Somatic Nervous System

The Somatic Nervous System (SNS) is primarily responsible for voluntary control of the body.
It facilitates skeletal muscle contraction through impulses originating in the Central Nervous System (CNS) that are transmitted to somatic motor neurons.
The point where these neurons connect with skeletal muscle fibers is known as the Neuromuscular Junction (NMJ).
Acetylcholine (ACH) is the neurotransmitter used in this system, acting on Nicotinic 2/Nm receptors to regulate several critical functions:
- Respirations through the contraction of the diaphragm and chest muscles.
- Maintenance of tone for movement.
- Maintenance of body posture.

Somatic neurons represent a single-neuron pathway; the entire distance from the CNS to the skeletal muscle fiber (the effector) is covered by one neuron.
These fibers are myelinated to ensure rapid conduction.
Acetylcholine (ACH) is the only neurotransmitter involved in this pathway.
Receptors located at the motor end plate of the skeletal muscles are specifically identified as Nicotinic-m receptors (Nicotinic-2/Nm).

Mechanism of Action (MOA): These agents inhibit skeletal muscle contraction by interfering with neuromuscular function.
Primary Therapeutic Uses:
- Treating painful injuries.
- Decreasing contractions or spasms resulting from trauma, overexertion, stress, or tension.
- Managing spastic diseases such as cerebral palsy and multiple sclerosis.
- Pre-operative applications and during surgeries.
Classification: Skeletal muscle relaxants are generally categorized as either centrally acting or peripherally acting.

Mechanism of Action (MOA): This is a two-step process.
- Step 1: The agent binds to the Nicotinic 2/Nm receptor and induces depolarization, which manifests as muscle fasciculations (twitching).
- Step 2: The agent alters the Nicotinic 2/Nm receptor so it can no longer respond to endogenous ACH, thereby preventing further contractions.
Clinical Uses:
- Intubation procedures.
- Adjunct therapy for general anesthesia.
Side Effects and Patient Teaching:
- Fasciculations leading to muscle pain.
- Jaw rigidity.
- Hypotension and arrhythmias.
- Potential for toxicity if the patient has abnormal cholinesterase levels.
- Risk of malignant hyperthermia: It is vital to obtain a family history before administration.
- Pediatric use: Should only be used in emergent situations in pediatric patients.
Antidote Information: There is no specific pharmacological antidote; treatment consists of supporting respirations until the drug is naturally metabolized by cholinesterase in the body.
Specific Example: Succinylcholine.

Mechanism of Action (MOA): These agents occupy the Nicotinic 2/Nm receptor sites, preventing ACH from binding. Consequently, no depolarization or contraction occurs.
Clinical Uses:
- Intubation.
- Adjunct therapy for general anesthesia.
- Mechanical ventilation support.
Side Effects and Patient Teaching:
- Muscle pain resulting from fasciculations.
- Jaw rigidity.
- Hypotension.
- Arrhythmias.
- Respiratory depression.
Examples of Non-Depolarizing Agents:
- Curare
- Cisatracurium
- Pancuronium
- Vecuronium

Mechanism of Action (MOA): These agents inhibit skeletal muscle contraction by blocking the release of calcium ( $Ca^{2+}$ ) within the muscle fiber itself. They do not affect the Nicotinic 2/Nm receptor or spinal cord conduction.
Clinical Uses:
- Spastic conditions such as cerebral palsy, multiple sclerosis, and spinal cord injuries.
- Treatment of malignant hyperthermia.
Side Effects and Patient Teaching:
- Hepatotoxicity: Features a Boxed Warning.
- Vomiting, dizziness, fatigue, and weakness.
Contraindications: Patients with existing liver disease.
Specific Drug Examples:
- Dantrolene
- Botulinumtoxin A or B (including generic variations: abobotulinumtoxin A, incobotulinumtoxin A, onabotulinumtoxin A, rimabotulintoxin B).

Mechanism of Action (MOA): These drugs decrease the number of impulses available to produce a contraction at motor neurons by blocking conduction within the spinal cord.
Clinical Uses:
- Treatment of spasms.
- Injury, tension, and overexertion.
Side Effects and Patient Teaching:
- Blurred vision and fatigue.
- Decreased mental alertness and lethargy.
- Dizziness and decreased muscle tone.
- Hypotension.
Safety Warnings:
- Avoid the consumption of alcohol.
- Patients may need to avoid operating a vehicle if the sedative effects are intense.
Examples of Centrally Acting Agents:
- Baclofen
- Carisoprodol
- Diazepam
- Methocarbamol
- Orphenadrine
- Tizanidine

Site 1 (CNS/Spinal Cord): This is the site for impulse conduction from the CNS through the spinal cord. Centrally acting muscle relaxants work here.
Site 2 (Neuromuscular Junction/NMJ): This is the location where depolarizing and non-depolarizing muscle relaxants function by interacting with the junctional folds and ACH receptors.
Site 3 (Skeletal Muscle Fiber): This is where direct-acting drugs work to inhibit calcium ion release from the cell body (sarcolemma/myofilaments).

Parasympathetic Neuron: Uses pre- and postganglionic fibers to reach Smooth or Cardiac muscle via Muscarinic receptors.
Sympathetic Neuron: Innervates targets via adrenergic receptors; however, the ganglion of both parasympathetic and sympathetic neurons contains Nicotinic-n (Nn) receptors.
Somatic Neuron: Directly innervates skeletal muscle via Nicotinic-m (Nm) receptors using ACH at the NMJ. No ganglion is present between the spinal cord and the effector muscle.