NERVE AND MUSCLE

• Neurons are the basic building blocks of the nervous system
• There are 100 billion neurons in the human nervous system
• The specialized function of neurons is the transmission of nerve impulses
• Parts of a neuron and their functions:

  1. Cell body:

  • Maintains the functional and anatomic integrity of the neuron

  2. Dendrite:

  • Receives messages in the form of impulses for the neuron

  3. Axon:

  • Transmits impulses to other neurons or to end organs

  4. Node of Ranvier:

  • Increases the speed of impulse conduction

  5. Terminal buttons:

  • Store synaptic granules secreted by the nerve

  6. Excitation:

  • Electrical phenomenon in nerve cells
• Types of stimuli that may excite a nerve:

  1. Mechanical

  2. Chemical

  3. Electrical

  4. Thermal
• After a nerve is stimulated, two types of physio-chemical disturbances are produced:

  1. Local, nonpropagated potentials called synaptic, generator, or electrotonic potentials

  2. Propagated disturbances called nerve impulses, which are the universal language of the nervous system
• Conduction in neurons:

  • An active, self-propagating process that requires energy expenditure by the nerve
  • The impulse moves along the nerve at a constant amplitude and velocity
  • Electrical potential changes occur in a nerve when it conducts impulses
• Resting membrane potential in neurons is -70 mV
• Definition of terms:

  1. Threshold intensity: Minimum amount of stimulus intensity that will produce an impulse and an action potential

  2. Subthreshold intensity: Intensity of stimulus that will not produce an action potential

  3. All or None Law: A stimulus at threshold intensity will produce an impulse; increasing it will not affect the produced impulse, and decreasing it will produce no impulse at all

  4. Stimulus of extremely short duration: Will not excite the nerve, regardless of intensity

  5. Weak stimulus: No response occurs, regardless of how long the stimulus is applied

  6. Accommodation: Process where the nerve adapts to the applied stimulus, resulting in no impulse or action potential being produced
• Ionic movement during the action potential:

  • When the cell membrane is depolarized, there is an increase in membrane permeability to Na+, leading to Na+ influx
  • Na+ influx further lowers the membrane potential and increases Na+ permeability
  • Na+ influx overcomes the repolarizing process, producing the spike potential
  • Repolarization occurs through increased K+ permeability, leading to diffusion of K+ out of the cell
  • Net transfer of positive charge out of the cell completes repolarization
• Saltatory conduction:

  • Jumping of depolarization from one node of Ranvier to the next, increasing the speed of impulse conduction
• Muscle cells can be excited to produce an action potential transmitted along their cell membrane
• Types of muscles:

  1. Skeletal muscle:

  • Comprises the great mass of the somatic musculature
  • Has well-developed cross striations
  • Generally under voluntary control

  2. Cardiac muscle:

  • Has cross-striations but functionally syncytial in character
  • Contracts rhythmically in the absence of external innervation

  3. Smooth muscle:

  • Lacks cross-striations
  • Functionally syncytial in character
• Morphology of skeletal muscle:

  • Made up of individual muscle fibers
  • Begins and ends in tendons
  • Muscle fibers are multinucleated, long, and cylindric in shape
  • Muscle fibers are made up of fibrils divided into individual filaments composed of contractile proteins
• Transverse tubules are continuous with the membrane of the muscle fiber, facilitating rapid transmission of the action potential from the cell membrane to all the fibrils in the muscle
• Sarcoplasmic reticulum forms an irregular curtain around each fibril between its contact with the T system and is concerned with Ca++ movement and cell metabolism
• There are two kinds of muscle response: electrical and mechanical
• A single action potential causes a brief contraction followed by relaxation, known as muscle twitch
• The duration of the muscle twitch varies with the type of muscle being tested: "Fast" muscle fibers are concerned with fine, rapid, precise movement, while "Slow" muscle fibers are concerned with strong, gross, sustained movements
• During muscle contraction, there is shortening of the contractile elements in the muscle brought about by the sliding of the thin filaments over the thick filaments
• The initial stage of contraction involves the action potential being transmitted to all the fibrils in the fiber via the T system, triggering the release of Ca++ from the sarcoplasmic reticulum
• The release of Ca++ initiates contraction by binding to troponin C, weakening the binding of troponin I to actin and permitting the tropomyosin to move, uncovering the binding sites for myosin so that ATP is released, leading to contraction
• Shortly after releasing Ca++, the sarcoplasmic reticulum begins to reaccumulate Ca++ and store it until the Ca++ concentration outside of the sarcoplasmic reticulum has been lowered sufficiently, ceasing the chemical interaction between myosin and actin and causing the muscle to relax
• If the active transport of Ca++ is inhibited, relaxation does not occur, resulting in a sustained contraction
• ATP is required for all muscle contraction events
• Steps in skeletal muscle contraction:

  1. Discharge of motor neuron

  2. Release of transmitter substance (acetylcholine) at the motor end plate

  3. Generation of end-plate potential

  4. Generation of action potential in muscle fibers

  5. Inward spread of depolarization along T tubules

  6. Release of Ca++ from lateral sacs of sarcoplasmic reticulum and diffusion to thick and thin filaments

  7. Binding of Ca++ to troponin C, uncovering myosin binding sites on actin

  8. Formation of cross-linkages between actin and myosin and sliding of thin on thick filaments producing shortening
• Steps in relaxation:

  1. Ca++ is pumped back into the sarcoplasmic reticulum

  2. Release of Ca++ from troponin

  3. Cessation of interaction between actin and myosin
• Types of skeletal muscle contraction:

  1. Isometric contraction: no appreciable decrease in the length of the whole muscle, no joint movement

  2. Isotonic contraction: contraction against a constant load with approximation of the ends of the muscle, joint movement
• Tetanus is a response to rapidly repeated stimulation occurring before any relaxation has occurred, where responses fuse into one continuous contraction
• Treppe (Staircase Phenomenon) occurs when a series of maximal stimuli is delivered to skeletal muscle just below the tetanizing frequency, resulting in an increase in tension developed during each twitch until uniform tension per contraction is reached
• Summation of contractions occurs when repeated stimulation before relaxation produces additional activation of the contractile elements, adding to the contraction already present
• Energy sources of metabolism in muscle include ATP, phosphocreatine, glucose, and glycogen
• Fatigue in muscles can result from the accumulation of lactic acid
• Rigor is a state of extreme rigidity in muscle fibers when they are completely depleted of ATP and phosphocreatine
• Denervation of healthy skeletal muscle causes muscle atrophy and abnormal excitability, leading to fibrillations
• Smooth muscle types:

  1. Visceral smooth muscle: occurs in large sheets, has bridges between individual muscle cells, functions in a syncytial fashion, found in the walls of hollow viscera, shows continuous, irregular contractions independent of its nerve supply

  2. Multi-unit smooth muscle: made up of individual units without interconnecting bridges, found in the iris of the eyes, involuntary, non-syncytial, sensitive to circulating chemical substances
• Cardiac muscle has a resting membrane potential of -80mV
• Impulses are transmitted from one nerve cell to another at synapses, which are junctions where the axon or some other portion of one nerve cell terminates on another nerve cell
• Types of junctional transmission:

  1. Chemical (most common)

  2. Electrical

  3. Both chemical and electrical (conjoint)
• Chemical mediators bind to receptors on the surface of the post-synaptic cell, triggering intracellular events that alter the membrane permeability of the post-synaptic neuron