A&P I Exam 3 - Skeletal & Smooth Muscle

3 different types of muscle

1). Skeletal

2). Smooth

3). Cardiac

Location of skeletal muscle?

Attached to bones, skin (some facial muscles) or other muscles (tendons)

Location of smooth muscle?

In the walls of hollow visceral (involuntary) organs (except the heart), eye muscles, airways, large arteries, etc.

Ex). stomach, skin, blood vessels, esophagus, intestines, etc. 

Location of cardiac muscle?

Walls of the heart (makes up most the mass of the heart)

Appearance of skeletal muscle?

1). Single, LONG, cylinder fibers

2). Striations

3). Multinucleated

Appearance of smooth muscle?

1). Single, spindle-shaped fibers. Arranged in sheets (since smooth muscle contracts as a single unit).

2). No striations

3). Uninucleated (one nucleus)

Appearance of cardiac muscle?

1) Branched, shorter fibers connected by intercalated discs

2). Striations

3). Uninucleated (one nucleus)

Function of skeletal muscle?

1). Contractions move the skeleton

2). Mostly voluntary, however, reflexes are involuntary.

Function of smooth muscle?

1). Propelling content in organs (digestion) & blood vessels, regulating blood flow, etc.

2). Involuntary

Function of cardiac muscle?

1). Covers the heart and pumps blood throughout the body

2). Involuntary

Other features of skeletal muscle?

1). A part of somatic nervous system (voluntary)

2). Contains neuromuscular junction (NMJ)

Other features of smooth muscle?

1). A part of autonomic nervous system (involuntary)

→ Includes sympathetic and parasympathetic divisions

2). Inherent tonicity: has an inherent muscle tone (steady state), thus does not require nerve stimulation (no NMJ).

3). Influenced by hormones, sympathetic and parasympathetic divisions of ANS, ECF conditions, etc.

4). May respond to stretch (e.g., uterus stretches when drinking lots of fluids, childbirth, etc.). 

Other features of cardiac muscle

1). A part of autonomic nervous system (involuntary)

→ Includes sympathetic and parasympathetic divisions

2). Inherent contractility: does not require nerve stimulation (no NMJ). It beats on its own as it has its own intrinsic conduction system (e.g., why heart transplants work). 

3). Influenced by hormones, sympathetic and parasympathetic divisions of ANS, etc. 

Identify this type of muscle

Skeletal muscle

Identify this type of muscle

Smooth muscle

Identify this type of muscle

Cardiac muscle

4 characteristics of all muscle tissue

1). Excitability

→ Conductivity

2). Contractility

3). Extensibility

4) Elasticity

Excitability

(responsiveness) ability to receive and respond to stimuli

Contractility

ability to shorten forcibly when stimulated

Conductivity

ability to set off a wave of excitation that travels along muscle fiber

Extensibility 

ability to be stretched 

Elasticity

ability to return to original shape after being stretched (recoil)

4 Main functions of skeletal muscle

1). Movement: SKM contractions move skeleton

2). Posture: 

3). Joint stability

4). Heat production (thermogenesis): SKM produces 20-30% of body heat at rest and up to 85% during exercise. 

Anatomical Organization of SKM

1a). Epimysium (CT layer)

1b). Individual whole muscle

2a). Perimysium (CT layer)

2b). Fascicles

3a). Endomysium (CT layer)

3b). Muscle fibers (cell)

4). Myofibrils

5). Sarcomeres

6). Myofilaments 

Epimysium

Outer most connective tissue layer that surrounds an entire individual muscle.

Ex). surrounds the deltoid, rectus abdominis, etc.

Muscle (organ)

Individual muscles, including the deltoid, rectus abdominis, etc.

Perimysium

Middle connective tissue layer that surrounds individual fascicles.

Fascicles

Bundles of 20-30 muscle fibers

Epimysium

Inner most connective tissue layer that surrounds individual muscle fibers

→ the boundary for the extracellular environment (since it surrounds the muscle cell).

Muscle fiber (cell)

The fundamental contractile unit of skeletal muscle

Myofibril

Threadlike structure within muscle fiber that are arranged in sarcomeres.

Sarcomere

Region of myofibril between 2 Z discs (functional unit).

Myofilaments

Thick and thin filaments composed of various proteins (e.g., myosin, actin, tropomyosin, and troponin) that work together to produce muscle contractions.

→ smallest unit of muscle tissue

Sarco- and Myo-

Prefixes that mean muscle

6 specialized structures of muscle cell (fiber)

1). Sarcolemma

2). Sarcoplasm

3). Transverse (T)-Tubules

4). Sarcoplasmic Reticulum

→ Terminal cistern

5). Triad

6). Myofibrils

Additional Structures

1). Mitochondria

Sacrolemma

The plasma membrane of a muscle fiber (cell) that is involved in the propagation of action potentials. 

Sarcoplasm

The cytoplasm of a muscle fiber (cell) that contains organelles and myofibrils.

Transverse (T)-Tubules

Extensions of the sarcolemma that extend into sarcoplasm and cut across the transverse plane (horizontal) in a muscle fiber (cell).

Sarcoplasmic Reticulum

A specialized endoplasmic reticulum (ER) in muscle fibers (cells) that are specialized in carrying calcium ions.

Terminal Cisternae

Enlarged areas of the sarcoplasmic reticulum that store calcium ions. They reside on both sides of the T-tubules; thus, they also cut across the transverse plane (horizontal).

Triad

A structure formed by two terminal cisterns and one T-tubule. Cuts across the transverse plane (horizontal) in the muscle fiber (cell). 

Repeating subunits that compose myofibrils

Sarcomeres

Components of Sacromeres

1) Thick filaments

2). Thin filaments

3). Banding patterns

→ I band

→ A band

→ H zone

→ Z disc

→ M line

Components of thick filament

1). Myosin (protein)

2). Titin core (protein)

Myosin molecule components

1). 2 myosin heads

→ 2 binding sites to actin

→ 2 binding sites for hydrolyzing ATP

2). 2 twisted myosin tails

Components of thin filament

1). Actin (protein)

2). Tropomyosin (protein)

3). Troponin (protein)

G-action

Individual actin subunits (globular form). Location of the binding site for myosin heads.

F-Action

Chains of g-action subunits (filamentous form). 2 chains of f-action twisted together form the thin filament backbone.

Tropomyosin

Structure: double-stranded protein wrapped around F-action.

Function: “bike chain.” At rest, it covers myosin binding sites, thus preventing myosin from binding to actin at rest.

Troponin

Structure: 3 subunits, attached to tropomyosin, which forms the troponin-tropomyosin complex. Also attached to g-actin.

Function: “bike lock".” Holds tropomyosin in place and contains binding sites for calcium ions.

Calcium Ions = KEY!

I Band

Section of all thin filaments (light)

→ I =”THIN” letter, thus contains only THIN filaments

A Band

Section of mostly thick filaments, with small thin filament overlap.

→ A = “AND” since contains thin AND thick

H Zone

Section of all thick filaments within the A band.

→ H = “THICK” letter, thus contains only THICK filaments

Z Disc

Structure: Vertical lines between sarcomeres or cut down the middle of I band.

Function: anchor points for thin and thick filaments

M Line

Runs down the middle of sarcomeres, A band and H zone.