Structure and Function of the Muscular, Neuromuscular,

Cardiovascular, and Respiratory Systems

Gary R. Hunter, PhD, CSCS, FACSM

Robert T. Harris, PhD

chapter 1

Structure and Function of the Muscular,

Neuromuscular, Cardiovascular, and

Respiratory Systems

Chapter Objectives • Describe the macrostructure and micro-

structure of muscle.• Describe the sliding-filament theory.• Describe the characteristics of different

muscle fiber types.• Describe the characteristics of the cardio-

vascular and respiratory systems.• We will focus on the muscular and

neuromuscular systems, review pages 13-19 on your own (heart and lungs)

The Muscular System

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Muscular System

• Macrostructure and Microstructure– Each skeletal muscle is an organ that contains

muscle tissue, connective tissue, nerves, and blood vessels.

– Fibrous connective tissue, or epimysium, covers the body's more than 430 skeletal muscles.

TendonMuscle Belly

Epimysium (deep facia)

Fasciculus

Endomysium

Sarcolemma

Single muscle fiber

Perimysium

Nucleus

Sarcoplasm

MyofibrilMyofilaments

(actin & myosin

Epimysium

• The epimysium encases the muscle• All of the various layers of CT

(endomysium, perimysium and the epimysium blend and are contiguous with the tendon).

• Tendon: Attaches the muscle to the bone (it is much stronger than muscle tissue)

Fasiculi

• Each fasiculi may consist of up to 150 fibers.

• Each muscle fiber is surrounded by a connective tissue known as endomysium.

• The fasiculi themselves are surrounded by connective tissue known as perimysium.– Think of this as the connective tissue within the

muscle

TendonMuscle Belly

Epimysium (deep facia)

Fasciculus

Endomysium

Sarcolemma

Single muscle fiber

Perimysium

Nucleus

Sarcoplasm

MyofibrilMyofilaments

(actin & myosin

Muscle Fibers

• Muscle fibers (also known as cells) are about the same diameter as a strand of human hair and can reach lengths as long as the muscle itself

• Muscle fibers are made up of many myofibrils bundled together

• Again, what myofilaments do we find in the myofibrils?

• Strategically arranged actin and myosin

MuscleFiber

Mitochondrion

Opening to T-

tubule

Myofibril Sarcolemma

T-tubuleSarcoplasmic

reticulum

Sarcoplasm

Muscle Contraction

• What is the smallest “contractile unit” of skeletal muscle?– The sarcomere

• Sarcomeres are lined up in series along the length of the myofibril – (remember: myofibrils make up each muscle fiber,

so simultaneous contraction of the sarcomeres within all of the myofibrils leads to muscle cell shortening)

Figure 1.4

Muscular System

• What is the Sliding-Filament Theory of Muscular Contraction?– The sliding-filament theory states that the actin

filaments at each end of the sarcomere slide inward on myosin filaments, pulling the Z-lines toward the center of the sarcomere and thus shortening the muscle fiber.

– Sliding Filament Theory

Figure 1.5

(Review) Nervous system activation of a muscle

Tropomyosin

AP

Contraction of a Myofibril

– (a) In stretched muscle the I-bands and H-zone are

elongated, and there is low force potential due to reduced cross-bridge–actin alignment.

– (b) When muscle contracts (here partially), the I-bands and H-zone are shortened.

– (c) With completely contracted muscle, there is low force potential due to reduced cross-bridge–actin alignment.

Neuromuscular SystemMotor Unit

• The nervous system works via electrical impulses (action potentials), it signals the muscle to contract by sending action potentials to the muscle

• What is a motor unit?– A motor unit consists of a motor neuron and the

muscle fibers it innervates.– There are typically several hundred muscle fibers in

a single motor unit.

Figure 1.2

The Nervous System is Creepy

Key Point

• The discharge of an action potential from a motor nerve signals the release of calcium from the sarcoplasmic reticulum into the myofibril, this allows the “cross bridges” of the myosin to attach to actin.

