PHYS20008 Lecture Notes - Lecture 10: Alpha Motor Neuron, Skeletal Muscle, Myocyte
12 Jun 2018
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Lecture 10
PHYS20008 - HUMAN PHYSIOLOGY
LECTURE 10
SKELETAL MUSCLE
SKELETAL MUSCLE
•Skeletal muscle cells are excitable, can
fire APs (action potentials), and firing an
AP causes it to contract.
•Same principles about APs we’ve already
learned apply here.
•A motor unit refers to a single alpha
motor neuron and all the muscle fibres it
innervates.
•Skeletal muscle is voluntary (whereas
cardiac muscles aren’t for example).
•A single alpha motor neuron goes into
the muscle and branches out to innervate
a bunch of muscle fibers. Muscles have
multiple motor units.
•Eg. Red alpha motor neuron comes
down and innervates red muscle
fibres.
•Smallest units recruited first, largest last.
•We increase the force of a muscle contraction
not by getting individual fibres to contract
harder, but by recruiting addition motor units.
•The muscle fibres are in parallel, so we can contract a small proportion and the rest will move too
in cohesion.
•Motor units have different flavours.
•Fast twitch/slow twitch. There are about 15 different fast twitch fibres.
•1 unit will only have one type of twitch fibres. Eg. The red one above will be all fast or all
slow.
•Slow twitch fibres are more fatigue resistant, while fast twitch fibres are more powerful.
•Eg. Core muscles would want slow twitch ones because they need to be working all the time.
•Eg. Hand muscles would want fast twitch ones because they need to do fancy shit.
MUSCLES: MADE OF PARALLEL FIBRES
•Reggie’s muscles are organised as such:
•Bundles of muscle fibres. Muscle fibre =
muscle cell. One cell with a membrane used
to fire APs.
•All human skeletal muscles are a mixed
collection of more than one fibre type (in
contrast to other vertebrate species).
•These fibre types differ in rate of
myofibrillar ATPase, identity of
contractile and regulatory proteins as
well as proteins of the SR and T-
tubules.
•The most basic division of fibre types is
made on twitch and fatigue
characteristics.
•Single unit of a muscle contraction is a
twitch. Rarely do we do this !
Lecture 10
PHYS20008 - HUMAN PHYSIOLOGY
!
deliberately.
•Slow twitch fibres have a high density of
capillaries. High twitch have less.
•Therefore lots of blood coming into slow
twitch fibres, which is why high twitch
fatigue easily.
CONTRACTILE CHARACTERISTIC OF
MAMMALIAN MOTOR UNITS
BASIC STRUCTURE OF MUSCLE FIBRES
•Inside the muscle fibre
we have
individual
smaller fibres
called
myofibrils,
which are lined
in parallel and
surrounded by
sarcoplasmic
reticulum,
encasing each
myofibrils. The
SR contains
high Ca2+
volumes. The
release of Ca2+
from the SR is
what leads to a
contraction.
•Ca2+ is
released from the
SR and binds to these filaments and the tiny little purple bits (myosin heads?) ratchet/latch? on
the string of pearls yellow filament. This pulls the ends closer together, making the muscle shorter,
making it contract.
