Functions of (Skeletal) Muscles

1.Movement

2. Posture and Muscle Tone

3. Heat Production

Movement

Muscles move bones by pulling on them

•Length of skeletal muscle shortens as muscle fibers

contract moves bones (levers)

•Usually only insertion (more movable) bone moves

at end of muscle

•Shortening of muscle pulls insertion bone towards

origin (more stationary) bone

•Origin bone stays in place while insertion bone moves towards it.

•Origin and Insertion can change depending on activity

Voluntary Muscle Movement Smooth

Results from muscles working in pairs

Prime Movers, and Synergists / Antagonists

Prime Mover main muscle responsible for mvmt

Synergists help prime movers

Antagonists work opposite of prime movers &

synergists

As prime movers & synergists contract

opposing antagonists relax

When antagonist muscles contract, they produce movement opposite to prime movers & synergists

Example: biceps brachii (prime mover)

brachialis (synergist/helper)

triceps brachii (antagonist)

Example: triceps brachii (prime mover)

biceps brachii & brachialis

(antagonists)

Combined movement smooth

Posture

Muscle tone maintains posture through

Tonic Contractions don’t movedon’t move body

parts but keeps them in position

Few muscle fibers shorten, muscle as a whole doesn’t shorten so no mvmt occurs

Good posture body parts help in position that favor best function

Good posture: good appearance

head/chest held high

chin, abdomen, buttocks in

knees slightly bent

Heat Production

Important for body to maintain constant Body Temperature

Metabolic Rxs / Cellular Fxs

maintain homeostasis

Hypothermia ↓ BT

Fever/Hyperthermia↑ BT

Affects cellular activity/homeostasis (enzyme/protein Fx impaired)

Muscle contraction produces most of heat required to maintain BT

Sweat releases heat

*integument

Sudoriferous/Eccrine sweat glands

Motor Unit = motor neuron + muscle cells attached to it

Muscle cells stimulated by nervous impulse that enters muscle fiber through motor neuron

Neuromuscular Junction: Point of contact between motor neuron & muscle fiber

Nerve impulses pass through neuromuscular junction and cause release of neurotransmitters (chemicals) that cause muscle to shorten/contract

The contractile system for muscles is organized into a number of distinct parts, each of which is controlled by a single motor neuron and each motor neuron controls a large number of muscle fibers.

Action potential/impulse

FatigueWhen muscles stimulated repeatedly w/out adequate rest

strength of contraction ↓

Prolonged periods of exercise

ATP stores are depleted

O2 can’t keep up, muscles produce

lactic acid through

(anaerobic respiration/glycolysis)

O2 debt repaid by rapid breathing

Systems that play an essential role in movement

Respiratory

Circulatory

Excretory

Nervous

Muscular

Skeletal

Muscle functions rely on other systems. No part of the body lives by or for itself!

Now, can you explain HOW?

Muscle Stimulus

Muscle cell stimulated contracts completely

ALL OR NONE

All cells in a muscle don’t contract simultaneously

Principle of Graded Response

Sub-minimal Stimulus no contraction

Minimal or Threshold Stimulus strength of contraction ↑ according to graded response

Maximal Stimulus every muscle cell/fiber in entire muscle responds (from all or none)

??Supramaximal Stimulus no additional effect on contraction b/c all muscle fibers already contracting

5 Types of Skeletal Muscle Contraction

1. Tonic Contraction muscle tone & posture

(no movement)

2. Isometric Contraction ↑ tension w/in muscle but no ↓ in muscle length

Results in larger muscles

No movement

No muscle shortening

But ↑ internal tension

3. Isotonic Contraction Produces movement at

joint and muscle shortens

Ex. walking, running, breathing, lifting, twisting

Insertion end of muscle moves toward

point of origin

(except for facial isotonic contractions)

4.Twitch Contraction quick, jerky contraction

1/10th second delay in response to stimulus

Muscles don’t shorten at stimulation but fraction of a second later

Reaches peak contraction then gradually resumes former length

Steps of a Twitch Contraction

stimulus latent delay contraction relaxation

5.Tetanic Contractions (tetanus)

More sustained than twitch

Produced by series of stimuli bombarding muscle in rapid succession (~ 30 / minute)

Bacteria from puncture wound tetanus or “lock jaw” results in severe cramping, almost continuous tetanic muscle contractions

Clostridium tetani (gram + bacillus, anaerobic)