chapter 9a

BIOL 2301 ANATOMY AND PHYSIOLOGY I

CHAPTER 9A – MUSCLES AND MUSCLE TISSUE

MEMBRANE POTENTIALS & ACTION POTENTIALS

#### Joel Quiros, Ph.D.

9.1 – INTRO TO MUSCLE

• Muscle provides support and structure to the body.
• Muscles exhibit varying degrees of flexibility.
• Muscles account for nearly half of the body’s mass, playing a critical role in movement and posture.

MUSCULAR SYSTEM INTRODUCTION

Key Characteristics

• Muscle transforms chemical energy into mechanical energy, allowing the body to exert force.
• Terms related to muscle:
    - Myo-
    - Mys-
    - Sarco- (e.g., sarcoplasm: cytoplasm of muscle cells)
• There are three types of muscle tissue:
    1. Skeletal Muscle
    2. Cardiac Muscle
    3. Smooth Muscle
• Only skeletal and smooth muscle fibers are elongated and are referred to as muscle fibers.

COMPARISON OF MUSCLE TYPES

Table 9.3: Comparison of Skeletal, Cardiac, and Smooth Muscle

MUSCLE FUNCTIONS

1. Produce movement
      - Skeletal muscle is responsible for locomotion and manual manipulation.
2. Maintain posture and body position
      - Skeletal muscles continually adjust to counteract the pull of gravity.
3. Stabilize joints
      - Muscles contribute to joint stability.
4. Generate heat
      - Muscle contractions play a significant role in maintaining normal body temperature.

• Additional muscle functions include protecting organs, forming valves, controlling pupil size, and causing "goosebumps."

9.4 – SKELETAL MUSCLE FIBER PHYSIOLOGY

• Muscle contraction results from action potentials (signals) initiated by motor neurons originating in the spinal cord or brain.

HOW DO MUSCLES MOVE?

• The process of movement begins in thought (planning) and is executed as action.

PHYSIOLOGY OF SKELETAL MUSCLE FIBERS

• Membrane Potentials
• Ion Channels
• Action Potentials
• Neuromuscular Junction
• Excitation-Contraction Coupling
• Cross-Bridge Movement
• Muscle Relaxation
• General Concepts pertaining to Anatomy & Physiology.

MEMBRANE POTENTIALS

• Cells exhibit polarization, indicating a separation of charged particles (voltage) across the cell membrane.
• Voltage is present solely at the membrane.
• Resting Membrane Potential (RMP):
    - Voltage difference between the inside and outside of a cell, typically ranging from -50 to -100 mV depending on the cell type.
    - The negative sign indicates a negatively charged inner membrane compared to the outside.

ESTABLISHING MEMBRANE POTENTIAL

Two Key Factors

1. Concentration difference of ions across the plasma membrane (PM)
      - Dominated by the Na+ - K+ pump:
        - Inside of the cell has a lower concentration of Na+ and a higher concentration of K+.
        - Outside has higher Na+ and lower K+.
2. Membrane permeability characteristics:
      - The cell membrane is relatively permeable to K+ but not to Na+ or negatively charged particles.
      - K+ can diffuse out freely, causing a loss of positive charge and a negative charge inside.
      - Movement of K+ is influenced by the principle of opposite charges attracting.
      - Other ions like Ca²⁺ and Cl⁻ as well as proteins also contribute to RMP.
      - Even with these differences, intracellular and extracellular solutions are electrically neutral and isotonic overall.

ION CONCENTRATION DIFFERENCES

Extracellular Fluid

• Cations:
    - K⁺ = 5 mEq/L
    - Na⁺ = 142 mEq/L
    - Ca²⁺ = 5 mEq/L
    - Others = 5 mEq/L
    - Total = 155 mEq/L
• Anions:
    - Proteins = 16 mEq/L
    - Cl⁻ = 103 mEq/L
    - Others = 36 mEq/L
    - Total = 155 mEq/L

Intracellular Fluid

• Cations:
    - K⁺ = 148 mEq/L
    - Na⁺ = 10 mEq/L
    - Ca²⁺ = <1 mEq/L
    - Others = 41 mEq/L
    - Total = 200 mEq/L
• Anions:
    - Proteins = 56 mEq/L
    - Cl⁻ = 4 mEq/L
    - Others = 140 mEq/L
    - Total = 200 mEq/L

Key Takeaway

• The differences in intracellular and extracellular ion concentrations arise primarily from the Na⁺ - K⁺ pump and the permeability of the plasma membrane.

