Muscle Contraction: From Nerve Signal to Movement

Introduction

Skeletal muscle allows us to move, react, and maintain posture—all by contracting and relaxing in response to nervous signals. This section begins with the physiology of skeletal muscle contraction, from the initiation of a signal to the relaxation phase, followed by an overview of key muscles of the body and their functions. You’ll explore what happens at the cellular level to make a muscle fiber contract and how this complex system relies on neural communication, ion exchange, and specific protein interactions. Refer to visuals throughout this chapter for guidance (see Figure 1).

Motor Units and Control of Muscle Force

A motor unit consists of a motor neuron and all the muscle fibers it innervates. The number of fibers per motor unit determines the type of control the muscle has. Muscles that perform fine movements, like those controlling the eyes and fingers, have fewer fibers per neuron. This allows for precise motor control. In contrast, large muscles like the quadriceps have motor units with many muscle fibers, allowing greater force but less precision (see Figure 2).

Neuromuscular Junction and Signal Transmission

Contraction begins at the neuromuscular junction (NMJ)—the point where a motor neuron meets a skeletal muscle fiber (see Figure 3). The NMJ includes the axon terminal, the synaptic cleft (a small gap), and the motor end plate. When a nerve impulse reaches the axon terminal, it triggers the release of the neurotransmitter acetylcholine (ACh). ACh diffuses across the cleft and binds to receptors (AChRs) on the muscle membrane, opening ion channels.

This allows sodium (Na⁺) ions to enter the cell and potassium (K⁺) ions to exit, creating an electrical change known as depolarization (see Figure 4).

Excitation and Calcium Release

The electrical change travels along the sarcolemma and into transverse (T) tubules, which carry the signal deep into the muscle fiber. This activates calcium channels on the sarcoplasmic reticulum (SR), releasing stored calcium ions (Ca²⁺) into the cytoplasm (see Figure 5). Calcium binds to the protein troponin, causing it to shift the position of tropomyosin, which uncovers the active binding sites on the thin filament actin (see Figure 6).

The Cross-Bridge Cycle and ATP Use

With actin binding sites exposed, myosin heads from the thick filament attach to actin, forming a cross-bridge. Myosin must be energized by ATP, which allows the head to “cock” back into position. Upon binding, the myosin head pivots in a power stroke, pulling actin inward. A new ATP molecule binds to detach the myosin head and re-cock it, allowing the cycle to repeat rapidly—up to 10 times per second during sustained contraction (see Figure 7).

Muscle Relaxation

To stop contraction, the nerve signal ceases, halting ACh release. Any remaining ACh is broken down by the enzyme acetylcholinesterase (AChE). Simultaneously, calcium pumps use ATP to reabsorb calcium back into the SR. Without calcium, troponin and tropomyosin return to their original positions, covering actin’s binding sites and ending contraction (see Figure 8).

Muscle Twitch and Tension Regulation

A muscle twitch is a single contraction and relaxation cycle triggered by one neural stimulus. It has three phases: latent (delay), contraction (tension increase), and relaxation (return to baseline). This can be measured in a myogram, which plots tension over time (see Figure 9).

When stimuli are repeated before a muscle fully relaxes, twitches summate, producing stronger contractions. Incomplete relaxation leads to incomplete tetanus—a sustained, forceful contraction. In pathological cases, such as tetanus infection, muscles enter complete tetanus, where they remain contracted without relaxing (see Figure 10).

Major Skeletal Muscles of the Body

Muscles of Facial Expression

• Orbicularis oculi encircles the eye and enables blinking and squinting (see Figure 11).

• Buccinator compresses the cheeks, aiding in blowing and chewing.

• Orbicularis oris surrounds the mouth and allows for puckering, speaking, and facial expressions.

Muscles of the Head and Neck

• Frontalis elevates the eyebrows and wrinkles the forehead.

• Occipitalis, located at the back of the head, pulls the scalp posteriorly (see Figure 12).

• Sternocleidomastoid connects the sternum, clavicle, and mastoid process. It rotates and laterally flexes the neck.

• Temporalis and masseter elevate the mandible during chewing. Temporalis also moves the jaw side-to-side.

Muscles of the Torso

• Pectoralis major flexes, adducts, and medially rotates the arm at the shoulder (see Figure 13).

• Rectus abdominis runs vertically along the anterior trunk and enables flexion of the spine (e.g., sit-ups).

• External obliques support trunk rotation and lateral flexion, aiding in twisting motions and forced expiration.

Back and Shoulder Muscles

• Trapezius controls scapular elevation, retraction, and rotation.

• Latissimus dorsi enables shoulder extension, adduction, and medial rotation.

• Deltoid abducts the arm and assists in shoulder flexion and extension (see Figure 14).

Muscles of the Arm

• Biceps brachii flexes the forearm and assists in supination.

• Triceps brachii, located on the posterior arm, extends the forearm (see Figure 15).

• Brachioradialis supports elbow flexion and acts as a synergist to the biceps.

Forearm Muscles: Rotation of the Arm

• Supinator turns the palm upward (supination).

• Pronator teres turns the palm downward (pronation). These muscles enable fine adjustments for tasks such as writing and tool use (see Figure 16).

Muscles of the Thigh: Quadriceps Group

The quadriceps femoris group includes four muscles that extend the knee (see Figure 17):

• Rectus femoris (central and superficial)

• Vastus lateralis (lateral)

• Vastus medialis (medial)

• Vastus intermedius (deep to rectus femoris)

These are essential for walking, running, squatting, and jumping.

Posterior Thigh and Gluteal Region

• Biceps femoris, part of the hamstring group, extends the hip and flexes the knee.

• Gluteus maximus is a large hip extensor and stabilizer during standing, walking, and climbing (see Figure 18).

Muscles of the Lower Leg

• Tibialis anterior dorsiflexes the foot (pulls toes upward) and is active during heel strike in walking (see Figure 19).

• Gastrocnemius and soleus work together to plantar flex the foot (push toes downward). Gastrocnemius is important for quick, powerful movements, while soleus supports endurance activities.

Lesson Video:

Learn about muscle contraction and the major muscles of the body by watching this 62-minute video:

Practice Questions

Use these practice questions to assess your knowledge before you move on to the next section.