Lecture 6: Anatomy and Excitation

Types of muscles and their subtypes

Striated: Skeletal muscle tissue, cardiac muscle tissue

Smooth: Smooth muscle tissue

Skeletal muscle tissue function

Moves the skeleton

Cardiac muscle tissue function

Pumps blood through the heart

Smooth muscle tissue examples

Blood vessels, digestive tract

Aponeuroses

A flat, broad sheet of dense, fibrous connective tissue that acts like a tendon to anchor muscles to bones, cartilage, or other muscles

How is the skeletal muscle connected to the skeletal system

Tendons or aponeuroses

Tendons vs aponeuroses

Tendons: Narrow and/or rounded

Aponeuroses: Broad/flat

How are skeletal muscles organized?

Surrounded, supported, and separated by layers of fasciae

Fascial compartment

A defined space within the body that contains groups of muscles, nerves, and blood vessels wrapped tightly in a tough layer of fascia

Name for muscle cell

Myofibres

Myofibrils

Specialized organelles inside striated muscle cells

Myosatelite cells

Stem cells for repairment of muscle tissue found within endomysium

Fascicle

Bundles in the individual muscle containing muscle fibres

Surrounded by a perimysium

Muscle organization hierarchy

Individual muscle, fascicle, myofibre, myofibrils

Epimysium

Dense irregular connective tissue

Surrounds the entire muscle

Perimysium

Dense irregular connective tissue

Surrounds bundles (fascicles)

Endomysium

Aerolar connective tissue

Surrounds individual muscle fibres

Myofibrils

Assemblies of special protein filaments that allow muscles to contract

Sarcolemma

Cell membrane of myofibres

Sarcoplasm

Cytoplasm of myofibres

Sarcoplasmic reticulum

Endoplasmic reticulum of myofibrils

Surrounds myofibril

Stores and releases calcium

Thick and thin filaments

Protein filaments within myofibrils that interact to produce muscle contraction

Transverse tubules (T-tubules)

Part of the sarcolemma (folds inward, cylindrical-like structure)

Triad

Swelling site/area of the sarcoplasmic reticulum where the t-tubule surrounds it

Terminal cisternae

Swelling of the sarcoplasmic reticulum on either side of the t-tubule

Myofiber characteristics

Contains many nuclei

Superficial nuclei

Stretches from the muscle origin to its insertion

How are myofibres produced and repaired

Fusion of many myoblasts during development during embryonic development

Repaired in adulthood by myosatellite cells

How do myosatellite cells repair?

Myosatellite proliferation and fusion with existing myofibres

How are myosatellite cells formed?

During embryonic development, some of the myoblasts don’t fuse and stick to the endomysium

Contraction

Myofibres/Myofibrils becoming shorter or producing tension

Excitation

Sarcolemma generating an electrical signal that precedes contraction

Excitation-Contraction (E-C) coupling

Mechanisms by which excitation triggers contraction

What does each mature myofiber have?

One neuromuscular junction part of the motor unit (no matter how long)

Motor unit

Combination of motor neuron and muscle cells

Neuromuscular junction

Made up of the axon terminal, synaptic cleft, and the motor end plate

Connecting point between a neuron and myofibre (not actually touching

Axon terminal

End of motor neutron

Motor end plate

Specialized region of the sarcolemma of myofibre

Synaptic cleft

Space between axon terminal and motor end plate

Where does excitation occur

NMJ

Steps to excitation

Electrical signal sent in the form of an action potential

Exocytosis of acetylcholine into synaptic cleft

Acetylcholine binds to receptors on motor end plates

Electrical activity spread along the entire length of myofibre (propagation)

Transmembrane potential (V_m)

Difference in electric potential between the interior and the exterior of a cell

All cells have a negative transmembrane potential at rest (inside slightly negative)

Excitable cells

Cells able to change transmembrane potential (muscle and neurons)

What happens to (V_m) when excitable cell is excited

(V_m) will be less negative than at rest

Potentials

Changes in a cell's membrane potential (Vm) that allow cells to send and receive electrical signals (refers to the signal part)

Graded potential

Membrane potential becoming less negative

Action potential

Good at spreading electrical activity/spreading signal across a long cell

How do Vm signals occur

Proteins that allow ions to rapidly cross plasma membrane

What alters the rate of transport

Transporter protein type (e.g. ion channel, co-transportor, atp pump) Fastest → Slowest

Membrane potentials are generated by which transport proteins

Ion channels for speed

How are potentials created

Movement of ions through ion channels

Vm signals/potentials are described in terms of changes from the ___

Resting membrane potential (-70 mV)

Depolarization

Moving away from resting potential

Becoming less negative

Repolarization

Return to resting potential

Becoming more negative

Hyperpolarization

More negative than resting membrane potential

Leak channels (ungated)

RMP created by leak channels

Gated channels

Ligand-gating (binding of a mol.)

Voltage-gating (changes in membrane potential)

Mechanical-gating (mechanical force)

Open and close depending on “gating” factors

Acetylcholine

Neurotransmitter. Released by motor neuron into synaptic cleft. Interact with receptor on motor end plate. Causes the opening of ion channels that allow sodium into the myofibre

What happens when ACh binds to an ACh-receptor?

Opens up gate and allows sodium to cross the cell membrane

Steps of excitation of a myofibre at the NMJ

Myofibre at rest

Release of neurotransmitter (ACh)

ACh binding and NMJ excitation

Spread of excitation as an action potential

Enzymatic removal of ACh

Electrical signal reaches triad

Conformational change in DHPR

Pull open RYR

Release calcium

Voltage gated ion channels

Found in sarcolemma

Allows excitation propagation through myofibre from NMJ

How does excitation propagation work

Near NMJ depolarization (excitation) spreads

Spreading triggers additional voltage gated channels permeable to sodium to open, creating action potential which propagates throughout the entire length of the myofibre’s sarcolemma

T-tubule role in action potentials

propagates the action potential into the vicinity of the sarcoplasmic reticulum, triggering internal calcium release

Dihydropyridine receptor (DHPR) and Ryanodine receptor (RYR)

DHPR: Voltage gated. In t-tubule

RYR: In Sarcoplasm

Electrical signal causes conformational change

Causes RYR to be pulled open

Calcium release