Muscle Tissue and Motor Units

Major Muscle Questions

• Several questions are important in understanding muscle function.
• What types of muscle tissue exists.
• How are they the same and how do they differ.
• The cellular structure of a voluntary muscle.
• How does this structure facilitate function.
• How an action potential of a motor neuron results in a muscle contraction.
• How do we achieve variable control over all muscles (i.e., large contraction vs. small contraction).
• What is the source of energy for muscle contraction and how do we replenish this energy source.

Muscle Cell Excitation

• Each muscle cell must be excited by a branch of a motor neuron.
• Muscle cells do not excite each other.
• A motor unit is defined as about 200 muscle cells that are excited together.
• A neuron branches about 200 times.
• Branching can vary and be as small as three muscle cells.

Muscle Motor Units

• Muscles have many motor units.
• The first dorsal interosseous muscle predicted to have about 40,000 muscle cells arranged into 120 motor units

Motor Unit Size

• Each motor unit has about 333 cells (40,000 cells divided by 120 motor neurons = 333 cells).
• Muscle cells of one motor unit spread throughout the entire muscle
• Contraction of one motor unit leads to a weak contraction of the entire muscle.
• The contraction of subsequent motor units will increase the strength of muscle contraction, also known as recruitment.
• Not all motor units are activated at the same time with progressive activation allowing varying strengths of contraction.

Motor Unit Size and Function

• The size of motor units depends on muscle function.
• Muscles that move the eye have many small motor units, allowing for fine control.
• The deltoid muscle has fewer larger motor units for powerful but less precise movements.

Generating Appropriate Muscle Contractions

• Somatic sensory neurons called proprioceptors and interneurons inform the cerebellum through action potentials about contraction strength
• If the contraction is not strong enough, more muscle cells are recruited.
• Interneurons from the brain initiate action potentials, involving planning in the forebrain, also known as the motor cortex.
• This also includes special sense input, like sight and balance.
• Sensory neurons send action potentials to the central nervous system (CNS).
• Sensory neurons tell the brain if the "load" has been moved and is integrated with information from the forebrain in the hindbrain, also known as the cerebellum.
• Motor neurons are excited by interneurons and communicate with muscle cells at the NMJ, or neuromuscular junction.

Sarcomere Length and Tension

• The starting length of a sarcomere predicts the amount of tension that is generated
• Myosin and actin overlap are important to create tension.
• An optimum sarcomere length exists to maximize contraction.

Muscle Energetics and ATP

• Skeletal muscle contraction, especially during exercise, requires ATP to function.
• ATP hydrolysis is required to prepare myosin in an extended conformation to interact with actin.
• The body’s supply of ATP is limited and it is important to regenerate ATP from ADP to sustain contractions.

Mechanisms to Sustain Contractions

• Three mechanisms exist to replenish ATP from ADP to sustain contractions:
• Aerobic respiration
• Anaerobic respiration
• Phosphorylation using creatine phosphate (CP)
• The best mechanism depends on the type of exercise.

Mechanisms, Duration, and Intensity

• The mechanism a cell uses depends on the duration and intensity of the exercise.
• Intensity decreases over time, meaning that endurance exercise occurs with lower intensity.
• All events occur in the muscle cell and occur at overlapping times.
• Specific mechanisms are favored based on the energy demand of the cell.

Aerobic Respiration

• Aerobic respiration converts ADP to ATP by metabolizing glucose, fatty acids, and proteins in the mitochondria.
• Produces a lot of ATP and is the most efficient ATP production method.
• Aerobic respiration can use many food sources, like glucose, fatty acids, and proteins.
• Oxygen is required, and is ideal for sustained contraction if sufficient oxygen is present

Anaerobic Respiration

• Glucose is metabolized to produce lactic acid and ATP in anaerobic respiration.
• Anaerobic respiration consists of fewer steps, meaning is faster
• Anaerobic respiration produces less ATP than aerobic respiration and can only use glucose.
• Lactic acid is produced and oxygen is not required.
• The time sustaining contraction is 30-90 seconds (high intensity, low duration).
• Lactic acid production causes a “burning” sensation associated with intense exercise, also known as "feel the burn!".

Overworking and DOMS

• Overworking muscles and ignoring the burn can cause delayed onset muscle soreness, or DOMS.
• The mechanism of DOMS is not fully understood, though it is thought to be a combination of damage to muscle cells and a buildup of molecules related to damage.
• DOMS is defined by increased blood flow and inflammation of the tissue, resulting in tenderness and loss of strength.

Creatine Phosphate

• ADP converted to ATP using creatine phosphate (CP).
• At rest, the muscle stores energy from ATP in the form of creatine phosphate.
• The formula is: ATP + Creatine = ADP + Creatine Phosphate
• During exercise, creatine phosphate is used to convert ADP back to ATP as: ADP + creatine phosphate = creatine + ATP

Creatine Phosphate Details

• Creatine phosphate is only one step, and produces only 1 ATP,
• Creatine phosphate is very fast, but with low ATP yield, it requires no oxygen.
• Time sustaining contraction lasts around 15 seconds (high intensity, short duration – very start of exercise).

Endurance and Resistance Exercise

• How muscles respond to exercise depends on the type of exercise.
• Endurance exercise increases blood flow to muscles, number of mitochondria, and myoglobin in muscle.
• Resistance exercise increases the size of muscle fibers, number of mitochondria, and glycogen stores in muscle.