Locomotion

1
Locomotion

• Lecture 3 (cont.)

2
Contractile proteins

• Thin filaments actin
• Thick filaments -- myosin

3
Actin
4
Myosin

• Myosin head has enzymatic activity
• ATP ADP Pi

5
Structure of Skeletal Muscle
6
So

• Organization of actin and myosin

7
(No Transcript)
8
Sliding Filament Theory

1. Actin and myosin highly organized
2. Actin anchored to Z-line or Z-disk
3. Actin and myosin interdigitated
4. Contraction occurs as actin and myosin filaments
   slide past each other.
5. Result sarcomere length shortens

9
(No Transcript)
10
Mechanism of Contraction

1. Attachment at start of cycle, head of myosin is
   tightly attached onto action filament
   (cross-bridging). This attachemnt is calcium
   dependent.
2. Release ATP binds to head of myosin. This
   reduces affinity of myosin for actin.
3. Cocked myosin head moves a distance of 5nm and
   cleaves ATP into ADP P
4. Force generation release of P from myosin head
   allows head to attach to next actin. Release of
   ADP from head allows tight binding to actin and
   change in angle of myosin head.

11
So

• What events initiate contraction?
• A rapid release of Ca into muscle fiber
  cytoplasm causes contraction.but how?

12
New Terminology

• Sarcolemma plasma membrane of a muscle fiber
• Sarcoplasmic reticulum(SR) endoplasmic
  reticulum of muscle fiber
• Transverse tubules (T-tubules) invaginations of
  sarcolemma into cytoplasm

13
Muscle Fiber
14
OK.What causes the muscle to contract?

1. A nerve cell (neuron) produces an electrical
   impulse from brain or spinal cord.
2. Neuron releases chemical at end of axon called a
   neurotransmitter acetylcholine (ACh).

15
A Neuron
16
(No Transcript)
17
Synapse

• A synapse is a junction between a neuron and
  another cell (target).
• Neuron (axon) does not make contact with target
  cell.
• Axon releases ACh into synaptic cleft.

18
Synapse
19

• 3. ACh binds to receptor within sarcolemma of
  muscle fiber.

20
(No Transcript)
21
4. Opens ion channels
22
Muscle Fiber
23

1. Causes electrical impulse along sarcolemma
2. Into t-tubules
3. Impulse across to SR

24
Sarcoplasmic Reticulum (SR)

• ER
• Sequesters Ca
• Active transport
• SR releases Ca into cytoplasm around
  myofibrils.
• Ca allows actin to bind to myosin
• But it is not that simple!

25
Two Regulatory Proteins

• A. Tropomyosin lies along the thin filament of
  actinblocks myosin binding

26

• B. Troponin
• Ca binding protein
• Attached to tropomyosin

27
(No Transcript)
28
Release of Ca from SR

1. Ca binds to troponin
2. This causes tropomyosin to move away from myosin
   binding site on actin.
3. This allows head of myosin to bind with actin.
4. This starts the sliding filament cycle until
   Ca is sequestered into SR.
5. Ca was released to cause contraction.why would
   Ca be re-sequestered?
6. The role of ATP and rigor mortis