• Using the energy from ATP, tension can now be developed in muscle.

Section Outline

• Neuromuscular System – Activation of Muscles – Muscle Fiber Types – Motor Unit Recruitment Patterns During Exercise – Preloading – Proprioception

• Muscle Spindles • Golgi Tendon Organs

Neuromuscular System

• Activation of Muscles• What chemical is released following an action potential?

– Arrival of the action potential at the nerve terminal causes the release of acetylcholine. Once a sufficient amount of acetylcholine is released, an action potential is generated across the sarcolemma, and the fiber contracts.

– What determines the level of muscular control.– Hand vs. Thigh

• Muscles that function with great precision may have as few as one muscle fiber per motor neuron.

• Muscles that require less precision may have several hundred fibers served by one motor neuron.

Neuromuscular System

ACTIVATION OF MUSCLES (neuromuscular junction)

Once enough acetylcholine

crosses the neuromuscular

junction, the action potential is

transmitted and the muscle

contracts

This occurs because the

sarcolemma (muscle cell

membrane), then T-tubule

depolarizes and the sarcoplasmic

reticula releases calcium

All-or-none principle

• All of the muscle fibers in the motor unit contract and develop force at the same time. There is no such thing as a motor neuron stimulus that causes only some of the fibers to contract. Similarly, a stronger action potential cannot produce a stronger contraction.

Stimulated Motor Unit

• Twitch, twitch summation, and tetanus of a motor unit: – a = single twitch– b = force resulting

from summation of two twitches

– c = unfused tetanus– d = fused tetanus

Neuromuscular System

• Muscle Fiber Types – Type I (slow-twitch) – Type IIa (fast-twitch)

• Type IIab (fast-twitch); now named as Type IIax• Type IIb (fast-twitch); now named as Type IIx

– Type IIx are “true” fast-twitch fibers and are highly if not 100% glycolytic in nature

– All other Type II fibers are oxidate and glycolytic hybrids

Table 1.1

MUSCLE FIBER TYPES

• Postural muscles are typically composed of a higher percentage of type 1 fibers

• Whereas larger muscles (quadriceps) have a mixed composition

Key Point

• Motor units are composed of muscle fibers with specific morphological and physio-logical characteristics that determine their functional capacity.

• Motor neurons, motor units and/or muscle fibers can be characterized as type .. Motor units are always the same type (you won’t find a motor neuron innervating both type I and type II fibers)

Neuromuscular System

• Motor Unit Recruitment Patterns During Exercise– The force output of a muscle can be varied through

change in the frequency of activation of individual motor units or change in the number of activated motor units.

Table 1.2

Neuromuscular System

• Preloading– Occurs when a load is lifted,

since sufficient force must be developed to overcome the inertia of the load

• Proprioception– Information concerning

kinesthetic sense, or conscious appreciation of the position of body parts with respect to gravity

– Processed at subconscious levels

Neuromuscular System

• How Can Athletes Improve Force Production?– Recruit large muscles or muscle groups during an

activity.– Increase the cross-sectional area of muscles

involved in the desired activity. – Preload a muscle just before a concentric action to

enhance force production during the subsequent muscle action.

– Use preloading during training to develop strength early in the range of motion.

Key Point

• Proprioceptors are specialized sensory receptors that provide the central nervous system with information needed to maintain muscle tone and perform complex coordi-nated movements.

Neuromuscular System

• Proprioception – Muscle Spindles

• Also known as intrafusal fibers • Muscle spindles are proprioceptors that consist of several

modified muscle fibers enclosed in a sheath of connective tissue.

Muscle Spindle

• Figure 1.7 – When a muscle is stretched, deformation of the

muscle spindle activates the sensory neuron, which sends an impulse to the spinal cord, where it synapses with a motor neuron, causing the muscleto contract.• Example: If you are being stretched by your trainer and

they push the muscle a little too far/fast .. What is your natural reaction?

Figure 1.7

Neuromuscular System

• Proprioception – Golgi Tendon Organs (GTO)– Where are GTOs located?

