral palsy

✅ Used in multiple sclerosis

✅ Used before surgery

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One-Line Summary

Skeletal muscle relaxants decrease muscle contractions and spasms by blocking signals either in the brain/spinal cord (central acting) or directly at the muscle (peripheral acting). 💪➡😌

Peripheral Skeletal Muscle Relaxants

These drugs work outside the brain and spinal cord , directly at the muscle or neuromuscular junction.

Think:

🧠 Brain sends signal → Nerve → Muscle

Peripheral muscle relaxants block the signal near the muscle .

3 Types to Know

• Depolarizing Agents
  

• Non-depolarizing Agents
  

• Direct acting Agents

Peripheral Skeletal Muscle Relaxants

1. Depolarizing Agents

What do they do?

They overstimulate the muscle first, then the muscle becomes unable to contract.

Think of it like:

📞 Phone keeps ringing and ringing and ringing...

Eventually the phone stops responding.

The muscle gets "stuck" and cannot contract normally.

What's happening?

1. Drug attaches to muscle receptor

2. Muscle briefly twitches

3. Receptor stays activated

4. Muscle becomes relaxed/paralyzed

Easy Memory

Depolarizing = First stimulate, then paralyze

• Stimulates first

• Causes brief twitching

• Then paralysis

• Example: Succinylcholine

Peripheral Skeletal Muscle Relaxants

2. Non-Depolarizing Agents

What do they do?

These drugs simply block the signal from the start.

The message never reaches the muscle.

Normal:

🧠 → Nerve → ACh → Muscle contracts

With drug:

🧠 → Nerve → 🚫 ACh blocked → No contraction

Easy Memory

…. = Block the message

ACh blocked

↓

No contraction

↓

Muscle relaxes

• Blocks ACh receptors

• Prevents contraction

• Causes relaxation/paralysis

• Examples: Rocuronium, Vecuronium

Peripheral Skeletal Muscle Relaxants 1

. Direct-Acting Agents

What do they do?

These drugs work directly on the muscle fiber itself .

They don't mainly block the nerve signal.

Instead they tell the muscle:

"Don't contract so strongly."

Easy Memory

Think:

💪 Muscle is the problem

💊 Drug goes directly to the muscle

💪 ➜ 😌

• Works directly on muscle

• Reduces muscle contraction

• Example: Dantrolene

Peripheral Skeletal Muscle Relaxants 1

types

Type	What It Does
Depolarizing	Overstimulates muscle, then paralysis
Non-depolarizing	Blocks ACh signal to muscle
Direct-acting	Works directly on muscle fiber

Peripheral muscle relaxants work near the muscle: depolarizing agents overstimulate then paralyze, non-depolarizing agents block the nerve signal, and direct-acting agents work directly on the muscle fiber. ✅

Depolarizing Agents (Super Easy Version)

💪 "Turn the muscle ON so much that it gets tired and can't move anymore."

What Depolarizing Agents Do Step 1: Stimulate the muscle

The drug attaches to the Nm (Nicotinic 2) receptor just like ACh.

Drug → Nm receptor

↓

Muscle stimulated

↓

Muscle twitching

These little muscle twitches are called:

Fasciculations

Step 2: Muscle becomes unable to contract

The drug stays attached to the receiver.

The receiver becomes "worn out" and stops responding.

Now even when the body releases ACh:

🚫 Receiver won't listen

🚫 Muscle won't contract

Result:

💪 Muscle paralysis/relaxation

Depolarizing Agents (Super Easy Version)

Why are they used?

1. Intubation

Intubation = placing a breathing tube

Doctors want muscles relaxed.

Relaxed muscles

↓

Easier tube placement

2. General Anesthesia

Used during surgery.

Patient asleep

+

Muscles relaxed

↓

Safer surgery

Depolarizing Agents (Super Easy Version)

Side Effects

Fasciculations

Before paralysis, muscles twitch.

Twitching

↓

Soreness

↓

Muscle pain

Jaw Rigidity

Jaw muscles become stiff.

Hypotension Hypo = Low

Hypotension = Low blood pressure

Arrhythmias Arrhythmia = Abnormal heartbeat

Malignant Hyperthermia

A rare but dangerous reaction.

Can cause:

🔥 Very high fever

👨‍👩‍👧 Family history

Because it can run in families.

Pediatric Warning

Should only be used in emergencies in children because of serious risks.

Peripheral Skeletal Muscle Relaxants 

• Depolarizing Agents

No Antidote Antidote

…= Drug that reverses another drug

Depolarizing agents:

🚫 No antidote

Doctors must

✅ Support breathing ✅ Wait for the drug to wear off

How Does the Drug Leave the Body?

An enzyme called:

Cholinesterase

breaks down the drug. Drug in body

↓

Cholinesterase breaks it down

↓

Muscle function returns

• No antidote-support respirations until drug is metabolized by cholinesterase in the body

• Depolarizing Agents

easy summary

MOA

1. Bind Nm receiver

2. Cause fasciculations (twitching)

3. Receiver stops responding

4. Muscle paralysis occurs

Uses

✅ Intubation

✅ General anesthesia

Side Effects

✅ Fasciculations

✅ Muscle pain

✅ Jaw rigidity

✅ Hypotension

✅ Arrhythmias

✅ Malignant hyperthermia

Important Facts

✅ No antidote

✅ Support respirations

✅ Metabolized by cholinesterase

✅ Check family history for malignant hyperthermia

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One-Sentence Summary

Depolarizing agents first make muscles twitch (fasciculations), then keep the Nm receptor activated so the muscle can no longer respond to ACh, causing temporary paralysis for procedures like intubation and surgery. 💪 → ⚡ → 🚫 → 😌

1. Peripherally Acting/ I.       Skeletal Muscle Relaxants

• Depolarizing Agents

• DRUG

succinylcholine (no antidote)

Non-Depolarizing Agents (Super Easy Version)

Think of these drugs as:

🚫 "Blocking the muscle's door so ACh can't get in."