MEMBRANE POTENTIAL MECHANISM

1. K⁺ diffuses down its steep concentration gradient out of the cell via leakage channels.
2. Loss of K⁺ results in a negative charge on the inner face of the plasma membrane.
3. K⁺ also moves inward due to attraction from the negative charge created.
4. A negative membrane potential (approximately -90 mV) is established when K⁺ movement in equals K⁺ movement out.

ALTERING THE RESTING MEMBRANE POTENTIAL

• To transmit electrical signals from the brain to muscles, it is necessary to alter the resting membrane potential.
• This is achieved by changing ion permeability, which occurs at chemical synapses.

REFRESHER ON ION CHANNELS

• Leak Channels: Always open, allowing free movement of ions in/out based on diffusion.
• Ligand-Gated Channels: Open in response to ligand binding, permitting ion movement.
• Voltage-Gated Channels: Open in response to changes in membrane voltage.

ACTION POTENTIALS (AP)

• Defining Characteristics:
    - Reversal of the charge difference across the plasma membrane, resulting in a positively charged interior.
    - This change can last a few milliseconds and can propagate down neurons to muscle cells.
• Phases of AP:
    1. Depolarization:
       - Reduction in negative charge—inside cell becomes more positive.
    2. Repolarization:
       - Membrane potential returns to the original resting value.

STIMULUS AND THRESHOLD FOR AP

• Cell stimulation causes voltage-gated Na⁺ channels to open, leading to Na⁺ influx, making the interior more positive.
• If the threshold (all-or-none point) is reached, an action potential occurs, resulting in further Na⁺ channel openings.
• If the threshold is not reached, no action potential occurs, and the cell returns to resting membrane potential.

RESTORING THE RESTING MEMBRANE POTENTIAL

• Once inside the cell becomes sufficiently positive, voltage-gated Na⁺ channels close, and voltage-gated K⁺ channels open to restore the resting membrane potential.
• K⁺ exits the cell, causing the charge within to become negative again.

ACTION POTENTIAL SUMMARY

• The process involves:
    - Na⁺ channels opening during depolarization,
    - closing down and K⁺ channels opening during repolarization,
    - returning to resting potential afterwards.

NEUROMUSCULAR JUNCTION / SYNAPSE

Intersection of Nervous and Muscular Systems

• The neuromuscular junction involves the axon terminals and the innervated area of the sarcolemma (muscle cell membrane).
• Key Terms:
    - Presynaptic: before the synapse.
    - Postsynaptic: after the synapse.
    - Presynaptic Terminal: axon terminal.
    - Synaptic Cleft: space between the two membranes.
    - Postsynaptic Membrane: muscle plasma membrane in the junction.
    - Synaptic Vesicles: contain acetylcholine (ACh), a neurotransmitter responsible for signal transmission.
    - Sarcolemma: plasma membrane of muscle fiber.
    - Neurotransmitter (NT): substance released from a presynaptic membrane.

REFRACTORY PERIOD (RP)

• The refractory period refers to a time frame where the sensitivity of a recently stimulated area decreases, and it consists of two parts:
    - Absolute Refractory Period: The initial phase where the neuron is completely insensitive to stimuli, occurring during depolarization. This is when Na⁺ channels are open and prevents new action potentials from being generated, ensuring one-way impulse transmission.
    - Relative Refractory Period: The subsequent phase following the absolute RP where a stronger-than-normal stimulus can initiate another action potential. This occurs during repolarization when Na⁺ channels are closed and K⁺ channels are open.