• Golgi tendon organs are proprioceptors located in tendons near the myotendinous junction.

• They occur in series (i.e., attached end to end) with extrafusal muscle fibers.

Golgi Tendon Organ

• What happens when you lift something that is far too heavy?– When an extremely heavy load is placed on the

muscle, discharge of the GTO occurs.– The sensory neuron of the GTO activates an

inhibitory interneuron in the spinal cord, which in turn synapses with and inhibits a motor neuron serving the same muscle.

Figure 1.8

Neuromuscular System

• Older Muscle – Muscle function is reduced in older adults. – Reductions in muscle size and strength are

amplified in weight-bearing extensor muscles.– Muscle atrophy with aging results from losses in

both number and size of muscle fibers, especially Type II muscle fibers.

– Inactivity plays a major role but cannot account for all of the age-related loss of muscle and function.

Section Outline

• Cardiovascular System – Heart

• Valves • Conduction System • Electrocardiogram

– Blood Vessels• Arteries • Capillaries • Veins

– Blood

Cardiovascular System

• Heart– The heart is a muscular organ made up of two

interconnected but separate pumps.• The right ventricle pumps blood to the lungs.• The left ventricle pumps blood to the rest of the body.

Heart and Blood Flow

• Figure 1.9 (next slide)– Structure of the human heart and course of blood

flow through its chambers

Figure 1.9

Cardiovascular System

• Heart– Valves

• Tricuspid valve and mitral (bicuspid) valve• Aortic valve and pulmonary valve• Valves open and close passively, depending on the

pressure gradient

– Conduction System • Controls the mechanical contraction of the heart

Electrical Conduction System

• Figure 1.10 (next slide)– The electrical conduction system of the heart

Figure 1.10

Cardiac Impulse

• Figure 1.11 (next slide)– Transmission of the cardiac impulse through the

heart, showing the time of appearance (in fractionsof a second) of the impulse in different parts of the heart

Figure 1.11

Cardiovascular System

• Heart– Electrocardiogram

• Recorded at the surface of the body• A graphic representation of the electrical activity of the

heart

Electrocardiogram

• Figure 1.12 (next slide)– Normal electrocardiogram

Figure 1.12

Depolarization leading to

atrial contraction

Depolarization

leading to ventricular

contraction

Repolarization of

ventricles

Cardiovascular System

• Blood Vessels – Blood vessels operate in a closed-circuit system. – The arterial system carries blood away from the

heart.– The venous system returns blood toward the heart.

Distribution of Blood

• Figure 1.13 (next slide)– The slide shows the arterial (right) and venous (left)

components of the circulatory system.– The percent values indicate the distribution of blood

volume throughout the circulatory system at rest.

Figure 1.13

Cardiovascular System

• Blood Vessels– Arteries – Capillaries– Veins

Cardiovascular System

• Blood– Hemoglobin transports oxygen and serves as an

acid–base buffer.– Red blood cells facilitate carbon dioxide removal.

Key Point

• The cardiovascular system transports nutrients and removes waste products while helping to maintain the environment for all the body’s functions. The blood transports oxygen from the lungs to the tissues for use in cellular metabolism, and it transports carbon dioxide from the tissues to the lungs, where it is removed from the body.

Section Outline

• Respiratory System – Exchange of Air – Exchange of Respiratory Gases

Respiratory System

• Figure 1.14 (next slide)– Gross anatomy of the human respiratory system

Figure 1.14

Respiratory System

• Exchange of Air– The amount and movement of air and expired gases

in and out of the lungs are controlled by expansion and recoil of the lungs.

Expiration and Inspiration

• Figure 1.15 (next slide)– The slide shows contraction and expansion of the

thoracic cage during expiration and inspiration, illustrating diaphragmatic contraction, elevation of the rib cage, and function of the intercostals.

– The vertical and anteroposterior diameters increase during inspiration.

Figure 1.15

Respiratory System

• Exchange of Respiratory Gases– The primary function of the respiratory system is the

basic exchange of oxygen and carbon dioxide.