10.4 Nervous System Control of Muscle Tension

Muscle Contraction Types and Characteristics

Muscles don't just shorten when they contract. Contraction really means the muscle is generating tension, and that can happen while the muscle shortens, lengthens, or stays the same length. The type of contraction depends on the relationship between the force the muscle produces and the external load it's working against.

Types of Muscle Contractions

Isotonic contractions involve movement at the joint because muscle length changes while tension stays relatively constant. There are two subtypes:

• Concentric contraction: The muscle shortens as it generates force. Think of your biceps brachii contracting to curl a dumbbell upward. The muscle force exceeds the load, so the muscle shortens.
• Eccentric contraction: The muscle lengthens while still generating tension. This happens when the external load is greater than the force the muscle produces, but the muscle is still actively contracting to control the movement. Your biceps during the lowering phase of that same curl is a classic example.

Isometric contractions generate force without any change in muscle length. The muscle produces tension, but it isn't enough to move the load. Holding a heavy box in front of you with your arms bent is isometric: your biceps are working hard, but the joint angle isn't changing.

Length-Tension Relationship

The amount of tension a sarcomere can produce depends on how much overlap exists between thick (myosin) and thin (actin) filaments.

• Optimal overlap occurs at a sarcomere length of roughly 2.0–2.2 μm. At this length, the maximum number of cross-bridges can form, so tension production is highest.
• Too short: When the sarcomere is overly compressed, the thin filaments from opposite sides start to overlap each other and the thick filaments butt up against the Z-discs. Fewer productive cross-bridges form, so tension drops.
• Too long: When the sarcomere is stretched out, there's less overlap between thick and thin filaments, meaning fewer cross-bridges can form and tension decreases.
• Passive tension also plays a role. The elastic components of the muscle (especially the protein titin) resist stretching, so passive tension increases as the muscle is pulled beyond its resting length.

Muscle Twitch and Tension Control

A single muscle twitch is the response of a muscle fiber to one action potential. It's brief and weak on its own, but understanding its phases is the foundation for understanding how the body produces smooth, sustained contractions.

Phases of a Muscle Twitch

1. Latent period: The short delay between the arrival of the action potential at the muscle fiber and the beginning of tension development. During this time, the action potential travels along the sarcolemma and into the T-tubules, triggering calcium release from the sarcoplasmic reticulum (SR). No visible contraction occurs yet.
2. Contraction phase: Calcium binds to troponin, tropomyosin shifts, and cross-bridges begin cycling. Tension rises as the muscle shortens (or attempts to shorten against a load).
3. Relaxation phase: Calcium is actively pumped back into the SR by $\text{Ca}^{2+}$-ATPase pumps. Cross-bridge cycling stops, tension drops, and the muscle returns toward its resting length.

Mechanisms of Tension Control

The body rarely uses single twitches. Instead, it modulates tension through the timing and frequency of stimulation.

• Wave summation: If a second stimulus arrives before the muscle has fully relaxed from the first twitch, the second contraction builds on the residual tension of the first. The result is a higher peak tension than either twitch alone. The faster the stimuli arrive, the more tension accumulates.
• Incomplete tetanus: When stimuli come rapidly enough that the muscle only partially relaxes between each one, you get a jerky but elevated plateau of tension.
• Complete tetanus: When stimuli arrive so fast that there's no relaxation at all between them, the individual twitches fuse into one smooth, sustained contraction at maximum tension. This is the normal way your muscles produce steady force during voluntary movements.
• Treppe (staircase effect): When a resting muscle receives repeated stimuli at a constant frequency (with full relaxation between each), the first few twitches progressively increase in strength. This is thought to result from a buildup of calcium in the sarcoplasm and warming of the muscle fibers, which improves enzyme efficiency.

Neuromuscular Control

The nervous system controls how much force a whole muscle produces by varying which and how many motor units are activated.

Motor Units

A motor unit is one motor neuron plus all the muscle fibers it innervates. When that motor neuron fires, every fiber in the unit contracts. This is the all-or-none principle applied at the level of individual muscle fibers: each fiber either contracts completely or not at all. There's no partial contraction of a single fiber.

Motor unit size varies depending on the precision needed:

• Small motor units (e.g., 2–3 fibers per neuron in the muscles controlling eye movement) allow fine, precise control.
• Large motor units (e.g., hundreds of fibers per neuron in the quadriceps) generate powerful but less precise movements.

The Neuromuscular Junction

The neuromuscular junction (NMJ) is the synapse between a motor neuron's axon terminal and the muscle fiber's motor end plate. Here's how the signal crosses:

1. An action potential arrives at the axon terminal.
2. Voltage-gated calcium channels open, and $\text{Ca}^{2+}$ flows into the terminal.
3. Calcium triggers synaptic vesicles to fuse with the membrane and release acetylcholine (ACh) into the synaptic cleft.
4. ACh binds to nicotinic receptors on the motor end plate, opening ligand-gated ion channels.
5. $\text{Na}^+$ rushes into the muscle fiber, depolarizing it and generating a muscle fiber action potential.
6. Acetylcholinesterase in the synaptic cleft rapidly breaks down ACh, ending the signal.

Recruitment

The body controls overall muscle force through recruitment, which means activating additional motor units. At low force levels, only a few small motor units fire. As more force is needed, larger motor units are progressively recruited. This follows the size principle: smaller motor neurons (with lower thresholds) are activated first, and larger ones are added as demand increases. This gives you smooth, graded increases in force rather than sudden jumps.