NMS Lecture 6 - NMJ + Muscle Anatomy

3 types of muscle

Smooth muscle, Cardiac muscle, Skeletal muscle

Smooth muscle

Involuntary movement, seen in GI tract

Cardiac Muscle

Striated muscle found in walls of heart to propel blood. Involuntary control

Skeletal Muscle

Muscle attached to the skeleton (striated), voluntary contractions

What is a skeletal muscle made out of

Number of muscle cells and fibers that run lengthwise down the muscle

Endomysium

Membrane that sits over each muscle cell and ioslates each muscle cell from another

Role of motor neuron

Activate the muscle it is attached to

Motor unit

The motor neuron, its axon, and all the muscle fibers it activates

How many synapses does a muscle fiber have

One - the neuromuscular junction

How many synapses does a post-synaptic cell have

10-40 000 inputs

Neuromuscular junction

Synapse between the efferent (motor) neuron and the muscle fiber

What type of synaptic transmission does a neuromuscular juction have

Direct-gated chemical synaptic tranmission

Motor end plate

Region of muscle fiber plasma membrane underneath the terminal portion of the axon

Steps of synaptic transmission to a muscle

1. Action potential travels down efferent neuron to pre-synaptic terminal

2. Depolarized terminal causes opening of voltage-gated calcium channels

3. Calcium enters the terminal, causing synaptic vesicles to fuse to the membrane andn release neurotransmitter

4. Acetylcholine (ACH) released by the neuron

5. ACH binds to the post-synaptic nicotinic receptors on the muscle

6. Nicotinic receptors open and cause sodium to enter the muscle cells

7. Local depolarization occurs, the muscle is brought to threshold and opens voltage gated sodium channels.

8. Action potential produced in the muscle

Differences between synaptic transmission at the NMJ and a central synapse

• One action potential in motor neuron generates one action potential in a muscle (no summation like in CNS)

• Each muscle cell is innervated by 1 pre-synaptic axon (not 40 000)

• No inhibitory NT, only ACH

Transverse tubule (T-tubule)

Muscle extracellular space - positively charged compared to intracellular space

Events at the neuromuscular junction (steps)

1. ACH from efferent neuron released into synaptic cleft and binds to muscle receptors

2. Muscle cell is depolarized at the motor end plate due to sodium entering the cell through chemically gated ion channels

3. Current flows between the end plate and the adjacent membrane at rest, which depolarizes the membrane to threshold and opens voltage gated Na channels

4. Action potential generated

5. Action potential travels along the surface of the muscle fiber, outwards in both directions towards the ends of the fiber, and down the T-tubules

   

Function of the T-tubule

Site of excitation-contraction coupling

Excitation-contraction coupling

The process in which an action potential in the T-tubule initiates calcium release for muscle contraction

Dihydropyridine (DHP) receptor

Voltage gated calcium channel in the T-tubule that is physically bound to a ryanodine receptor

Ryanodine receptor

Physically connected to the DHP receptor and acts as a calcium channel

Steps of Excitation-Contraction coupling

1. Action potential travels down the T tubule

2. Action potential activates DHP receptor

3. DHP receptor changes conformation, and opens the ryanodine receptor connected via the foot process

4. Calcium rushes out of the sarcoplasmic reticulum through the ryanodine receptor and enters the cytosol

5. Calcium interacts with the contractile elements of the muscle cell underneath the sarcoplasmic reticulum

Comparison of excitation-contraction coupling between skeletal and heart muscle

No physical coupling of receptors in heart muscle. The heart uses calcium-induced calcium release

Calcium induced calcium release (steps)

1. When an AP occurs, L-type calcium receptors open and release calcium

2. Calcium binds to ryanodine receptors, releasing sarcoplasmic reticulum calcium into the cytosol

Structure of a muscle cell/fiber

Composed of myofibrils, bundles of myofilaments. Myofilaments are made out of actin and myosin

Structure of a myofilament

T-tubules and sarcoplasmic reticulum are wrapped around the sarcomere

Structure of a sarcomere

Structural unit of a myofilament

• Bound by Z lines (interconnecting protein networks)

• Made up of actin (thick filament) and myosin (thin filament)

• Actin and myosin are what allow the muscle to contract

• H zone is the area in the middle of the sarcomere, only containing actin

What contracts in a muscle

The size of the actin/myosin filaments does not change. The H zone shrinks during contraction