Unit Two: Lecture Content

Explain the anatomical structures involved in delivering the signal to contract from the nervous system to the functional unit of contraction within a skeletal muscle fiber. 

-Sarcolemma: the plasma membrane of a skeletal muscle fiber (cell of skeletal muscle)

-Motor end plate: More zoomed out view of a motor neuron axon and its interface with a skeletal muscle fiber via multiple branches of axon terminals. It includes several neuromuscular junctions.

-Neuromuscular Junction: Zooming in on one specific axon terminal and its interface with the plasma membrane of the skeletal muscle fiber.

-NMJ is a specific type of synapse (synapses connect a neuron and the next cell in line).

-The synaptic knob: is the end of an axon terminal that is expanded -synaptic vesicles: Spheres of membranes inside the synaptic knob. Membranes are filled with proteins (neurotransmitters) which are chemical messengers that leave a neuron to send a message over a short distance.

-The specific chemical messenger (neurotransmitter) used is Acetylcholine (ACh).

• The Synaptic Cleft: the space between cells at the neuromuscular junction which allows for an on/off switch at the neuromuscular junction

-In the plasma membrane of SMF there are acetylcholine receptors:

Integral proteins;bind acetylcholine and receive the message it is time to contract. 

1.)  ACh gets the message it is time to contract via NMJ

2.) The stimulus generates a wave of electricity-the wave of electricity travels along the length of the plasma membrane of the skeletal muscle fibers

3.) Transverse tubules: tunnels through and across(invaginations with the PM) the SMF: conduct wave of electricity through the SMF (signals it is time to contract).

4.) Sarcoplasmic Reticulum: which is the smooth ER of the SMF

-CH:arranged in a mesh like pattern around each myofibril and up against every T-tubule

-FX: secrete calcium when wave of electricity reaches it via T-tubule 

5.) Mitochondria: is fit in everywhere it can be.It produces ATP whenever oxygen is present

6.)Nuclei: SMF are multinucleate and peripherally positioned.

7.) Myofibrils: Contractile organelles

• Compare and contrast at least three (3) characteristics among the three types of skeletal muscle fibers (SMF). Explain the composition of skeletal muscle fibers in the gastrocnemius muscles of an Olympic sprinter. 

• Slow oxidative SMF: smaller, fewer myofibrils, less strength

• -Darker, (more myoglobin which is the muscle equivalent of hemoglobin fx: to carry oxygen)

• -aerobic (uses oxygen to make ATP)

• -fatigue resistance high endurance

• Fast Oxidative SMF: Intermediate in size and endurance

• -Still uses oxygen to create ATP

• Fast Glycolytic SMF: larger, more myofibrils, more strength

• Less endurance, fatigue prone

• Uses anaerobic respiration to make ATP, when little oxygen can get to muscle

• What do they all have in common?

• All three are found in every muscle, but in differing proportions.

• So, an olympic sprinter would have more fast oxidative SMF in their gastrocnemius than an olympic jogger

• An olympic marathoner would have more slow oxidative SMF in their thighs/legs

• In the abdominal muscles there is more slow oxidative for endurance as well