Unlike depolarizing agents, these drugs DO NOT stimulate or activate the muscle first.

The drug sits on the Nm (Nicotinic 2) receptor .

Drug sits on receptor

↓

ACh cannot attach

↓

No signal reaches muscle

↓

No contraction

↓

Muscle relaxes

Think of it like:

🔑 ACh = Key

🚪 Nm receiver = Lock

💊 Drug = Gum stuck in the lock

Now the key can't fit!

ACh ❌

Cannot enter receptor

Result

💪 No contraction

😌 Muscle relaxation/paralysis

• Non-Depolarizing Agents

Uses

1. Intubation

Intubation = placing a breathing tube.

Doctors want muscles relaxed.

2. General Anesthesia

Used during surgery./ muscle relax/ easier surgeyr

3. Mechanical Ventilation

Mechanical ventilator = breathing machine.

The patient must stay relaxed so the machine can breathe for them.

Non-Depolarizing

NO stimulation

NO twitching first

Just BLOCKS ACh

🚫 "ACh is locked out."

Drug blocks Nm receptor

↓

ACh cannot bind

↓

No contraction

↓

Muscle relaxes

• Non-Depolarizing Agents

Side Effects

Fasciculation = Muscle twitching

Muscle Pain

Twitching can cause soreness.

Jaw Rigidity

Jaw becomes stiff.

Hypotension = Low blood pressure

Arrhythmia = Abnormal heartbeat

Respiratory Depression

Respiratory = Breathing

Depression = Slowed down

Because these drugs relax muscles, they can also relax the muscles needed for breathing.

Patients may need:

✅ Oxygen

• Non-Depolarizing Agents
  
  QUICK STUDY REVIEW

MOA

✅ Occupy Nm receptors

✅ Block ACh from binding

✅ No depolarization

✅ No muscle contraction

✅ Muscle relaxation/paralysis

Uses

✅ Intubation

✅ General anesthesia

✅ Mechanical ventilation

Side Effects

✅ Fasciculations, ✅ Muscle pain, ✅ Jaw rigidit✅ Hypotension

✅ Arrhythmias ✅ Respiratory depression

Non-depolarizing agents work by blocking the Nm receptor so acetylcholine cannot bind, preventing muscle contraction and causing muscle relaxation for procedures such as intubation, surgery, and mechanical ventilation. ✅💪🚫😌

• Non-depolarizing Agents

• DRUG

• Skeletal Muscle Relaxants

  1. Peripherally Acting

rocuronium bromide - reversed by neostigmine or sugammadex

Direct-Acting Muscle Relaxants (SUPER EASY VERSION)

Think of this drug as:

💊 ➡ 💪 Works directly on the muscle

It does NOT work in the:

🚫 Brain

🚫 Spinal cord

🚫 Nm (Nicotinic 2) receiver

Instead, it goes right to the muscle fiber itself.

These drugs block the release of calcium inside the muscle.

Drug

↓

Blocks Ca++

↓

No calcium released

↓

Muscle cannot contract

↓

Muscle relaxes

Think of Calcium as the "ON Switch"

Normal:

🔘 Calcium ON
↓
💪 Muscle contracts

With drug:

🚫 Calcium blocked
↓
💪 Muscle relaxes

Peripheral Skeletal Muscle Relaxants

BIG DIFFERENCE

Depolarizing Agents

Work at the Nm receptor.

Non-Depolarizing Agents

Block the Nm receptor.

Direct-Acting Agents

Ignore the receiver completely.

Go directly to the muscle.

Depolarizing → Nm receptor
Non-depolarizing → Nm receptor
Direct-acting → Muscle fiber itself

Peripheral Skeletal Muscle Relaxants

Uses

• Direct Acting Agents

These drugs are used when muscles are too tight or spastic.

Cerebral Palsy

Brain damage causes muscles to stay tight.

Brain problem
 ↓
Muscles stay tight
 ↓
Drug relaxes muscles

Multiple Sclerosis (MS)

Damaged nerves send abnormal signals.

Spinal Cord Injury

Signals become abnormal.

Malignant Hyperthermia

Direct-acting agents (like dantrolene) help relax the muscles.

• Direct Acting Agents
  
  Side Effects

Hepatotoxicity Hepato = Liver Toxicity = Damage

Meaning:

🚨 Can damage the liver

This is the MOST IMPORTANT side effect.

Box Warning

A box warning means

⚠ Serious warning from the FDA

Pay close attention.

vomiting, dizziness, fatigue, weakness
Contraindication

Liver Disease

Because the drug can damage the liver:

🚫Do NOT use in patients with liver disease.

Direct-Acting Muscle Relaxants

easy memory

MOA

✅ Work directly on muscle fibers

✅ Block calcium (Ca++) release

✅ No calcium = No contraction

✅ Do NOT affect Nm receivers

✅ Do NOT affect spinal cord conduction

Uses

✅ Cerebral palsy

✅ Multiple sclerosis

✅ Spinal cord injuries

✅ Malignant hyperthermia

Contraindication

🚫 Liver disease

Direct-acting muscle relaxants work directly on the muscle fiber by blocking calcium release, preventing muscle contraction and relaxing muscles in conditions such as cerebral palsy, multiple sclerosis, spinal cord injuries, and malignant hyperthermia. 💊 → 🚫 Ca++ → 💪 Relaxed Muscle.

• Direct Acting Agents

• DRUG

dantrolene (Dantrium®)