Muscle physiology and lifespan changes

Physiology of the muscular system

Muscle tissues have four characteristics in common:

• Excitability: Ability to react to a stimulus
• Contractility: Ability to shorten in response to a stimulus
• Extensibility: Ability to be extended in response to a stimulus
• Elasticity: Quality of being elastic

Primary functions

The muscular system has four primary functions:

1. Muscle contractions provide muscle tone and posture: Partial, sustained contraction (muscle tone) keeps the body upright against gravity - detailed in the muscle tone and posture section below.
2. Maintains the body temperature: A by-product of the energy (adenosine triphosphate, or ATP) used for muscle contraction is heat production. All types of muscle contractions produce heat, with the skeletal muscles providing the most heat. About 85% of body heat comes from muscle contractions. When a person is cold, shivering (contractions of random skeletal muscles) produces heat. This helps maintain the body temperature.
3. Provides joint stability: Muscles and tendons stabilize joints during movement and at rest. The tendons extend over the joint, stabilizing the joint. Joint stability depends on the strength and coordination of joint muscles. Muscle weakness can lead to joint instability.
4. Controls passageways in the body: Sphincters are ringlike muscles that open and close body structures, regulating the flow of substances. Involuntary sphincters are found in the digestive tract, urethra, and the iris of the eye. The outer anal sphincter and urethral sphincter are voluntary and allow for a bowel movement and the passage of urine. These two sphincters are made of skeletal muscles.

Muscle contractions

Before a skeletal muscle can contract, it must be stimulated by an impulse that comes from the brain or spinal cord. The impulse moves away from the brain toward the muscle via a nerve cell called a motor neuron. The point of contact between the nerve ending and the muscle fiber is called a neuromuscular junction (NMJ), a type of synapse. At the synapse, there is a very small gap called a synaptic cleft. Neurotransmitters are released by the motor neuron in response to a nerve impulse.

For example, acetylcholine (ACh), a neurotransmitter, must travel across the synaptic cleft to continue the stimulus and generate a muscle contraction. The released ACh triggers a change in the permeability of the individual muscle fiber. Then, sodium ions flow into the fibers, depolarizing the membrane; this depolarization triggers calcium ions to be released from their storage area in the muscle fiber. When calcium is released, the thin actin fibers slide between the thick myosin fibers, causing the muscle to shorten or contract. Muscles need calcium and energy in the form of adenosine triphosphate (ATP) to contract. The muscle will relax once the ACh is inactivated by acetylcholinesterase, and the calcium ions are sent back to their storage areas of the muscle.

Muscle fatigue

When you start exercising, your muscles feel strong. After repeating the same movements without rest, the muscles start to feel weaker, the strength decreases, and the ability to contract may be lost. This is called muscle fatigue.

For muscles to contract, they need ATP, which is produced through aerobic respiration using oxygen and glucose. If insufficient oxygen is available at the site for use, energy will be created through anaerobic respiration. A by-product of anaerobic respiration is lactic acid. As the lactic acid builds up in the muscle, it changes the pH of the muscle tissue. This causes a burning sensation in the muscle and leads to fatigue. As the person slows the activity, the breathing depth and rate remain at a high level. Extra oxygen is required (by increased respirations) until the lactic acid has been oxidized. The extra amount of oxygen required to rid the muscles of lactic acid is called oxygen debt. This is an example of homeostasis. The extra oxygen helps return the energy and oxygen reserves to the normal resting level.

Watch out: Aerobic respiration (using oxygen) is how muscles normally produce ATP. When oxygen supply can’t keep up with demand, muscles shift to anaerobic respiration, which produces ATP quickly but generates lactic acid as a by-product. It’s the lactic acid buildup - not a lack of oxygen itself - that causes the burning sensation and fatigue.

Muscle tone and posture

Even when we are not actively moving, our muscles are in a state of partial contraction called muscle tone. Nerve impulses help maintain muscle tone, so muscles are ready to act when needed. Muscle tone is an important factor in proper posture. When sitting or standing, the posture (or the position of the body) is maintained by skeletal muscles. Muscle tone works against gravity to keep the body in a stationary position.

Types of muscle contractions

Besides muscle tone and posture, there are four other types of muscle contractions:

• Twitch: A quick, fine movement of a small area of muscles in response to a stimulus.
• Tetanic: Sustained and steady contraction response to a stimulus. Muscles can shorten, lengthen, or remain a constant length during a tetanic contraction. Lifting a heavy object with one hand is an example of a tetanic contraction. Tetanic contractions can occur with isotonic and isometric contractions.
• Isotonic: Muscle contraction that usually produces movement at a joint. The muscle usually shortens and thickens (bulges), and a task is done. Examples include walking, running, lifting weight, and twisting.
• Isometric: Muscle contraction that does not produce movement. There is no change in muscle length, but muscle tension increases. Example: pushing against a wall - there is no movement, but muscle tension increases.

Life span changes

Over the lifespan, changes occur in both the skeletal and muscular systems. The following sections discuss the changes that occur.

Skeletal system changes

When the skeleton is developing prior to birth, cartilage and fibrous structures are present. Over time, these structures are replaced with bone matrix, and the bones change size due to the continual remodeling process carried out by bone cells - osteoblasts build new bone tissue, while osteoclasts break down (resorb) old bone tissue.

During childhood, bones grow rapidly. The prime time to build bone mass or density is from childhood to young adulthood. Bone density can increase through a calcium-rich diet and regular weight-bearing exercise. Poor nutrition, inactivity, smoking, and excessive alcohol intake can reduce bone density.

The following are common skeletal system changes that occur with age:

• Bones lose calcium and other minerals, which reduces bone mass or density. Bone density starts to decrease around age 30 to 40 in both males and females. It accelerates in women after menopause. The loss of bone density makes the bones more brittle, leading to osteoporosis and fractures.
• The disks between the vertebrae wear and tear with age. They can dehydrate, and the cartilage can stiffen, causing the disk to bulge.
• Loss of height from the compression and curving of the spinal column occurs, which can cause a more stooped posture.
• Joints become stiffer and less flexible. Synovial fluid decreases, and cartilage may wear away, causing degenerative changes. This can lead to inflammation, stiffness, pain, and deformities.

Muscular system changes

Men can see muscle changes in their 20s, whereas women are usually in their 40s when changes become obvious. With age, muscle tissue is replaced more slowly and may be replaced with tough, fibrous tissue. This makes the extremities look thin and bony. Muscles have less of an ability to contract. Lean body mass decreases. With the muscle mass changes, older people experience a loss of strength and endurance. They can experience fatigue and reduced activity tolerance. Fasciculations are more common with age. People who are unable to move an extremity may get muscle contractures.