Download Muscle Structure

Muscle Structure
Skeletal muscle is made up of individual muscle fibres (muscle cells) grouped
together to form bundles (fasciculi).
Each element of the muscle is covered by connective tissue to help provide
shape and add strength.
 The muscle fibre is covered by the endomysium
 The fasciculi are covered by the perimysium
 The muscle itself by the epimysium
Each individual muscle fibre contains many myofibrils.
Each myofibril is an arrangement of separate units connected end-to-end
called sarcomeres.
The sarcomeres contain two protein filaments called actin and myosin.
When the muscle contracts the actin and myosin filaments slid over each
other, rather like a pair of patio doors, and this shortens the sarcomere.
Muscle Stimulation
All skeletal muscle reacts to an electrical stimulus, which is conducted from
the brain to the muscle via nerves.
Motor/efferent neurones (nerves) carry information from the central nervous
system to the muscle.
Motor neurones are made up of three parts – dendrites, axon and cell body.
The stimuli are received from the CNS by the dendrites, and passed on via
the axon to the axon terminal that connects with the motor end plate of the
muscle.
As there are so many muscle fibres it would take a lot of internal ‘wiring’ to
connect them all to a separate motor neurone, so instead one motor neurone
branches off and stimulates between 5 and over 1000 muscle fibres. This is
called a motor unit. The number of muscle fibres in a motor unit depends on
the type of work the muscle performs and the degree of muscular control
required. Large muscles have thousands of fibres in the motor unit, whereas
small muscles have few fibres per motor unit.
Once the motor unit is stimulated by a nerve impulse, then all the fibres in it
will contract simultaneously. If the nerve impulse is sufficiently strong enough
to initiate contraction, all the fibres in the unit will contract maximally. If the
nerve impulse is not strong enough, non of the muscle fibres will contract.
This is known as the ‘all or none’ law – contraction of a motor unit will occur
maximally or not at all.
Strength of Contraction
The human body possesses an amazing ability to perform tasks with the
appropriate amount of force, from gently wiping a bead of sweat from the
forehead with a finger, to throwing an opponent with maximal effort during a
judo match.
There are two ways in which the athlete can vary the strength of a muscle
contraction.
1. Vary the number of motor units recruited in a contraction
If a large number of motor units are stimulated to contract, a large portion of
the muscle will contract, thus producing a large, powerful movement. If only a
small number of motor units are stimulated to contract, only a small portion of
the muscle will contract, with a resultant smaller movement of less force.
2. Vary the frequency with which the motor units are activated
The frequency with which motor units are activated is a process called
summation.
Spatial summation
All the motor units in the muscle do not contract simultaneously, but they
‘share’ the workload by some motor units contracting while others are relaxed.
This spreads the ATP consumption throughout the muscle, and therefore the
fatigue is spread too.
Wave summation
For contraction to be sustained, continual impulses must be received. If a
second nerve impulse is received by the muscle very quickly after the first
one, there will not have been time for the first contraction to have been
completed and the muscle to have relaxed before the second nerve impulse
arrives. The effect of this is that the total contraction is increased, as the next
contraction will begin before the first one has finished. This accumulation of
contractions to make the overall contraction stronger will continue as long as
a nerve impulse is being received at the muscle, until maximal contraction or
fatigue.