MO Figure

Muscle Function at

the Cellular Level

Michael Patrick O'Neill/Science Source.

• Like nervous tissue, muscles are excitable

or "irritable”they have the ability to respond to a stimulus

• Unlike nerves, however, muscles are also:

Contractible (they can shorten

in length)

Extensible (they can extend or

stretch)

Elastic (they can return to their

original shape)

Functions of Muscular Tissue

• Muscle makes up a large percentage of the

body’s weight

• Their main functions are to:

Create motion – muscles work with nerves,

bones, and joints to produce body movements

Stabilize body positions and maintain posture

Store substances within the body using

sphincters

Functions of Muscular Tissue

Location Function Appearance Control

Skeletal

skeletonmovement,

heat, posture

striated, multi-

nucleated (eccentric),

fibers parallel

voluntary

Cardiac

heartpump blood

continuously

striated, one central

nucleusinvoluntary

Visceral

(smooth muscle)G.I. tract,

uterus, eye,

blood vessels

Peristalsis,

blood pressure,

pupil size,

erects hairs

no striations, one

central nucleusinvoluntary

Three Types of Muscular

Tissue

(b) Cardiac muscle (c) Visceral smooth muscle

(a) Skeletal muscle

Three Types of Muscular Tissue

Location Function Appearance Control

Skeletal

skeletonmovement,

heat, posture

striated, multi-

nucleated (eccentric),

fibers parallel

voluntary

Cardiac

heartpump blood

continuously

striated, one central

nucleusinvoluntary

Visceral

(smooth muscle)

G.I. tract,

uterus, eye,

blood vessels

Peristalsis,

blood pressure,

pupil size,

erects hairs

no striations, one

central nucleusinvoluntary

Skeletal Muscle

Skeletal Muscle

Skeletal muscle fibers are very long “cells” - next to

neurons (which can be over a meter long),

perhaps the longest in the body

The Sartorious muscle contains

single fibers that are at least

30 cm long

A single skeletal muscle fiber

Skeletal Muscle

Sarcolemma

Motor neuron

Skeletal Muscle

The terminal processes of a motor

neuron in close proximity to the

sarcolemma of a skeletal muscle fiber

A muscle fiber consists of a single cell.

• The cell is long and multinucleated.

• The cytoplasm is called the sarcoplasm.

• Contains a specialized ER, called the

sarcoplasmic reticulum, that stores

calcium.

• Contains myofibrils made up of thick and

thin filaments.

Skeletal muscle fibers

The Skeletal Muscle Fiber

Increasing the level of magnification, the myofibrils are

seen to be composed

of filaments

Thick filaments

Thing filaments

A scanning electron micrograph of a sarcomere

• The basic functional unit of skeletal muscle

fibers is the sarcomere: An arrangement of

thick and thin filaments sandwiched between

two Z discs

The Skeletal Muscle Fiber

The “Z line” is really a Z disc when considered in 3

dimensions. A sarcomere extends from Z disc to Z disc.

• Muscle contraction occurs in the sarcomeres

The Skeletal Muscle Fiber

• Myofibrils are built from three groups of

proteins

Contractile proteins generate force during

contraction

Regulatory proteins help switch the contraction

process on and off

Structural proteins keep the thick and thin

filaments in proper alignment and link the

Muscle Proteins

• The thin filaments are comprised mostly of

the structural protein actin, and the thick

filaments are comprised mostly of the

structural protein myosin

• However, in both types of filaments, there

Muscle Proteins

• In the thin filaments actin proteins are strung

together like a bead of pearls

• In the thick filaments myosin proteins look

like golf clubs bound together

Muscle Proteins

In this first graphic, the myosin binding sites on the actin

proteins are readily visible.

