Biology 20Y - Motor Systems

Skeletal Muscle

• voluntarily controlled

• multinucleated

• striated

• individual skeletal muscle cells: muscle fibres

• attached to bones by tendon

• carry out body movement

Smooth Muscle

• involuntarily controlled

• not striated

• mononucleated

• smooth myocotes (smooth muscle cells)

• blood vessels, digestion, respiratory tract

Cardiac Muscle

• involuntarily controlled

• striated

• mononucleated

• cardiomyocytes (muscle tissue)

• the heart

functions of muscular tissues

1. producing body movement

2. stabilizing body positions

3. storing/moving substances throughout body (blood circulation, respiration, etc)

4. generating body heat (using ATP to do work, shivering)

the four muscle functions stem from…

the fact that muscles can contract (tensile forces)

properties of muscle tissue

1. electrical excitability

2. contractility

3. extensibility

4. elasticity

electrical excitability

ability to respond to stimuli by generating electrical impulses

• muscle cells primarily contract when they are stimualted by action potenial

action potential

electrochemical impulse travelling down nerve cell

contractility

ability to contract forcefully when stimulated by action potential. Contraction produces tension to attachment site (overcomes resistance)

extensibility

ability to stretch without damage

elasticity

ability to retain shape after being stretched out

• smooth and cardiac muscles more elastic than skeletal

Components of Skeletal Muscle Tissue

muscle cells are called myocytes

• skeletal myocytes are muscle fibers

• cytoplasm: sarcoplasm (glycogen and myoglobin)

• smooth endoplasmic reticulum: sarcoplasmic reticulum (store and release Ca ions to regulate muscle)

• cell membrane: sarcolemma

myoglobin

protein that binds oxygen in muscle cells, aiding in energy production.

T system

system of T-tubules that run perpindicular to sarcoplasmic reticulum

• electrical impulses travel through T-tubules

types of muscle fibers

1. slow twitch (type I)

2. fast twitch (type II)

Slow Twitch (Type I)

less power, dont tire easily, slow contraction, high myoglobin content (more O2)

Fast Twitch (Type II)

Type IIa: fast contraction, high myoglobin

Type IIb: fast contraction, low mitochondria, low myoglobin

generates explosive power, tires quickly

Myofibril

A long, thread-like structure found in muscle cells, composed of myofilaments that facilitate contraction.

• made of myofilaments. Myofilaments are arranged in contracile units called sarcomeres

Actin Filaments

thin filament

Myosin filaments

Thick filament

Elastic filament

large protein called titin

Sarcomeres

composed on actin and myosin filaments and titin, boundaries are called Z-disks

muscular contraction

myosin and actin filaments silde over eachother to shorten length of muscular contraction

A band

extends entire length of thick filaments (myosin)

I Band

contains thin filaments (no thick)

H zone

contains thick filaments (no thin)

Muscle Protein

a type of protein that makes up the muscle fibers and is essential for muscle contraction, including actin and myosin.

Troponin

Ca 2+ ions bind to troponin ( change shape), released by sarcoplasmic reticulum

The Sliding filament model of muscular contraction

Myosin filaments walk actin filaments towards M-line

• thin filaments move inwards towards center of sarcomere

• casues sarcomere to shorten

• filaments are not shortening: they slide over eachother (overlap)

Muscle Contraction cycle

1. action potential reaches sarcoplasmic reticulum

2. sarcoplasmic reticulum releases Ca 2+ ions into sarcoplasm

3. Ca 2+ ions bind to troponin

4. troponin changes shape, pulling on tropomyosin

5. tropomyosin moves (uncovering myosin binding sites on actin filament)

Steps of contraction cycle

1. ATP hydrolusis (into ADP + Pi), myosin head includes ATP and ATPase (enzyme)

2. Myosin heads bind to actin, forming cross-bridges. (ADP + Pi are released)

3. Power stroke: myosin heads pull actin filaments towards M-line

4. detachment of myosin from actin: cross bridge reemains intact until myosin head bind to ATP (detach from actin filament) - ATP come from mitochondria

Muscular dystrophy

A group of genetic disorders characterized by progressive weakness and degeneration of skeletal muscles. It results from mutations that disrupt the production of proteins needed for healthy muscle function.

Anyotophic lateral sclerosis (ALS)

A progressive neurodegenerative disease affecting nerve cells in the brain and spinal cord, leading to loss of muscle control and eventual paralysis.

Myostatin Deficiency

A genetic condition resulting in reduced levels of myostatin, a protein that inhibits muscle growth, leading to increased muscle mass and strength.