The Muscular System

Three Types of Muscle

Every muscle in the body falls into one of three types, and the differences are worth knowing because they explain why some things are under your control and some are not.

Skeletal muscle is what you think of as muscle. It makes up most of the body's mass, it works on conscious command, and it fatigues.

Smooth muscle lines the hollow structures. It squeezes blood vessels to raise blood pressure, narrows or widens the bronchioles, and pushes food along the digestive tract. You have no conscious control over any of it.

Cardiac muscle is unique. It is striated like skeletal muscle but involuntary like smooth muscle, and it has a property neither of the others has: automaticity, the ability to generate its own electrical impulse without any signal from a nerve. That is why a heart continues beating in a patient whose brain is no longer sending it anything.

How a Muscle Contracts

Skeletal muscle is built from bundles of fibers, and each fiber is packed with two overlapping protein filaments, actin and myosin.

A nerve impulse arrives at the muscle and releases acetylcholine at the junction between them. That triggers calcium release inside the fiber. Calcium exposes binding sites on the actin, myosin grabs on and pulls, and the filaments slide past each other. The muscle shortens. Every one of those steps requires ATP, which is the cell's energy currency.

Two consequences follow from that.

Muscle contraction requires oxygen, because making ATP efficiently requires oxygen. Without it the muscle switches to anaerobic metabolism, produces far less ATP, and generates lactic acid as a byproduct. That is the burn you feel in a sprint, and it is also what is happening system wide in shock.

Relaxation also requires ATP, because the myosin has to let go. That is the reason for rigor mortis: once the supply is gone the filaments stay locked and the body stiffens.

Muscles Pull, They Never Push

A muscle can only shorten. Every movement therefore needs muscles working in opposing pairs.

The muscle producing the movement is the agonist or prime mover. The one opposing it is the antagonist. Bending the elbow, the biceps is the agonist and the triceps is the antagonist. Straightening it reverses the roles.

The fixed end of a muscle is its origin, usually the proximal attachment. The moving end is its insertion, usually the distal attachment. The origin stays put and the insertion moves toward it.

Muscle Tone

Even at rest, some fibers in every skeletal muscle are contracted. That constant low level tension is muscle tone, and it is what holds your head up, keeps your jaw closed and maintains your posture without any conscious effort.

Tone is a useful finding. A patient who is flaccid has lost it, which points at the nervous system rather than the muscle. A patient whose muscles are rigid has too much of it.

Muscles Worth Being Able to Name

You do not need all six hundred. You do need the ones that come up in assessment and treatment.

Breathing. The diaphragm is the primary muscle of respiration, a dome of skeletal muscle separating the chest from the abdomen. It flattens when it contracts, which enlarges the chest and pulls air in. It is innervated by the phrenic nerve, which comes off cervical roots C3, C4 and C5. That is where the memory line comes from: C3, 4 and 5 keep the diaphragm alive. A spinal cord injury above that level takes the diaphragm with it.

The intercostal muscles between the ribs lift the rib cage. The sternocleidomastoid and the scalenes in the neck, and the pectoralis and abdominal muscles, are accessory muscles. A patient using them is telling you the ordinary work of breathing is no longer enough.

Trunk and limbs. The rectus abdominis and the oblique muscles form the abdominal wall. The latissimus dorsi and the trapezius cover the back. The deltoid caps the shoulder and is a standard intramuscular injection site. The biceps and triceps work the elbow. The quadriceps on the front of the thigh and the hamstrings on the back work the knee, and the vastus lateralis portion of the quadriceps is the preferred injection site in an infant. The gastrocnemius is the calf.

Injuries to Muscle and Connective Tissue

Strain is an overstretched or torn muscle or tendon. Pain on use, local tenderness, usually no deformity.

Sprain is an overstretched or torn ligament, which means the injury is at a joint. Swelling, bruising, instability and pain on weight bearing.

Strain is muscle, sprain is joint. That distinction gets tested and it is worth locking in.

Contusion is bleeding into the muscle from blunt force. Rupture is a complete tear, and the muscle belly may bunch up visibly.

Compartment syndrome happens when swelling inside a muscle compartment raises the pressure past what the arteries can overcome. Muscle within the compartment starts to die. Pain out of proportion to the injury is the earliest and most reliable sign, and it typically appears well before any pulse disappears. Waiting for a lost pulse means waiting until the muscle is already gone.

When Muscle Breakdown Becomes a Whole Body Problem

Rhabdomyolysis is the breakdown of skeletal muscle, releasing its contents into the bloodstream. It follows crush injury, prolonged immobility such as a patient down on the floor overnight, extreme exertion, seizures and severe hyperthermia.

Three things come out of the dying muscle and each causes its own problem.

• Potassium, which is normally kept inside cells. A large release raises the blood potassium enough to produce lethal cardiac rhythms.
• Myoglobin, the oxygen carrying pigment of muscle. It clogs the kidneys and turns urine dark, and it is a common cause of acute kidney injury.
• Acid, which worsens the metabolic picture already present.

The dangerous moment in a crush injury is often the moment the weight comes off. While the compression is on, none of that can reach the central circulation. Release opens the route, and the patient who looked stable under the rubble can arrest shortly afterward. Anticipate it, do not be surprised by it.

What to Take Away

Three muscle types, and only cardiac muscle can generate its own impulse. Contraction and relaxation both cost ATP, which is why oxygen matters to muscle and why the body stiffens after death. Strain is muscle, sprain is ligament. Pain out of proportion is compartment syndrome until proven otherwise, and a crushed limb is most dangerous at the moment it is freed.