10.5: The Muscles

Neuromuscular junction (NMJ)

The signal from lower motor neurons communicate with the muscle at the neuromuscular junction (NMJ). The NMJ is similar to other chemical synapses, however the postsynaptic cell is a muscle cell separated by about 30 nm. The postsynaptic site is the sarcolemma, the cell membrane of muscle cells, which has several folds to increase surface area.

Figure 10.26 Electron micrograph image of an NMJ (top; T = axon terminal, M = muscle cell). Cartoon depicting the phases of signaling at an NMJ (bottom). The action potential (1) allows release of aceylcholine (2), activating nAChRs (3), allowing depolarization of muscle cells (4) and the contraction (5).

The axons of the lower motor neurons synthesize and release acetylcholine. Densely expressed in the sarcolemma are nicotinic acetylcholine receptors (nAChRs), ionotropic receptors that allow sodium influx and subsequent muscle cell depolarization upon acetylcholine binding. This depolarization results in muscle cell contraction.

Muscle anatomy

As mentioned above, motor functions can either be voluntary (moving your arm above your head) or involuntary (muscle contraction that leads to bowel motility, or heart beating). Although the nervous system influences both types of muscle activity, most of our discussion revolves around deliberate, voluntary skeletal muscle movement. The main action of muscles is to contract, a physical change of their shape where they become wider and shorter. For example, as you flex your arm, your bicep changes from long and thin to short and thick.

Different muscles have different characteristics driven by their shape and composition. For example, some muscles can be active for a long time without getting tired (maintaining your posture in your chair as you read this) while others can exert a lot of force but get tired quickly (lifting weights in the gym).

In studying muscles, some of the key measurements are where they connect to the bone, how long they are, how thick they are, and what type of muscle fibers they are made of. These measurements can be used to calculate how much force a muscle can generate and how quickly the joint can move.

Clinical connection: Myasthenia gravis (MG)

Myasthenia gravis (MG) is an autoimmune disorder characterized by muscle weakness, resulting in difficulty with speech, trouble with movement and swallowing, drooping eyelids, and double vision. Each year, an estimated 20 out of a million people get diagnosed with MG.

The muscle weakness seen in MG results from immune system-mediated destruction of the nAChRs expressed at the NMJ. Thus, when the lower motor neuron releases ACh, the muscle cells are unable to detect this release, so they fail to contract appropriately.

One therapeutic strategy involves inhibition of acetylcholinesterase, the enzyme that degrades ACh. This causes the synaptic ACh to remain in the synapse longer, increasing the chance that receptors get activated. Alternatively, autoimmune diseases like MG can be improved with immunosuppressant therapy. With successful treatment, MG usually does not result in changes in lifespan.

Figure 10.27 A patient with MG may show weakness of facial muscles.

Individual muscles, such as your biceps brachii, are made up of several muscle fascicles, which in turn are made up of muscle fibers. Muscle fibers are the individual cells in which contraction occurs. The functional units for contraction are called sarcomeres. It is the aggregated activity of hundreds of thousands of sarcomeres within each muscle fiber that generates the force with which you walk your dog or chew your food.

The contraction within sarcomeres happens between two proteins actin and myosin. Actin are the thin filaments that form the scaffolding, and myosin are thick filaments that pull the actin together, shortening the sarcomere and contracting the muscle. Although each sarcomere is small (1.5 to 3.5 µm), many are stacked back to back along the length of each fiber, pulling against each other to contract through the whole range of your joints. Many sarcomeres bundled in parallel provide the combined force that give muscles strength. The strength in all of your muscles comes from these tiny threads pulling against each other.

Figure 10.28 In a sarcomere, myosin (purple) and actin (red) overlap when the muscle flexes.

Muscle types

There are two key characteristics of muscles: their structure (striated due to the presence of sarcomeres or smooth), and whether they can be voluntarily controlled. Based on these characteristics, there are three types of muscles in humans and other vertebrates.

	Striated	Smooth
Voluntary	Skeletal
Involuntary	Cardiac	Smooth

Skeletal muscle is what most people think of when they think “muscle.” They attach to bones with tendons, and exert force on your skeleton to create movement and exert force on objects.

Skeletal muscles are also called “voluntary muscles” because they are the muscles that move when you choose to make a movement. They are voluntarily contracted (and in reflexes) to move your body by moving your skeleton. There are two types of skeletal muscles.

Fast twitch muscles generate a lot of force quickly, but also tire quickly. They are used mostly in high intensity exercise like lifting weights and sprinting. Slow twitch muscles generate less force, but can work for a long time. They are used in endurance exercise like jogging. Many exercises use a combination of both.

Muscles come in several different shapes, defined by the arrangement of their fibers and how they connect to tendons. The different shapes allow for different properties: some can change length quickly, others change shape less but are stronger. Another consideration for shape is the geometry of the joint they cover: for example, the pectoralis muscle reaches from your chest across your shoulder, and so the muscle is wide and flat where it connects to your sternum, and narrows to a point where it connects to your upper arm.

Figure 10.29 Different types of muscle as seen at 400x magnification. Muscle fibers are seen running from left to right. In skeletal and cardiac muscles, striations can be seen perpendicular to the muscle fibers.

Skeletal muscles work to move the body through a combination of cooperation and opposition. The agonist muscle is the main mover, like the biceps brachii when you flex at the elbow. It’s movement is supported by synergistic muscles, including the brachialis and brachioradialis. These can also be fixators, providing stability and preventing or allowing rotation of the wrist while flexing at the elbow. Antagonist muscles are those that move in the opposite direction to the agonist. For the elbow, the antagonist is the triceps bracii, which lengthens during the bicep flex. Another example of an agonist-antagonist pair are the hamstrings to flex the leg, and the quadriceps to extend it.

Figure 10.30 Skeletal muscles may be paired and produce opposing effects on a particular motion.

Smooth muscles are the muscles embedded within organs like your stomach and intestines, blood vessels, and bladder. They are also known as “involuntary” muscles because they are not under direct conscious control.

Cardiac muscle can be thought of as a hybrid between skeletal muscles and smooth muscles, in that they are striated like skeletal muscles but not under conscious control.