Lec 17+ 18 Musculoskeletal System

Musculoskeletal system 

1. Muscle types

   1. Skeletal muscle

      1. Responsible for locomotion, facial expressions, shivering(helps us thermoregulate), breathing

      2. Striated

   2. Cardiac muscle

      1. Makes up heart. Responsible for pumping blood

      2. Striated

   3. Smooth muscle

      1. Involuntary generation of forces in hollow organs (gut, bladder, blood vessels)

      2. Not striated

Bundle fibers-connective muscle fibers-connectibe tissue-Single muscle fiber-nucleus-cell membrane-myofibrils-single myofibril-actin filament- myosin filament

Each muscle fiber is a multinucleated cell containing numerous myofibrils, which are highly ordered assemblages of thick myosin and thin actin filaments 

Sacromeres- unit of contraction 

Molecular mechanisms of muscle contraction

1. Neurotransmitters released from nerve to start process of contraction

1. Neuron- nerve cell

   1. Dendrites 

      1. Collect electrical signals

   2. Cell body

      1. Integrates incoming signals and generates outgoing signal to axon

   3. Axon

      1. Passes electrical signals to dendrites of another cell or to an effector cell

   4. Axon terminals

Whole process is an action potential -which begins at dendrites

Myelin- lipid based structures 

signal jumps from one spot to the next, increases speed because this is an insulated electric wire

Motor unit- single motor nerve and the associated muscle fibers that are innervated upon stimulation from the nerve

As action potential from axon terminal hits cell it goes down the t tubule

Globular head of myosin interacts with actin

Actin made up of 3 parts

1. Actin monomer- individual protein segments stitched together

2. Tropomyosin

3. Troponin

green calcium bind to troponin like little hands on cables to pull cable off, making active sites visible

1. Walk along or Ratchet Theory of Muscle Contraction

   1. Prior to contraction, myosin heads bind ATP and cleave it (1 phosphate comes off) but leave the products attached to the head (ADP+Pi) 

      1. This liberates but stores energy like a cocked spring

   2. Flooding myofibrils with Ca2+ that was stored in the sarcoplasmic reticulum . Ca2+ binds to troponin and exposes myosin binding sites

   3. Exposure of active sites on actin monomer(1st image blue circles) results in immediate binding with myosin head

   4. Binding of myosin head causes it to tilt forward the M band = power stroke

   5. Release of ADP +Pi. New ATP binds

   6. Head re;eases and ATP is cleaved, cocking the head back

SR dumps calcium

Calcium binds to troponin

Troponin exposes myosin binding site leading to muscle contraction

How do we stop the ratcheting?

1. Continued presence of Ca2+ will enable another binding of myosin head with actin active sites

2. How do we get rid of Ca2+?

   1. Calcium pumps on the surface of the SR are constantly pumping Ca2+ out of the cytoplasm and back into the SR