Skeletal Muscle
The most highly specialised for contraction. Consists of large fibres:
multinucleate with relatively prominent cross-striations
Innervation. Each fibre receives a single axon of an alpha motor
neurone forming an endplate; a specialised form of neuromuscular
junction, characterised by much folding of the post-synaptic
(muscle) surface to give a large surface area for transmission.
One spinal motor neurone can innervate 100s of muscle fibres —
the motor unit. There are many motor units in a muscle. Hence
increasing power due to ‘recruitment’.
Function. Fast, powerful, ‘all or nothing’ (‘twitch’) contraction.
Muscle Fibre-Types. Two main types of skeletal muscle fibre are
recognised with varying physiological properties according to
enzyme profiles. The principal enzymes used are succinic
dehydrogenase (SDH), a mitochondrial oxidative enzyme and
myofibrillar ATPase (mATPase). The Table below summarises the
enzyme and physiological differences between Type I and Type II
fibres. The fibre-type is determined largely by the neurone
innervating it. The constituent fibres of a motor unit are scattered
throughout a muscle (‘mosaic’ not ‘random’). The property of an
anatomical muscle depends upon the proportions of I and II.
Table. Some Properties of Types I and II Muscle Fibres
Enzyme activity
Type I
Type
II
mATPase activity (anaerobic)
low
high
Oxidative capacity (aerobic)
high
low
Speed of contraction
slow
fast
NERVE
Adapted to the conduction of electrical impulses (action potentials).
Transmission between one nerve terminal and another or an end-organ
is by chemical transmission across the synaptic gap.
It is important to distinguish between anatomical nerves (with names);
small nerves and nerve bundles (no names); nerve cell bodies; and
nerve processes (nerve fibres, axons, dendrites).