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Human A&P BIO 304 · American River College ← Course home (Week 4)

BIO 304 . WEEK 4 . TUESDAY . LAB WORKBOOK

Sliding Filament and the Cross-Bridge Cycle

How calcium, ATP, actin, and myosin convert chemical energy into mechanical force.

Print this page. You will draw your own diagrams from the directions below, then hand-label the structures listed. Drawing by hand is the integrity mechanism for this course.

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Part 1 of 2

Anatomy Lab

1A. What you will draw

You will draw the cross-bridge cycle as a 4-step loop. Draw a large square. At each corner, draw what myosin and actin look like at that step. Use arrows to show the direction of the cycle (clockwise).

Box A. The 4-step cross-bridge cycle

Directions

  1. Draw a large square that fills the box.
  2. Top-left corner: COCKING. Draw a myosin head in its high-energy, cocked position. Label it. Note that ATP has just been hydrolyzed to ADP plus Pi, both still bound to myosin.
  3. Top-right corner: BINDING. Draw the myosin head attached to actin. Note that calcium has bound troponin and tropomyosin has shifted to expose the actin binding site.
  4. Bottom-right corner: POWER STROKE. Draw the myosin head pivoted, pulling actin toward the M line. Note that ADP and Pi are released.
  5. Bottom-left corner: DETACHMENT. Draw the myosin head with a NEW ATP bound, released from actin. Note that ATP binding is required for detachment.
  6. Connect the corners with clockwise arrows. Mark every step where ATP is consumed or required.

Box B. Calcium release and reuptake

Directions

  1. Draw the sarcolemma at the top of the box, with an action potential arriving (use a small arrow).
  2. Draw a T-tubule diving down from the sarcolemma into the cell.
  3. Draw the sarcoplasmic reticulum wrapping a myofibril below.
  4. Add arrows showing Ca-squared-plus flowing OUT of the SR into the cytoplasm during stimulation.
  5. Draw a second small panel beside this one labeled Relaxation. Show the SR Ca-squared-plus ATPase pumping calcium BACK into the SR.
  6. Label every structure: Sarcolemma, T-tubule, Sarcoplasmic reticulum, Triad, SR calcium ATPase.

1C. Structures to label (15)

After you finish each drawing, label every structure below directly on your sketch.

  1. Myosin head (cocked)
  2. Myosin head (bound)
  3. Myosin head (post power stroke)
  4. Actin binding site
  5. Troponin
  6. Tropomyosin
  7. Calcium (Ca2+)
  8. ATP
  9. ADP + Pi
  10. Power stroke arrow
  11. Sarcolemma
  12. T-tubule
  13. Sarcoplasmic reticulum
  14. Triad
  15. SR Ca2+ ATPase

Part 2 of 2

Physiology Lab

2A. Sequencing puzzle: from nerve to power stroke

Below are 10 events involved in producing a single power stroke. They are listed in SCRAMBLED order. Rewrite them in the correct sequence in the numbered space provided. Start with the motor neuron action potential and end with the power stroke.

Scrambled events:

  1. Voltage-gated calcium channels open at the axon terminal.
  2. Myosin head pivots and pulls actin toward the M line.
  3. Acetylcholine binds nicotinic receptors on the sarcolemma.
  4. Action potential reaches the axon terminal of the motor neuron.
  5. Calcium binds troponin; tropomyosin shifts off the binding site.
  6. Sarcolemma depolarizes; action potential travels along T-tubules.
  7. Sarcoplasmic reticulum releases calcium into the cytoplasm.
  8. Acetylcholine is released into the synaptic cleft.
  9. Myosin head binds the exposed site on actin (cross-bridge forms).
  10. ATP is hydrolyzed; myosin head cocks into its high-energy position.

Your sequence (write the events in correct order):

2B. Synthesis questions

Answer each in 2 to 4 sentences. Use the language from this week's lecture and your drawings as evidence.

1. Rigor mortis sets in hours after death. Explain the molecular mechanism using your cycle drawing. Which step cannot proceed, and why?
2. Curare blocks the nicotinic acetylcholine receptor at the neuromuscular junction. At which step does the entire chain stall, and what is the patient's clinical picture?
3. Malignant hyperthermia is caused by a mutation that makes the SR calcium release channel hyperactive in response to certain anesthetics. Walk through the cycle and explain why body temperature climbs so rapidly.

3. What to submit

Complete both the Anatomy Lab (your own drawings, hand-labeled, plus the structures list) and the Physiology Lab (activity and synthesis questions). Photograph or scan every page and upload to Canvas before the deadline listed on the schedule. Hand-drawn, hand-labeled work is the integrity mechanism for this course. Typed or AI-generated diagrams are not accepted.