Muscles & Nerves
Dr. D.R.Johnson, Centre for Human Biology
Having looked at the bones of the skeleton and the joints
between them we logically move on to consider the muscles which
move the bones and then the nerves which control them. Muscle,
however does other things besides moving bones. It is a contractile
tissue, divided on histological structure into three types:
- skeletal or striated. Under direct (Voluntary) nervous control
- cardiac, also striated but specialised and confined to the
- smooth or visceral. Not under direct (voluntary) nervous
control. In walls of alimentary tract, blood vessels, arrector
pili - slow and sustained response.
Form and function
- Smooth muscle usually occurs in flat sheets, sometimes wrapped
around a viscus like a gut in circular and longitudinal layers,
or arranged as a sphincter to close off a tube (as in the anus).
- Skeletal: so called because often attached to bone, but not
invariably. This is what the layman thinks of as a muscle. Muscle
means little mouse in Latin, from the fancied resemblance of
the muscle body contracting beneath the skin. This contractile
body, the muscle proper, is usually attached to two bony points.
Attachments may be
Tendons are an integral part of muscle, virtually invariable
in length. Made of collagen fibres with occasional flattened
fibroblasts it is wonderfully boring stuff which resists stretch
and is flexible, so it can turn corners. Because it is so avascular
it appears white in life and heals very slowly: this is why damage
to the large tendon in the heel, the Achilles tendon is so crippling
for a sportsman (as it was, incidentally, for Achilles).
Tendons take the form of cords or strips, circular in cross section,
oval or flattened. They are made up of bundles (fascicles) of
collagen fibres, mainly parallel and often large enough to see
with the naked eye, and striated in appearance. Around the outside
is an epitendineum with elastic fibres, which obviously causes
a little drag as tendons run through connective tissue. Where
they have to move independently of other tissues various friction
reducing devices are used. The tendon may run over cartilage,
or over a sesamoid bone such as the patella, or a bursa may be
interposed. This bursa may be elongated and folded around the
tendon to form a sheath. A very flattened tendon is often called
an aponeurosis neurosis because it is white like nervous tissue.
This usually has the appearance of a flattened sheet of collagen
fibres, or often several sheets running on each other in different
directions like plywood.
A fleshy insertion is what it says, muscle joined to bone without
the intervention of a frank collagenous tendon or aponeurosis.
The collagen is still there, but in amongst the muscle fibres,
or forming a very short tendon
Origins and insertions
Muscles are often said to have an origin at one end and an insertion
at the other. The origin (the one that moves least on contraction)
is often proximal, the insertion distal. Often a muscle arises
from more than one place: it is then said to have two or more
heads (biceps, triceps). In some circumstances origin and insertion
can be interchanged, so it is easier to talk of attachments.
Forms of muscles
Wide functional variation in terms of size and shape according
to job done. The size of the functional component, the muscle
fibre varies from 10-60*m in diameter, and mm to 15-30 cm in
length. Diameter, length and arrangement of fascicles (bundles
of fibres) varies from muscle to muscle: fine bundles in precision
muscles, coarse ones in power muscles. Fascicles may be parallel,
or oblique or spiral according to position of attachments. Lets
look at some variants and see if we can explain them.
The simplest is probably the strap muscle which has a
fleshy, wide attachment at each end. We can make this long and
narrow, so long as the maximum length of the muscle fibre is
not exceeded. If it is, we need fibres in parallel, with tendinous
insertions between groups. The range of contraction depends on
the length of a muscle but its power depends on how many fibres
we can pack in. Strap muscles thus have a good range but low
power: to get more power we make the muscle fusiform i.e. three
dimensional. This often transforms the flat attachment into a
tendon with a circular cross section. The muscle fibres are often
concentrated at one end, but will work just as well if they are
digastric i.e. have two bellies. another way to increase power
is to produce more heads, in effect two or three or four muscles
pulling the same tendon.
Having more than one head results in muscle fibres pulling obliquely
on the tendon. This can often balance out, but in a unipennate
muscle, where fibres insert all along one side of a tendon the
resultant force is the result of two vectors: sideways force
is cancelled out in a bipennate or multipennate arrangement.
