Download Learning Objectives

DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16
This material will NOT be covered in the clinical skills session, so it is
essential you read these notes before then.
Key concepts
To follow the clinical skills session relating to the Peripheral Nervous system, it is
essential that you understand the difference between a spinal nerve (sometimes
also known as a nerve root or a spinal segment) and a peripheral nerve (often just
called “a nerve”).
Dermatomes and myotomes
Dermatomes are areas of the skin innervated by sensory fibres from individual spinal
nerve. Dermatomes are named after the nerve root which provides sensation e.g. C7
(middle finger area). Likewise myotomes are muscle/muscle groups supplied by motor
fibres from a single nerve root e.g. L5 (big toe extension). You will learn more about
myotomes in APE 4 ‘Muscles and Tendons’ session.
I.
Overview of embryological development
You can only really understand the nervous system if you know about the early
development of the body.
In a similar design to very primitive animals, the early human embryo is
also divided into segments. Along the back of the embryo, alongside
the developing spinal cord, you can see paired blocks of tissue known
as somites. The overlying skin is also in segments, though these are
not visible.
Each pair of somites will give rise as development proceeds to:
 a block of muscle (Myotome)
 connective tissue & dermis (Dermatome)
Each somite has a single link to the developing spinal cord, and this
link will become a SPINAL NERVE ROOT. Therefore, all organs &
tissues that subsequently develop from a particular somite will have a
nerve supply from that individual nerve root. Even if the area of skin is
stretched to a distant area of the body it will retain its link to the original
nerve root.
1
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16

With further development the pattern becomes a little more complicated. The
developing muscles and skin of each segment divide first into a dorsal block and a
ventral block.

Spinal nerves originally arise from the spinal cord as rootlets; the anterior rootlets
converge to form the anterior nerve root and the posterior rootlets converge to
form the posterior nerve root.





The anterior (ventral) nerve root comprises efferent (motor ) fibres from
the cell bodies anterior horn spinal cord gray matter to supply peripheral
muscles and effector organs;
The posterior (dorsal) nerve root comprises afferent (sensory) fibres
from cell bodies in the dorsal root ganglion (also known as a spinal sensory
ganglion) and links peripherally to sensory nerve endings and centrally to
the posterior horn of spinal cord (gray matter).
Anterior and posterior nerve roots unite outside the spinal cord, just
proximal to the intervertebral foramen, to form a mixed spinal nerve-i.e.
one that contains motor (efferent) and sensory (afferent) fibres.
The mixed spinal nerve root divides next into a dorsal primary ((posterior)
ramus & ventral primary (anterior) ramus after leaving the spinal canal.
Ramus=branch
o Do not confuse the rami with the dorsal root and the ventral root –
the roots lie within the spinal canal and combine proximal to the
division we are talking about here.)
The Dorsal Primary Ramus & Ventral Primary Ramus each therefore contain
both afferent (sensory) & efferent (motor) fibres.


The dorsal primary ramus provides the motor and sensory supply to the
posterior trunk,
The ventral primary ramus provides motor and sensory supply to the
anterior trunk & limbs.
2
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16

Ventral primary ramus
This pattern is straight forward in the thoracic
region where each dorsal primary ramus supplies
segmental muscles posterior to the vertebral
column and the overlying skin, and each ventral
primary ramus supplies a single intercostal space
with its muscles and the strip of overlying skin
ventrally (e.g. over chest).
Dorsal primary ramus
SUMMARY:


Spinal nerve contain afferent (sensory) & efferent (motor) & nerve fibres.
o The afferent fibres within the spinal nerve detect sensation from an
area of the skin known as a dermatome.
o The efferent fibres within the nerve supply muscle groups known as a
myotome.
Each nerve root (lie within the spinal canal) divides into a ventral & dorsal
primary ramus(branches)
o The primary ventral ramus contains nerves that relate to the anterior
trunk area and limbs
o The primary dorsal ramus contains nerves relating to the posterior
trunk area.
3
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16
II. Limb development:

Limbs develop from the ventral muscles and are therefore supplied by the ventral
primary ramus of these respective spinal nerve roots:
 Arms (C5 – T1)
 Legs (L1 – S2)

The limbs are initially small "buds" but get gradually longer. Each limb bud will
have a central core of tissue that will become bone and a transverse partition
separating a flexor muscle group compartment from an extensor muscle
group compartment:
Flexors
(anterior)
Extensors
(posterior)

The ventral primary rami (plural of ramus = rami) that supply and are linked to
he segments comprising the new limb, divide again:
 Anterior divisions of the ventral primary rami supply the flexor
compartment of muscles.
 Posterior divisions of the ventral primary rami supply the extensor
compartment of muscles.
Anterior division
(flexors)
Posterior division
(Extensors)
4
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16
III. Peripheral Nerves

The anterior and posterior divisions of each nerve root do NOT continue to run
separately down their compartment of the limb; instead they bunch into
peripheral nerves. E.g. ulnar nerve, femoral nerve etc.

The plexus is where the different nerve root components are assembled and
form the various peripheral nerves that supply the upper (brachial) & lower limbs
( sacrolumbar) respectively. You will study the brachial plexus and sacrolumbar
plexus in the dissection room sessions (DR).

Most peripheral nerves that originate from a plexus will therefore contain fibres
from multiple spinal nerve roots.

Each nerve root is still responsible for its dermatome and myotome, but a
peripheral nerve supply area covers multiple dermatomes.

