1.2: Nervous System

 

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)

Spinal Nerves

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. 

Principles of nerve supply

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. 

Nerve Supply of Body Wall

Posterior rami ¨ extensor muscles of spine & skull, also some overlying skin.

Anterior rami ¨ all other trunk muscles, limbs & skin. 

Posterior Rami

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.

Anterior Rami

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 

Neurovascular Plane (Pl 179)

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.

Sympathetic Fibres

All spinal nerves carry postganglionic sympathetic fibres; mainly vasoconstrictor, also sudo & pilomotor. 

Nerve supply of limbs

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. 

Limb Plexuses

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). 

Double Innervation

Eg muscles near pre/post-axial borders: lateral portion of brachialis (radial nerve), short head of biceps femoris (peroneal part of sciatic). 

Segmental Innervation of Skin

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).

Segmental Innervation of   Muscle

Need to know: i) what is the supply of this muscle; ii) what muscles supplied by this cord segment.

Principles:

(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

Spinal reflexes

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.

Autonomic Nervous System

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.

Sympathetic Nervous System

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.

Sympathetic Trunk

Theoretically there is a ganglion for each spinal nerve, but they fuse

¨ 3 cervical, 11 thoracic,  4 lumbar & 4 sacral ganglia. 

 (a) Efferent pathways

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).

(b) Afferent pathways

(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).

Parasympathetic Nervous System

Supplies only viscera, note however gonads & adrenals have no known parasympathetic supply. 

(a) Efferent pathways

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).

(b) Afferent pathways

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.

 

Plan of ganglia:

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).

 

Ciliary Ganglion (126)

¨ 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)

 

Pterygopalatine ganglion (127)

¨ 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 ganglion (127)

¨ 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)

 

Otic ganglion (128)

¨ 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).