Multipennates are fairly common compound muscles with a short
range but plenty of power.
Spiralised muscles are a special case which not only pull
the attachments together when they contract but try to untwist.
A similar twisting is sometimes arranged by wrapping the course
of a muscle around a bone.
Action of muscles
Muscles do not suddenly snap from a state of relaxation to one
of contraction. At a given time some functional units (motor
units, groups of fibres of various size) will be contracting,
some relaxing and some in stasis, the resultant providing muscle
tone. If the proportions doing each stays constant, so will muscle
tone, although individual units will cycle.
When an individual fibre contacts it tends to approximate its
ends, but whether or not this results in contraction depends
on the force generated and the forces opposing contraction. The
net result for the whole muscle may be contraction, relaxation
A muscle trying to initiate contraction is opposed by
- passive internal resistance of muscle
- ditto articular tissues
- opposing muscles
- opposing soft tissues
- inertia of whatever it is trying to move
If the force generated exceeds the sum of all these then the
limb is accelerated from rest: once moving a smaller force will
keep it moving. A muscle doing this is sometimes called a prime
mover or agonist. It is often opposed by antagonists
which can stop the movement. When both groups act together nothing
moves, or the movement is moderated or controlled. If the movement
is abolished the real result is that the joint across which the
muscles act will be stabilised - often cannot be done wholly
by close packing or gravity - which are preferable as they use
little energy. Movement is always opposed or aided by gravity,
and this is used wherever possible. In placing a weight on a
table the extensor of the arm is not triceps but gravity, controlled
by slow relaxation of the flexors.
The action of a prime mover often exerts a little unwanted movement.
For example the flexion of the fingers by long flexors also flexes
the wrists: this is opposed by wrist extensors.
In a simple arrangement of two bones joined by a muscle the pull
of the muscle can be resolved into
a. swing - tending to move the mobile bone
b. shunt - compressing the joint
c. spin - rotating the mobile bone
The relative size of each component is varied by moving the
attachments of the muscle. Obviously the largest swing is best
for initiating movement - spurt muscle: large shunt will allow
a mobile bone to be loaded by compressing the joint and large
spin can be used for prime movement or as a synergistic soaker
up of unwanted rotation.
Nerves and muscles
Control of muscle action is by the nervous system. Nerves are
in touch with muscles by motor end plates which convey a stimulus
to contract. They are also in touch with tendons via receptors
which measure stretch and connective tissue by other receptors
for pressure. These are referred to as efferent, if they are
taking a message to a tissue and afferent if they are taking
a message to the spinal cord and hence the brain. The brain and
spinal cord is referred to as the central nervous system (CNS),
and the connections as the peripheral nervous system (PNS). Different
terminology applies for similar structure according to where
bundle of axons tract nerve
accumulation of cell bodies nucleus (brain) ganglion
grey matter (spinal cord)
myelinating glial cell oligodendrocyte Schwann cell
The peripheral nervous system is thus largely made up of nerves
collections of cell processes, their insulating sheathes of myelin
and the cells that secrete it, Schwann cells and connective tissue.
The cell processes are referred to as efferent, if they are taking
a message to a tissue and afferent if they are taking a message
to the spinal cord and hence the brain. Almost all nerves contain
a mixture of afferent and efferent cell processes. At intervals,
ganglia, collections of cell bodies can be found. The CNS is
made up of the specialised brain, and less specialised spinal
cord. The spinal cord is made up largely of tracts, their insulating
sheaths of myelin and the cells that secrete it, oligodendrocytes.
In places nuclei, or accumulations of grey matter (collections
of cell bodies) can be found.
Central nervous system
Lets look first at the central nervous system.
Brain. Many invertebrate and vertebrate animals have a
nerve cord running from anterior to posterior. Almost invariably
this is larger at the anterior end, where sense organs are situated.
In vertebrates three sets of paired sense organs, covering smell,
sight and hearing/balance are located anteriorly and dorsally.