The regulatory proteins troponin and tropomyosin have

been added to the bottom graphic: The myosin binding

sites have been

covered

Muscle Proteins

In this graphic the troponin-tropomyosin complex has

slid down into the “gutters” of the actin molecule

unblocking the myosin binding site

The troponin-tropomyosin complex can slide back and

forth depending on the presence of Ca2+

Myosin binding site exposed

Muscle Proteins

• Ca2+ binds to troponin which changes the shape of

the troponin-tropomyosin complex and uncovers

the myosin binding sites on actin

Muscle Proteins

• Besides contractile and regulatory proteins, muscle

contains about a dozen structural proteins which

contribute to the alignment, stability, elasticity, and

extensibility of myofibrils

• Titan is the third most plentiful protein in muscle,

after actin and myosin - it extends from the Z disc and

accounts for much of the elasticity of myofibrils

• Dystrophin is discussed later as it relates to the disease

of muscular dystrophy

Muscle Proteins

• With exposure of the myosin binding sites on

actin (the thin filaments)—in the presence of

Ca2+ and ATP—the thick and thin filaments

“slide” on one another and the sarcomere is

shortened

The Sliding-Filament

Mechanism

• The “sliding” of actin on myosin (thick

filaments on thin filaments) can be broken

down into a 4 step process

The Sliding-Filament

Mechanism

Figure 1

Sliding filament model

Thick (myosin) and thin (actin) filaments slide past one another

during muscle contraction. As a result, the sarcomere shortens.

Figure 2

Sarcomeres

Z-discs form dark lines in electron micrographs and give

skeletal muscle a striated appearance.

C. F. Armstrong/Science Source.

Figure 3

Figure 4

Muscle anatomy

Figure 4a

Figure 4b

Muscle energy storage

Glycogen

• Glucose polymer. Glucose is used in

cellular respiration to make ATP.

Creatine phosphate

• Transfers a phosphate to ADP to form

ATP.

Figure 5

Muscle innervation

Muscle fibers can be innervated by a single neuron, which

increases contraction strength at the expense of fine control, or by

multiple neurons, which allows finer control but with little strength.

Figure 5a

Figure 5b

Figure 6

Excitation-contraction coupling

Figure 7

Regulation of contraction

In resting muscle,

tropomyosin covers

myosin-binding sites

on actin. The

troponin complex

holds tropomyosin in

place. Ca2+ binding

to troponin causes

tropomyosin to shift,

which exposes the

myosin-binding sites.

Myosin binds actin,

and the muscle

contracts.

Figure 7a

Figure 7b

Figure 7c

MO Figure

Skeletal System

and Locomotion

Art Wolfe/Science Source

Skeletal systems

Endoskeleton: inside the animal

Exoskeleton: outside the animal

Hydrostatic skeleton: composed of

pressurized water in internal compartments.

Figure 1

Hydrostatic skeletons

This Hydra moves using a hydrostatic skeleton.

© 2006 Nature Publishing Group Alvarado, A & Tsonis, P. Bridging the

regeneration gap: genetic insights from diverse animal models. Nature

Reviews Genetics 7, 873–884 (2006) doi:10.1038/nrg1923. Used with

permission.

Figure 2

Peristaltic movement

Caused by

alternating

radial and

longitudinal

muscle

contractions

against a

fluid-filled

coelom.

Figure 2a

Figure 2b

Figure 3

Exoskeletons

Arthropod exoskeletons contain chitin.

© 2002 Nature Publishing Group GDC seeks members of new Appointing Body. British Dental Journal 193, 431–

433 (2002), doi:10.1038/sj.bdj.4801590. Used with permission.

Figure 4

The human skeleton

Figure 5

Antagonistic muscle pairs

Coordinated

movement

enables flexion

and extension

of a limb.

Figure 5a

Figure 5b

Figure 6

Balance and locomotion

During walking and

running, the feet are

used for balancing,

pushing off, and

maintaining momentum.

Figure 7

Terrestrial locomotion

Energy stored in tendons enables this frog to jump.

Stephen Dalton/Science Source.