A peripheral nerve will also usually contain both afferent and efferent fibres i.e.
incoming (afferent =sensory) and outgoing (efferent = motor) messages.

A peripheral nerve will not mix anterior and posterior divisions of the ventral
primary ramus.

A peripheral nerve will normally supply either an extensor OR a flexor
compartment of muscles.
SUMMARY:
 Nerve roots divide and then anterior and posterior divisions combine in the
brachial plexus & sacro-lumbar plexus to form the peripheral nerves.
 Examples of peripheral nerves from the brachial plexus include median, ulnar
and radial nerve, and those from the sacrolumbar plexus include the femoral and
sciatic nerve.
 A peripheral nerve will contain fibres from several nerve roots.
 A peripheral nerve usually also contains both afferent and efferent fibres.
 A peripheral nerve will not mix anterior and posterior divisions of the primary
ventral ramus.
 Therefore, a peripheral nerve will normally supply either an extensor or a flexor
compartment.
5
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16
IV. Rotations-caudal and cranial concepts
To understand the position of muscle compartments and dermatomes in humans, you
need to consider the design differences between a lizard, a dog and a human. You will
recognise similar patterns, but with different rotations.
The difference between a lizard and a four-legged mammal like a dog is that the front
limb has rotated backwards (leaving the extensors facing caudally towards the tail) and
the hind limb forwards (with the extensors therefore facing cranially, towards the lizard’s
head).
6
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16
In humans the arms are also extended and supinated to the anatomic position and the
legs are extended at the hip and the knee.
Look at the orientation of corresponding parts of your upper and lower limbs e.g.
the thumb and the big toe, the knee and the elbow, the palm and the sole and work
out where the extensor and flexor compartments now lie in the upper and lower limbs.
We will return to this when we consider muscles in APE 4: Muscles and Tendons.
As a guide- extensors (shaded above) generally lie posteriorly e.g. triceps muscle
extends the forearm at the elbow joint , gluteus maximus extends the lower limb at the
hip (along with other hip extensors); flexor group muscles usually lie ventrally e.g. biceps
brachii muscle flexes the forearm at the elbow joint, quadriceps muscle flexes the knee
in the lower limb.
7
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16
The original linear dermatomes become stretched over the developing limb. As the limb
gets steadily longer some dermatomes will come to lie only on the distal limb (arm of
leg), leaving a discontinuity of segments on the trunk.
Now think about the effect of the design differences discussed above on the orientation
of the dermatomes.
 You will appreciate that embryologically, the big toe was originally cranial to
the little toe– as the thumb was, and remained cranial to the little finger.
The thumb and the big toe are therefore supplied by cranial dermatomes (C6 and
L5 respectively-see dermatome diagram activity 1).
 The extensor aspect of the lower limb has swung right round to face anteriorly,
so the dermatomes will spiral round inwardly on the limb rather than running in
parallel strips as in the upper limb (which has not rotated).
Embryological segments, and thus the corresponding spinal nerve root, are
represented on both anterior and posterior surfaces and in both flexor and
extensor muscle compartments.
8
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16
Clinical relevance of the difference between peripheral and spinal nerves

If you or injure a single nerve root it will result in potential pain, numbness or
weakness in the distribution of the nerve root i.e. the dermatome the nerve root
supplies. Early signs of this are "pins and needles" (paraesthesia). It is therefore
important to be able to know the nerve roots that supply each dermatome,
so that if you identify dermatomal patterns of deficit, you can deduce which
nerve roots are affected by pathology.

If, on the other hand, you injure a peripheral nerve the deficit will include
multiple dermatomes and / or myotomes but will usually be confined to either
the extensor or the flexor compartment of muscles. (The ulnar nerve is unusual
in that it wraps around onto the extensor surface a bit although it is an anterior
divisions -flexor compartment nerve.)
It may be helpful to think of the wiring in your house as an analogy.
 Imagine that you need each room to be supplied with lighting, power, telephone,
TV and computer.
 These services will be routed into the distribution box of the house separately
from the central supplier (spinal cord) – like different spinal nerve roots.
 The distribution box then acts like a limb plexus, splitting up each service into a
component to each room.
 All the different services to the same room will be bunched together in a single
conduit going in the required direction like a peripheral nerve.
o If your telephone is cut off because you didn't pay your bill the TV and
the lights will still work.
o If you drill through a conduit (peripheral nerve) doing DIY all the services
to that room will go out but they will all work in the rest of the house.
Remember that:
 Damage at central nervous system level (Spinal cord, nerve roots) affects
ALL muscles and dermatomes supplied by the damaged nerve roots i.e.
the more proximal the damage, the more widespread the effect/deficit
(spinal cord =central supplier in house analogy).
 Damage to the limb plexus (distributor box in house analogy) will affect
selected myotomes.
 Damage at peripheral nerve level (conduit in house analogy) will affect
multiple dermatomes and the muscle compartment(s) supplied by the
damaged peripheral nerve.
9
DSL PREREADING MATERIAL FOR APE 3: The Peripheral
Nervous System-key concepts 2015-16
References:
Moore L, Agur M R, Dalley A F (2010) Clinically Oriented Anatomy, sixth edition
Lippincott Williams & Wilkins
Acknowledgments
With thanks to Nicholas Bisson, Final Year Medical Student on SSC Medical
Education 2014 for his invaluable editorial input on this material.
10