nervous system

Spinal nerves

makeup

Formation

· formed by union of ventral and dorsal root

· ventral root carries efferent motor fibres

· dorsal root carries afferent sensory fibres

· union occurs in intervertebral foramen

· proximal to union is dorsal root ganglion

· is site of sensory nerve cell bodies

Division

· immediately after formation divides into anterior and posterior ramus

body wall

Posterior rami

· supply extensor musculature of trunk and overlying skin segmentally

· each nerve divides into medial and lateral branch

· both branches supply muscle

· only one branch reaches skin

Anterior rami

· supply flexor musculature of trunk segmentally

· rami of T1-L1 supply body wall segmentally

Limbs

Limb plexuses

· formed from anterior rami

· nerves become combined and rearranged in plexuses

Autonomic nerves

· all have

· preganglionic fibres with cell body in CNS

· postganglionic fibres with cell body in ganglion

sympathetic nervous system

Efferent fibres

· preganglionic fibres leave cord in anterior nerve roots of T1 to L2

· enter sympathetic ganglia

· enter via white ramus communicans

· fibres myelinated

· fibres may

· synapse with postganglionic cells in ganglion they have entered

· travel in sympathetic trunk and synapse with postganglionic cell in another ganglion

· leave ganglion and synapse in collateral ganglion

· fibres leave sympathetic ganglion to spinal nerve via grey ramus communicans

· fibres unmyelinated

· are fibres that will be distributed to

· blood vessels (vasomotor - cause vasoconstriction)

· sweat glands (sudomotor - cause sweating)

· arrector pili muscles (pilomotor - cause hair erection)

· every spinal nerve C1-S5 receives grey ramus

· only T1 to L2 have white ramus

Sympathetic ganglia

· sympathetic trunk extends along vertebral column

· from base of skull to coccyx

· theoretically each nerve has ganglion

· some have fused

· C1-4 - superior cervical ganglion

· C5,6 - middle cervical ganglion

· C7,8 - inferior cervical ganglion

· inferior cervical ganglion + T1 - stellate ganglion

· each sympathetic trunk ganglion has visceral branch

· cardiac branch in cervical region

· splanchnic branch elsewhere

· visceral branches descend to sympathetic visceral plexuses

· cervical, coeliac, superior and inferior hypogastric

· visceral plexuses joined by parasympathetic fibres

· fibres reach viscera by hitchhiking along arteries

· branches from all 3 cervical ganglia descend to cervical plexus

· branches from lower 3 thoracic ganglia descend to coeliac plexus

· branches from upper lumbar ganglia descend to superior hypogastric plexus

· sympathetic fibres for upper limb originate from T2 to T6

· sympathetic fibres for lower limb originate from T11 to L2

Sympathectomy

· in cervical sympathectomy for upper limb, T2 and T3 ganglia and rami and intervening trunk resected

· if T1 resected, Horner’s sydrome develops

· in lumbar sympathectomy, L3 and L4 ganglia and rami and intervening trunk resected

· if L1 resected, ejaculation compromised

parasympathetic nervous system

· distributed only to viscera

· not to limbs

· originate

· cranially

· sacrally

Stretch reflex

stretch receptor

· muscle spindle

· consists of

· capsule

· intrafusal muscle fibres (2-10)

· sensory and motor fibres

Intrafusal fibres

· specialised muscle fibres

· 2 types

1. nuclear bag fibre

· parallel to extrafusal fibres

2. nuclear chain fibre

· connect to sides of nuclear bag fibres

Sensory fibres

· 2 sensory fibres

· end on both bags and chains

1. primary (A alpha, Ia)

· annulospiral

· dynamic response (stretch and rate of stretch)

2. secondary (A alpha, Ib)

· flowerspray

· static response

· cell body in DRG

· synapses directly on anterior horn cell of extrafusal fibres (A alpha) of same muscle

Motor fibres

· gamma efferent fibres (A alpha)

· have motor endings on bag and chain fibres

· adjust length of muscle spindle

function

Passive stretch (DTR)

· sensory endings on muscle spindle distorted

· causes afferent sensory signal

· stimulates anterior horn cell (monosynaptic)

· causes muscle contraction

Sensitivity (reinforcement)

· dictated by muscle spindle length

· stimulation of gamma efferents causes contraction and shortening of intrafusal fibres

· results in increased sensitivity

· gamma efferents stimulated by central descending tracts

Central mediation

· alters gamma efferent discharge

· baseline central inhibition to dampen effects of change in muscle length

· gamma efferent discharge can be increased by

· anxiety

· stimulation of skin by noxious stimuli

· explains

1. flexor response to noxious stimulus

· selective contraction of flexors and inhibition of extensors)

2. reinforcement of reflexes

· increased afferent discharge from hands causes centrally mediated increase in gamma efferent discharge

· causes increased sensitivity of muscle spindles