CNS (brain and cord) and PNS (cranial / spinal nerves and ganglia)
ANS (autonomics) control homeostasis of
inner environment.
-
nerves are
basically either afferent, efferent, or interconnections.
Functional
unit = neuron. Fibres conduct impulses in one
direction; away from (axons) or towards (dendrites) cell body.
Similarly-functioning
cell bodies aggregate into nuclei
(in CNS) or ganglia
(outside CNS).
Processes from
aggregations run as tracts
(CNS) or nerves (PNS)
31 pairs: C1-8,
T1-12, L1-5, S1-5, 1 coccygeal.
Each formed by union
of dorsal & ventral roots
within intervertebral foramina.
-
Anterior
root contains efferent motor
fibres to skeletal muscle; T1-L2 and S2-4 also contain autonomic fibres.
-
Posterior
root contains sensory
afferent fibres with cell bodies in posterior root ganglion.
After forming, the spinal nerve divides into anterior
& posterior rami. Nerve plexuses (cervical, brachial,
lumbar and sacral) formed from anterior rami only; posterior rami donÕt form
plexuses.
Embryo
innervation never changes thereafter (cf blood supply); migrating structures
take their nerves Skeletal muscles are innervated by neuron pools: motor nerve
cell bodies in cranial nerve nuclei or in groups of anterior horn cells of
spinal cord.
Posterior
rami ¨ extensor muscles of spine & skull, also
some overlying skin.
Anterior rami ¨ all other trunk muscles, limbs & skin.
Divide into
medial & lateral branches; both supply muscle but only (either) one
supplies skin.
-
in the
upper half of body (to T6) the medial nerve ˆ skin, below that the lateral one does.
C1 has no
cutaneous innervation; posterior rami C7-8, L4-5 donÕt reach skin. All thoracic nerves do.
No posterior
ramus ever supplies limb skin
or muscle.
Motor: ¨ segmental supply to prevertebral flexors
eg scalenes, psoas, piriformis.
T1-12, L1 ¨ segmental supply to thoracic wall; lower
6 + L1 ¨ anterior abdominal wall.
For neck, only
C2,3,4 take part from cervical plexus; C5-T1 ¨ clothes upper limb
Between middle
& deepest of 3 muscle layers.
Nerves always superficial to arteries (spinal cord nearer to surface
than aorta). Spinal nerves pass
posterior to vertebral artery in neck, intercostal & lumbar arteries in
abdomen & lateral sacral artery in pelvis.
Sympathetic
trunk doesnÕt lie in neurovascular plane; runs vertically within arterial
circle.
The body wall is
supplied segmentally by spinal nerves: posterior strip supplied by posterior rami, lateral strip
by lateral branches of anterior rami, and ventral strip by anterior branches of
anterior rami.
All spinal
nerves carry postganglionic sympathetic fibres; mainly vasoconstrictor, also
sudo & pilomotor.
Limbs are supplied embryonically by lateral branches of anterior rami
Each limb has
flexor & extensor compartments, meeting at pre & post-axial borders of limbs. Veins mark these borders: cephalic (pre) and basilic (post)
in upper limb; LSV (pre) and SSV (post) in lower limb.
Nerves for
plexuses arise from enlargements of cord: cervical ¨ brachial plexus; lumbar ¨ lumbar & sacral plexuses. Every limb plexus divides into anterior
(¨ flexor compartment) & posterior (¨ extensor compartment) divisions.
Most caudal root
of limb plexus ¨ entirely
flexor (T1 in brachial plexus, S3 in sacral plexus).
Eg muscles
near pre/post-axial borders:
lateral portion of brachialis (radial nerve), short head of biceps femoris
(peroneal part of sciatic).
2 maps are
needed: one for skin supplied by a spinal nerve level (segmental, ie
dermatomes), & one by peripheral nerves. Adjacent dermatomes may overlap
considerably on the trunk.
Axial line = line of junction between discontinuous
spinal levels. Eg upper-limb anterior axial line: sternal angle ¨ over 2nd costal cartilage ¨ down middle of anterior forearm nearly
to wrist.
No overlap exists across axial lines ¨ best to test across sensation across
axial lines where overlap is less.
