1. Excitation-Contraction Coupling & Reflexes, Proprioception and Movement
PSK 4U Unit 4, Day 4

2. Excitation-Contraction Coupling
Muscles work by converting electrical and chemical energy into mechanical energy!
The process of muscle contraction as a whole is often referred to as “excitation-contraction coupling”
The “excitation” part describes the steps from when the electrical signal comes from the neuron and Ach is released, which in turn causes the release of calcium from sarcoplasmic reticulum which in turn exposes myosin binding sites on actin.
The “contraction” part describes the actual contraction portion (discussed as sliding filament theory)

3. Excitation-Contraction Coupling
Goes into a lot more detail on how:
SR is depolarized
Ach is recirculated

4. *CORRECTION*
The 10 steps of the Sliding Filament Theory are actually NOT all classified as part of the SFT:
Message is sent down axon
Axon terminal releases Ach
Ach receptors on sarcolemma receive Ach
This stimulates the release of Calcium ions inside the muscle fibre (from sarcoplasmic reticulum)
Calcium binds to troponin (on tropomyosin)
Tropomyosin (wrapped around actin) swivels off
This shows myosin binding sites
Myosin binds through the energy of ATP
Myosin attach, rotate, detach and attach again to “grab” actin and pull towards its centre – the powerstroke
When calcium is depleted, when ATP is gone, the muscle fibre passively returns to its resting state

5. EXCITATION *CORRECTION*
The 10 steps of the Sliding Filament Theory are actually NOT all classified as part of the SFT:
Message is sent down axon
Axon terminal releases Ach
Ach receptors on sarcolemma receive Ach
This stimulates the release of Calcium ions inside the muscle fibre (from sarcoplasmic reticulum)
Calcium binds to troponin (on tropomyosin)
Tropomyosin (wrapped around actin) swivels off
This shows myosin binding sites
Myosin binds through the energy of ATP
Myosin attach, rotate, detach and attach again to “grab” actin and pull towards its centre – the powerstroke
When calcium is depleted, when ATP is gone, the muscle fibre passively returns to its resting state
EXCITATION

6. CONTRACTION *CORRECTION*
The 10 steps of the Sliding Filament Theory are actually NOT all classified as part of the SFT:
Message is sent down axon
Axon terminal releases Ach
Ach receptors on sarcolemma receive Ach
This stimulates the release of Calcium ions inside the muscle fibre (from sarcoplasmic reticulum)
Calcium binds to troponin (on tropomyosin)
Tropomyosin (wrapped around actin) swivels off
This shows myosin binding sites
Myosin binds through the energy of ATP
Myosin attach, rotate, detach and attach again to “grab” actin and pull towards its centre – the powerstroke
When calcium is depleted, when ATP is gone, the muscle fibre passively returns to its resting state
CONTRACTION

7. SLIDING FILAMENT THEORY
*CORRECTION*
The 10 steps of the Sliding Filament Theory are actually NOT all classified as part of the SFT:
Message is sent down axon
Axon terminal releases Ach
Ach receptors on sarcolemma receive Ach
This stimulates the release of Calcium ions inside the muscle fibre (from sarcoplasmic reticulum)
Calcium binds to troponin (on tropomyosin)
Tropomyosin (wrapped around actin) swivels off
This shows myosin binding sites
Myosin binds through the energy of ATP
Myosin attach, rotate, detach and attach again to “grab” actin and pull towards its centre – the powerstroke
When calcium is depleted, when ATP is gone, the muscle fibre passively returns to its resting state
SLIDING FILAMENT THEORY

8. EC Coupling + SFT
In essence, excitation-contraction coupling is the process by which a nerve impulse is translated into muscle contraction.
Sliding filament theory is the mechanism that explains how muscles contract.
On a unit test/assessment, I’ll ask for the steps of EC Coupling and Sliding Filament Theory of Muscle Contraction. These are the 10 steps we worked on memorizing and understanding!

9. Rigor Mortis
Stiffening of the body beginning 3 to 4 hours after death
Deteriorating sarcoplasmic reticulum releases calcium
Calcium activates myosin-actin cross-bridging and muscle contracts, but can not relax.
Muscle relaxation requires ATP and ATP production is no longer produced after death
Fibers remain contracted until myofilaments decay

10. The Reflex Arc
Reflexes are an important part of all physical movement.
Autonomic reflexes are mediated by the autonomic nervous system and usually involve the action of smooth muscle, cardiac muscle and glands.
Somatic reflexes involve the stimulation of skeletal muscles by the somatic division of the nervous system.

12. The Reflex Arc
Stimulus causes sensory (afferent) neuron to generate a nerve impulse.
Impulse is carried to spinal cord where an interneuron interprets the signals and formulates response.
Motor neuron carries a response message from the spinal cord to the muscle (or organ).
The organ (or muscle, for example) carries out the response.

13. Proprioceptors
We know the process (excitation-contraction coupling) and the mechanism (sliding filaments) by which a nerve impulse translates into a muscle contraction, but what exactly determines:
the extent to which a muscle contracts,
the moment when a muscle relaxes, and
how muscles coordinate with other muscles and with other muscle groups in a particular area of the body?

14. Proprioceptors
There are specialized receptors located within tendons, muscles, and joints. These are called proprioceptors and provide sensory information about:
The state of muscle contraction
The position of body limbs
The body’s posture
Balance

15. The Stretch Reflex
Muscle spindles play an essential role in all physical movement.
Spindles run the length of a muscle fibre and send constant messages to the spinal cord.
Muscle spindles detect changes in muscle length and responds to changes by sending a message to the spinal cord.
The resulting muscle contraction allows for muscle to maintain proper tension or tone, for example, in order to maintain our posture.

16. The Stretch Reflex

17. Tension Reflex
Golgi tendon organs (GTOs) are sensory receptors that terminate where tendons join to muscle fibres. When a muscle stretches, so does the GTO receptor.
GTO are responsible for detecting changes in muscle tension.
Much like the stretch reflex, GTO sends a message to the CNS (central nervous system) to help protect the muscle from excessive tension that could injure the muscle, the joint, or both!
GTOs continually provide feedback to the CNS so it’s likely they play an important role in the development of strength and force, since in order to able to exert greater force, it is necessary to overcome the action of the GTO itself!

18. Tension Reflex

19. Tomorrow we’ll begin looking at the major muscles in the body:
Questions?
Tomorrow we’ll begin looking at the major muscles in the body:
How they are organized
Origin/insertion points
Agonist and antagonist pairs
Types of contraction
How they are named