Bio 231 @ Oit - Lecture 5

Front 1

What is a sarcomere?

Back 1

- Each myofibril appears as a series of repeating dark and light bands formed by the contractile proteins actin and myosin
- The functional unit of the muscle fiber of muscle cell is the sarcomere
- the sarcomere may be defined as the region of a myofibril between two successive z-lines (discs)

Front 2

Describe the organization of a sarcomere

Back 2

- Under the light microscope, the wide A-Band represents the regino of overlap between myosin and actin
- The narrow I-band contains only actin, there is no overlap
- The H-zone is the central region for each sarcomere, contains only myosin
- At rest the sarcomere is between 2-3 microns in length, can be stretched to 4 microns, upon contractions is only 1 micron

Front 3

How do the contractile proteins in muscle work?

Back 3

- a single myosin moleculeis two interwoven polepeptides possessing a tail and a globular head
- The thick myosin filament is composed of 300 myosin molecules
- Actin is composed of three smaller proteins;
-- g-actin
-- tropomyosin
-- troponin
- The g-actin contains the myosin binding sites, which are covered by the tropomyosin molecule
- Tropomyosin and troponin fuctino as regulatory proteins

Front 4

Describe the interaction of myosin and actin

Back 4

- G-actin contains the binding sites for the myosin heads
- Binding sites covered with the tropomyosin protein
- Associated with tropomyosin is the protein troponin
- The release of intracellular calcium ions bind to troponin
- Calcium binding to troponin removes the blocking function of tropomyosin

Front 5

How is cross bridge formation accomplished?

Back 5

- Energized myosin head attaches to actin, forming a cross-bridge
- the power stroke occurs when the myosin head pivots, pulls the actin myofilament inward
- Cross-bridge detachment occurs when new ATP attaches to myosin head
- As ATP is hydrolyzed to ADP and P, the myosin head returns to it's high energy "cocked" position

Front 6

Describe the internal tubules of muscle

Back 6

- Sarcoplasmic reticulum (SR) is a modified ER, contains intracellular calcium ions, essential for muscle contraction
- SR surrounds the myofibril similar to a sleeve around the arm
- The transverse tubules (t-tubles) are inward extensions of the sarcolemma; will carry the action potential deep into the cell and the SR
- the depolarization of the muscle cell initiates the releaseo f stored calcium ions from the SR (opens voltage gated ion channels)

Front 7

Give an overview of the sliding filament theory

Back 7

- Interaction between myosin and actin
- Calcium ions released for the SR will expose the myosin head binding site on the g-actin subunits
- ATP is necessary for the swiveling of each myosin head (power stroke)
- Actin thin filament pulled inward, thus shortening the sarcomere and each individual muscle fiber (muscle contraction)

Front 8

Describe the composition of a neuromuscular junction

Back 8

- Action potential propogates down the axon of motor neuron
- The AP eventually opens voltage-gated calcium ion channels; the influx of calcium ions will bind with the synaptic vesicles
- Synaptic vesicles contain the neurotransmitter acetylcholine (ACh)
- Release of the ACh through exocytosis
- ACh crosses the synaptic cleft and binds to the ACh receptor on the muscle cell, resulting in the depolarization of the muscle cell

Front 9

Describe the function of membrane potential

Back 9

- An unequal distribution of ions across the sarcolemma
- The intracellular fluid (sarcoplasm) has a higher concentration of potassium ions and a low concentration of sodium ions
- The extracellular fluid has a low concentratino of potassium ions and a high concentration of sodium ions (concentration gradient exists for these two ions)
- The unequal distribution of ions creates a membrane potential

Front 10

Describe the ion flow sequence in muscle

Back 10

- At rest the inside of the cell is slightly negative (-) (-70mv), the outside is slightly positive (+)
- The opening of sodium ion channels allows sodium ions to enter the cell, as more positive ions enter the cell the polarity of the membrane changes (depolarization)
- One the inside of the cell becomes positive (+20 mv) sodium channels close and potassium ion channels open
- The exit of intracellular positive potassiuminos to re-establishes polarity (repolarization)

Front 11

Describe the operation of sodium/potassium pumps

Back 11

- Repolarization restores the resting membrane potential (RMP) of the cell.
- Ion distribution has not been restored; too many sodium ions and too few potassium ions remain inside the cell.
- Sodium/potassium pumps are required to transport ions across the cell membrane and against a concentration gradient (active transport)
- A muscle cell has an abundance of mitochondria; generates the ATP to operate the pumps.

Front 12

What is meant by "action potential", with respect to neurons?

Back 12

- At rest the axon is polarized across the cell membrane (+/-)
- Stimulating an axon reverses this condition, causing depolarization (-/+)
- If the stiumulus (graded potential) is strong enough this triggers the opening of additional voltage gated ion channels (sodium) similar to the falling of dominos.
- The events of depolarization and repolarization defines an action potential
- The Na+/K+ pumps will return these ions to their proper location across the membrane

Front 13

How is an action potential propogated?

Back 13

- The opening of the first ion channel (triggered by the graded potential) will depolarize the adjacent region of the membrane, causing a second voltage-gated sodium ion channel to open.
- The depolarization wave continues down the axon opening sodium ion channels in succession (self-propagation)
- Depolarization continues at a constant velocity. As sodium ion channels are closing, potassium ino channels are opening, initiating the events of repolarization.
- The action potential propagates down the axon either by continuous conduction or saltatory conduction

Front 14

What are the three basic functions of the nervous system?

Back 14

- Sensory Input - various sensory recepts monitor changes in the internal and external environments
- Integration - involved with the processing and interpretation of incoming sensory information
- Motor output - directs an appropriate response to effectors; either muscle tissue or glands

Front 15

Describe the anatomical representation of the nervous system

Back 15

- Central Nervous System (CNS)
-- Brain
-- Spinal cord
- Peripheral Nervous System (PNS)
-- Cranial nerves (12 pair)
-- Spinal nerves (31 pair)

Front 16

Describe the functional representation of the nervous system

Back 16

- Central Nervous system (CNS)
-- Integration (brain and spinal cord)
- Peripheral Nervous System (PNS)
-- Afferent Division (all sensory)
-- Efferent Division (all motor)
--- Somatic Nervous System (skeletal muscle)
--- Autonomic Nervous System (cardiac and smooth muscle, and glands)
---- Sympathetic Division - excites or inhibits metabolic activity
---- Parasympathetic Division - restores normal function