Corresponding to these are three outpushings of the roof of the
brain containing groups of nerve cells. The primitive smell brain,
sight brain and hearing brain have undergone many changes but
are still recognisable in man. The smell brain has become the
cerebral hemispheres, the sight brain the tectum (less important
in mammals than birds) and the hearing brain the cerebellum.
Interestingly in the ventral midline a downpushing produces the
infundibular stalk - the link to the endocrine system.
Spinal cord The spinal cord is made up of two distinct
regions, grey matter and white matter which look like this. The
white matter is around the outside and the grey forms an H in
the middle. The grey matter is made up of accumulations of the
bodies of conducting cells, neurones. These cells have long processes
which may pass up and down the cord or out into the peripheral
nervous system. Through both white and grey matter run different
types of non-conducting glial cells, which provide nutrients
and wrap nerve fibres in the cord with myelin. The white matter
is made up of nerve cell processes, axons, wrapped in myelin
which appears white in fresh tissue: confusingly most fat stains
turn the white matter black.
Because there are afferent and efferent cell processes we
need a way in and out of the cord. The way in is dorsal, the
dorsal root which is a continuation of the top limb of the H
in each spinal segment. The way out, the ventral root is not
seen on this diagram because it is at a different level and tends
to be a series of rootlets. If we want to see them both at once
we have to make the section thicker. It is convenient in many
cases to have the wiring to and from a particular part of the
body running together, so the two roots join to form a mixed
spinal nerve which runs round beneath each rib in the thorax.
We will come to the bump in the dorsal root in a moment. The
spinal nerve covers a whole body segment, sending off motor sensory
or mixed branches as it does so.
We can now put in the simplest possible wiring diagram. This
consists of three neurons.
- a sensory neuron. The cell body of this is in the dorsal
root ganglion, the dorsal swelling I mentioned earlier, made
up of thousands of similar cell bodies. A long process comes
from a sense organ in the skin and a shorter one runs to the
spinal cord. Once in the spinal cord the possibilities are endless:
it could go up to the brain or down to another spinal level,
but lets keep it simple and say it synapses with
- a connector neuron whose body is in the dorsal horn and synapses
in turn with
- a motor neuron whose body is in the ventral horn, and whose
axon passes out along the ventral root to a muscle, which contracts.
So far so good, but we have only dealt with connection to
skeletal muscle. Smooth muscle is wired rather differently via
the autonomic nervous system. This is in two parts, sympathetic
and parasympathetic. Lets deal with the sympathetic and
look at a sense organ in a tendon. This uses all the familiar
bits of the circuit plus another part the sympathetic chain.
The sympathetic chains, or sympathetic trunks are made up of
(originally ) segmental ganglia and run antero-medially to the
spinal cord. Connections are nowadays made only in some parts
of the body - our example is thoracic, between T1 and L2.
The same three neurons are used, but in slightly different places.
The sensory neuron is exactly the same except that it synapses
not in the dorsal horn, but laterally. The thousands of synapses
make up a lateral horn in the thorax. The connector neuron
is longer, passing out of the cord and into the sympathetic ganglion
via the white ramus communicans.
There are three possible variants from here.
Firstly: the connector neuron synapses with the motor
cell body, and the axon leaves via the grey ramus communicans
to the smooth muscle.
Secondly: the connector neurone may travel along the cord
and synapse in another ganglion: it then leaves, not by a spinal
nerve but by a special pathway to travel with blood vessels to,
say, the heart.
Thirdly: the connector neurone does not synapse but runs
out of the sympathetic chain as part of a splanchnic nerve whose
preganglionic fibres synapse in a ganglion near the aorta before
supplying abdominal viscera. So only the proportions of the three
neurones and the location of their cell bodies have changed.
Outside the area served by direct connections to the spinal cord
the sympathetic fibres have to run up or down the sympathetic
chain first. The other part of the autonomic nervous system,
the parasympathetic, is more specialised. Whereas all the body
receives voluntary and sympathetic fibres parasympathetic innervation
is restricted to some viscera (excepting the adrenal and gonad).
Their connector neurones are restricted to certain levels in
the brain and to S 2,3,4. You will learn about this in detail
elsewhere in the course.
Return to Human Biology Course Notes
This page is maintained by Steve