Lesson 1, Topic 1
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Physiology of the Muscular System

April 11, 2024

Physiology of the Muscular System

Learning Objective: Examine the anatomy 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

Learning Objective: Describe the four primary functions of the muscular system.

The muscular system has four primary functions:

1. Muscle contractions provide muscle tone and posture. This function will be discussed in the following section.
2. Maintains the body temperature: A by-product of the energy (acetylcholinesterase, 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.

FIGURE 20.7  Muscles of the body. (A) Anterior view. (B) Posterior view.

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, causing 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.

• Attached to the bones of the skeleton by tendons. This allows for movement of the bones at the joints and enables the body to maintain posture.
• In the upper section of the esophagus to help with swallowing.
• In the eye to allow for eye movements.
• In the urinary and digestive systems. Sphincter muscles allow for the voluntary passage of urine and stool.

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 anaerobic respiration with 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.

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 (FIGURE 20.8).
• Isometric: Muscle contraction usually does not produce movement. There is no change in muscle length, but there is an increase in muscle tension. Examples include pushing against an immovable object, pushing against a wall, and pushing the palms together—there is no movement, but there is an increase in muscle tension.


Critical Thinking Application

Walter tells Dr. Kahn about his love of gardening. He loves being outdoors and feeling the dirt on his hands. Dr. Kahn palpates the sore muscle on Walter’s forearm and asks about the gardening activities that make the pain worse. Walter replies, “The activities that hurt the most are digging, pulling weeds, and pushing dirt in around the small plants.” Which of these three actions are an isotonic contraction of the muscle, and which are isometric?