Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Anatomy and Physiology

Chapter 12: Physiology of the Muscular System

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

General FunctionsMovement of the body and its

partsHeat productionPosture

2

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Function of Skeletal Muscle Tissue Characteristics of skeletal muscle cells

Excitability (irritability)—ability to be stimulated

Contractility—to contract/shorten, & produce movement

Extensibility—to extend/stretch, to return to their resting length

3

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Function of Skeletal Muscle Tissue

Muscle cells are called fibers because of their threadlike shape Contain many mitochondria and several

nuclei Sarcomere

Each myofibril consists of many sarcomeres Segment of myofibril between two

successive Z disks Contractile unit of muscle fibers

4

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 5

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Function of Skeletal Muscle Tissue Sarcolemma—plasma membrane of muscle

fibers Sarcoplasmic reticulum (SR)

T tubules—network of tubules and sacs found within muscle fibers

Continually pumps calcium and stores the ions for later release

6

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 7

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 8

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Function of Skeletal Muscle Tissue T tubules

Transverse tubules extend across the sarcoplasm

Membrane has ion pumps that continually transport Ca++ ions

Allow electrical impulses

9

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 10

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Myofilaments Each myofibril contains thousands of

thick and thin myofilaments 4 protein molecules make up

myofilaments Myosin

thick filament Myosin “heads” attracted to actin

Form cross bridges when attached to actin

11

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 12

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

4 different kinds of protein cont..2.) Actin—protein that forms thin filament

3.) Tropomyosin—protein that blocks the active sites on actin molecules

4.) Troponin—protein that holds tropomyosin molecules in place

13

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Mechanism of contraction Excitation and contraction

1. Skeletal muscle fiber remains at rest

2. Stimulated by a motor neuron• Neuromuscular junction(NMJ)—motor

neurons connect to the sarcolemma at the motor endplate

• “NMJ” is a synapse where neurotransmitter transmit signals

14

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Excitation and contraction (cont)

Acetylcholine: neurotransmitter released into the synaptic cleft

1. Stimulates the receptors

2. Initiates an impulse in sarcolemma

3. Triggers the release of calcium ions

4. Calcium binds to troponin, which expose active sites on actin

15

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 16

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 17

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 18

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 19

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 20

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 21

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Sliding filament model/theory

Active sites on actin are exposed, myosin heads bind to them

Myosin heads bend and “pull” the thin filaments past them

Each head releases, binds to the next active site, and pulls again

The entire myofibril shortens

22

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 23

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Whole Muscle Contraction

A single muscle fiber has an “all or nothing” response

But a whole muscle can vary its force of contraction.

Two characteristics of a whole muscle allow this

• Motor unit

• Recruitment

24

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Function of Skeletal Muscle Organs Motor unit

Motor unit= motor neuron + the muscle fibers it attaches

Could be a couple of fibers or an entire fascicle

• Recruitment: Smaller the number of fibers in a motor

unit, the more precise are the available movements

Larger the number of fibers in a motor unit, the more powerful the contraction available

25

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Energy sources for muscle contraction

ATP yields the energy required for muscular contraction

Muscle fibers resynthesize ATP from the breakdown of creatine phosphate (CP)

• Catabolic pathways Aerobic pathway

Presence of O2 from blood Slower than anaerobic pathway Supplies energy for the long term rather than

the short term

26

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Catabolic pathways (cont)

Anaerobic pathway Little to no O2 is available Very rapid, providing energy during

first minutes of maximal exercise Formation of lactic acid

Requires oxygen to convert back to glucose,

Soreness producing of an “oxygen debt” excess post-exercise oxygen consumption

(EPOC)27

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 28

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 29

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Responses of a Whole Muscle

• Twitch: Single muscle response in which muscle contracts and then fully relaxes

• Tetanus: Sustained muscle contraction caused by repeated stimulation

• Smooth, sustained contraction

• Tonus: Normal, continuous state of partial muscle contraction

30

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Speeds of response depends on which fibers are stimulated:Type I: slow-twitch (weakest force,

longest time)

Type II: fast-twitch (2 divisions)type IIa: intermediate speedtype IIb: fastest (greatest force, shortest time)

Skeletal Muscle Fiber Types

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Isotonic vs. Isometric contractions

Isotonic contraction: “same tension” Contraction where tone/tension of

muscle is the same as the length changes

Concentric—muscle shortens as it contracts

Eccentric—muscle lengthens while contracting

32

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 33

Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Isometric contraction

Isometric: “same length” o muscle length stays the same

while muscle tension increases

Most body movements occur as a result of both types of contractions

34