cht 9 pt 3

Motor Unit -
•

a motor neuron and all the cells it innervates

As small as 1 neuron –
• As large as 1 neuron –
• Average is 150

4 muscle cells

several hundred muscle cells

The cells in the motor unit are spread throughout the muscle –

stimulation of 1 motor unit causes a weak muscle contraction

Cells have All-or-None contraction -

when a muscle cell
is stimulated to contract it contracts to its full extent

Muscle tissue has a graded response to stimulus

 Force, velocity, and duration of contraction is variable Depends on how many motor units are stimulated and the
frequency of stimulation

Muscle Twitch –

stimulated isolated muscle

Myogram -

graphic record of muscle contraction

Muscle twitch –

response of motor unit to single action
potential

Strength
of contraction

Altered by recruiting
more motor units
Small motor units in a muscle stimulated first
and larger motor units later

Isotonic –

Muscle length changes, load moves

Concentric –

muscle shortens ex picking up book

Eccentric –

muscle lengthens ex walking uphill
•

Isometric –

tension builds to peak but no change
in muscle length ex lifting a piano!

ATP -s

Molecule that directly attaches to cross bridges to
them provide energy
Muscles only store enough ATP for 4-6 second

creatine-Phosphate

Provides enough ATP
for an additional
15 seconds

Anaerobic respiration

• Makes ATP very quickly
• Only makes 2 ATP
• Makes Lactic acid
(toxic)
• Can fuel muscles for up
to 45 sec

Aerobic respiration

• Requires O2 - 2 sources
Myoglobin and hemoglobin
• Makes CO2 (lungs remove)
• Makes a lot of ATP
• Can fuel muscles
for hours

Muscle

Muscle fatigue

- inability to contract, even when
stimulated to do so

Muscle Fatigue several factors:

Low ATP, build up of lactic acid, excess free phosphate
binding calcium ions, and/or Na+/K+ imbalances

Aerobic respiration

Anaerobic respiration

The smaller the
motor unit, the
_____ and more
_____ the
movements.

finer, delicate

Extraocular
muscles typically
have ____motor
units while the
_____postural
muscles have
____motor unit

small, large, large

When this neuron is
stimulated, all the
muscle fibers it
synapses upon will
be stimulated and
will _____as a
unit.

contract

motor unit