Limbs ÒborrowÓ
skin from the trunk \ some
innervation is from nerves outside the relevant limb plexus (C4 over deltoid,
T2 in axilla; T12, L1-3 in lower limb).
The lower
limb dermatome pattern is distorted: it is medially rotated and extended
from its fetal position
-
the
anterior axial line spirals from root of penis across scrotum, to midline back
of thigh and calf
-
a low
spinal may anaesthetise 2/3 of scrotum, but it takes 7 levels higher to get
anterior 1/3.
See Plate150 for dermatomes (interpret with
flexibility; and T1 does not supply trunk, S1 not buttock).
(1) Most
muscles are supplied equally by 2 adjacent segments of the cord
(2) Muscles
with a common 1¡ action on a
joint are supplied by the same segments
(3) Antagonists
sharing opposite action wrt (2) supplied by segments usually in numerical
sequence with muscles of (2)
(4) For
more distal joints the spinal centre is lower in the cord; count 1 lower for
each joint.
|
|
Hip |
Knee |
Ankle |
Flex
|
L2/3 |
L3/4 (extend) |
L4/5 |
|
Extend |
L4/5 |
L5/S1 (flex) |
S1/2 |
-
inversion and adduction are same as for flexion, eversion and abduction same as extension.
|
|
Shoulder |
Elbow |
Forearm |
Wrist |
Digits |
Hand |
Flex
|
C5 (add) |
C5,6 |
C6 (supinate) |
C6,7 |
C7,8 |
T1 (all) |
Extend
|
C6,7,8(abd) |
C7,8 |
C7,8 (pronate) |
C6,7 |
C7,8 |
|
-
these are
clearly less regular than the rules suggest (probably changed to allow finer
movements).
|
Muscles
& movement to best test each spinal cord level: |
|
C4
|
Diaphragm |
|
C5 |
Deltoid
(shoulder abduction) |
|
C6 |
Biceps (elbow
flexion, biceps jerk) |
|
C7 |
Triceps (elbow
extension, triceps jerk) |
|
C8 |
Fingers
(extension / flexion) |
|
T1 |
APB (abduct
thumb) |
|
T7-12 |
Anterior
abdominal wall |
|
L1 |
Lowest
internal oblique, transversus. |
|
L2 |
Psoas major
(hip flexion) |
|
L3 |
Quads (knee
extension, knee jerk) |
|
L4 |
Tibialis ant
& post (inversion) |
|
L5 |
EHL (extend
great toe) |
|
S1 |
Gastrocnemius
(plantarflex, ankle jerk) |
|
S2 |
Small foot
muscles |
|
S3 |
Perineal
muscles, anal reflex |
-
if the nerve root is damaged, sweating is normal, if
the root to the plexus is, sweating is lost
Planter reflex: UMN, normal if <1 yr old.
Abdominal reflex: muscle contraction on light stroking; absent in UMN lesions
Anal reflex: contraction of external sphincter on pinprick of perianal skin.
Bulbocavernosus reflex: sphincter contraction on squeezing glans penis.
1¡ difference between somatic &
autonomic pathways: somatic nerve fibres leaving CNS run uninterrupted to
muscles; autonomic pathways outside the CNS are interrupted by ganglionic
synapses.
Preganglionic
cell bodies are in CNS: parasympathetic
in CN nuclei or lateral
horn cells of sacral segments; sympathetic in lateral horn cells of thoracolumbar
segments.
Postganglionic
cell bodies are in
ganglia: parasympathetic in
4 cranial ganglia or in walls of target organs, sympathetic in ganglia of sympathetic trunk.
Preganglionic
fibres enter ganglia via white rami communicans ¨
(1) Synapse
with postganglionic cells in that ganglion, or
(2) Run
up/down trunk to synapse in another ganglion, or
(3) Pass
through that ganglion & synapse in a collateral ganglion eg coeliac
If they synapse
inside that ganglion (1), postganglionic fibres exit via grey rami
communicans (the more proximal of the two rami communicans) which pass
to spinal nerves as vaso-, sudo- & pilomotor fibres.
Theoretically
there is a ganglion for each spinal nerve, but they fuse
¨ 3 cervical,
11 thoracic, 4 lumbar & 4
sacral ganglia.
