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 heart
• 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
  • tendon
  • aponeurosis
  • fleshy

  
  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 or stasis.

A muscle trying to initiate contraction is opposed by

1. passive internal resistance of muscle
2. ditto articular tissues
3. opposing muscles
4. opposing soft tissues
5. inertia of whatever it is trying to move
6. load
7. gravity

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.

Muscle mechanics
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 you are:

                              CNS                         PNS

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.

1. 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
2. a connector neuron whose body is in the dorsal horn and synapses in turn with
3. 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.

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