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

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)

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

*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 *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

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

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

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!

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

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.

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.

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?

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

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.

The Stretch Reflex

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!

Tension Reflex

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