Every spinal
nerve receives a grey ramus
(postganglionic) hence fused ganglions may give off >1 branch
-
cervical,
lower lumbar and spinal ganglia do not have white rami communicans (no spinal
input)
Each ganglion
has a visceral branch:
Cervical (and
upper 4 thoracic) ¨ cardiac
branches to cardiac
plexus
Thoracic ¨ splanchnic nerves (greater 5-9, lesser 10-11, least 12) ¨ coeliac plexus
Upper lumbar ¨ Lumbar splanchnic n. ¨ superior hypogastric plexus
Sacral ¨ sacral splanchnic n. ¨ inferior hypogastric plexus
All trunk
ganglia also ¨ vascular
branches; cervical to internal & external carotid arteries (superior),
inferior thyroid artery (middle) and vertebral (stellate), thoracic to aorta & branches, lumbar to lateral sacral arteries.
Limb vessels get
their supply from adjacent peripheral nerves (supply does not run along the
artery).
(1) via
arteries and spinal nerves, or:
(2) from
viscera by hitching
along sympathetic nerves & plexuses
ˆ to cell
bodies in posterior root ganglia (via white rami communicans).
Supplies only
viscera, note however gonads
& adrenals have no
known parasympathetic supply.
Cranial preganglionic
fibres have cell bodies
in Edinger-Westphal nucleus, superior & inferior salivary nuclei &
dorsal motor nucleus of vagus.
Postganglionic
cell bodies for first 3
of these nuclei are in parasympathetic ganglia (see below); vagal
postganglionic cell bodies are in walls of viscera supplied.
Sacral
preganglionic cell bodies are
in lateral grey horn of S2-4; form the pelvic splanchnic nerves which enter inferior hypogastric plexus
& run with its branches to supply all cloacal derivatives & hindgut as
far as splenic flexure (postganglionic cell bodies are in the walls of these
viscera).
Run in facial,
glossopharyngeal & vagus nerves; cell bodies are in sensory ganglia of
respective nerves.
-
pass to the
tractus solitarius,
which connects to nearby brainstem for regulation of function.
Sacral
afferents run in pelvic
splanchnics; cell bodies are posterior root ganglia of S2-4.
-
some of
these make local reflexes (eg bladder emptying) others carry pain upwards.
Parasympathetic
(motor) root from
preganglionic cell in brainstem
Sensory root = peripheral process of cell body in trigeminal ganglion.
Sympathetic
root = postganglionic
fibre from superior cervical
ganglion travelling with ECA or ICA
Essential
root of all ganglia is motor one (these synapse in it, the others merely pass
through).
¨ Eyeball
(sphincter papillae and ciliary muscles)
Parasympathetic
root: via CN III; originates
in EW nucleus (by a branch to the inferior oblique muscle).
Sympathetic
root: fibres from
superior cervical ganglion (branch along ICA)
Sensory root: to cell bodies in trigeminal ganglion
(along branch of nasociliary nerve)
¨ lacrimal,
nasal & palatal glands
Parasympathetic
root: (motor) fibres
from superior salivary nucleus (via nerve of the pterygoid canal and the
greater petrosal nerve from the nervus intermedius part of facial nerve)
Sympathetic root: from superior cervical ganglion (via
ICA, deep petrosal nerve and the nerve of the pterygoid canal)
Sensory root: to cell bodies in trigeminal ganglion
(via maxillary nerve branches)
-
a few
fibres also carry taste,
follow the greater petrosal nerve (cell bodies in geniculate ganglion), ie via CN
VII
¨
submandibular & sublingual glands
Parasympathetic
root: from superior
salivary nucleus (via nervus intermedius, facial nerve, chorda tympani, lingual
nerve.)
Sympathetic
root: from superior
cervical ganglion (fibres run with the facial artery)
Sensory root: cell bodies in trigeminal ganglion (via
branch of lingual nerve)
¨ parotid
gland
Parasympathetic
root: from inferior
salivary nucleus via glossopharyngeal nerve and its tympanic branch to the
tympanic plexus than to the lesser petrosal nerve.
* Additional
motor root from nerve to
medial pterygoid passes through otic ganglion without synapse to supply tensor
palati & tensor tympani.
Sympathetic
root: from superior cervical ganglion (fibres
run with middle meningeal artery)
Sensory root: cell bodies in trigeminal ganglion (via
auriculotemporal nerve).