This document discusses the physiology of nerve fibers and spinal anesthesia. It begins by describing the different types of nerve fibers (A, B, C fibers), their functions, and myelination. It then explains the mechanism of spinal anesthesia, including how local anesthetics act in the subarachnoid space to block nerve fibers. Finally, it discusses factors that can affect the level and duration of the spinal blockade, such as drug properties, patient factors, and procedural details.

1.  Physiology of nerve fibers
 Mechanism of action
 Systemic effects
 Types of spinal anesthesia
 Monitoring of blockade
 Factors affecting spinal anesthesia

2.  Neurons are basic elements of all rapid signal
processing within the body
 Afferent nerve fiber: transmit impulse from
peripheral receptors to CNS
 Efferent nerve fiber: CNS to periphery

3. On the basis of :fiber DM and velocity of conduction
(larger DM larger conduction)
 Type A ,B ,C
 Type A further divided into α/β/γ/δ

4.  Type A-α1a innervate muscle spindle
 Type A-α1b innervate Golgi tendon organ
both A- α afferents are important for muscle reflex
and muscle tone control (Proprioception)
 Type A- β transmit touch nd pressure signal from
cutaneous mechanoreceptors(meisners nd pacinian
corpuscles)
 Type A- γ for skeletal muscle tone
 Type A- δ Transmit touch and fast pain and
temperature(warm) From free nerve ending

5.  Type B are autonomic preganglionic
sympathetic fibers
 Type C transmit slow pain and
temperature(cold) , Postganglionic
sympathetic fibers

6.  Type A &B are myelinated and Type C
unmyelinated
 Order of Dm and velocity A>B >C
 Sensory information from the somatic
segments of the body enters the spinal cord
through the dorsal roots of the spinal nerves

8.  Spinal Anesthesia in which the injection of
small amounts of local anesthetics into the
subarachnoid space (CSF) at the level below
(L2),where the spinal cord ends, anesthesia of
the lower body part below the umbilicus is
achieved

9.  Pharmacokinetics :is what the body does to a
drug , absorption ,distribution
,metabolism,elimination
 Pharmacodynamics: is what the drug does to
the body

10.  Large length of the nerve roots and nerves are
bathed in CSF
 Rootlets, anterior and posterior spinal root
nerves and periphery of spinal cord
 Less drug diffuse into DRG and centre of spinal
cord
 Spinal anesthesia is achieved with a small dose
and volume of LA resulting in dense sensory and
motor block.

11.  Lipophilic drugs act at whiter mater ( amides
& ester)
 Hydrophilic drugs act at grey mater ( quat.
Ammonium derivative of lidocaine)
 Clonidine induces hyperpolarization at the
ventral horn of the spinal cord
 Receptor for adenosine and potassium
channel openers are also demonstrated.

12.  Blockage of the anterior nerve root fibers
result in blockage of efferent fibers(somatic
and autonomic fibers)
 Blockade of posterior nerve root fibers result
in blockade of sensory and visceral impulses

13.  Speed of blockade depends on size,surface
area&degree of myelination of nerve fiber
 Smaller nerves more sensitive to LA(high
surface area) less myelination more sensitive
 Concentration of LA highest at point of
injection , then LA travels away by diluted
with CSF

15.  Relative sensitivity to the effect of LA on nerve
fibers observed in block of height is called
differential blockade
 ie sympathetic blockade is 2-6 dermatome
segment higher than sensory(order of more
cephalad -cold -pinprick-touch )
 Sensory is 2 dermatome higher than motor

16.  Anatomic and geometric arrangement of
individual fibers in a nerve bundle
 DM of nerve fibers
 Difference in firing rate of fibers
 Variability in longitudinal spread of LA
 Effects of ion channel
 Choice of LA

17.  Drug in CSF diffuse through pia mater & penetrate
through space of virchow robin (extension of
subarachnoid space accompanying blood vessels that
invaginate spinal cord from pia mater )to reach DRG
 Portion of drug diffuse outward through arachnoid
and dura mater
 Some taken up by blood vessels of pia and dura

18.  Primary mechanism :diffusion of drug from
high concentration to low
Elimination
 Depends on nonneural tissue uptake (vascular)
 Time for regression inversely correlated with
csf volume
 Greater spread of drug expose to larger
vascular absorption so short duration of action
 Lipid soluble LA bind to epidural fat to form
depot , so slow vascular absorption

19.  sympathetic nervous system (SNS) is described as thoracolumbar
since sympathetic fibers exit the spinal cord from T1 to L2.
(cardioaccelator fibers are T1 to T4 )
 Main neurotransmiter is norepinephrin and adrenal medulla secrete
epinephrine in sympathetic stimulation
 The parasympathetic nervous system (PNS) has been described as
craniosacral since parasympathetic fibers exit in the cranial and sacral
sacral regions of the CNS.

20.  Sympathetic fibers are small, myelinated, and easily
blocked. During neuraxial blockade, sympathetic
block occur prior to sensory, followed by motor.
 End result of neuraxial blockade is a decreased
sympathetic and somatic (sensory and motor) with an
unopposed parasympathetic tone & compensatory
reflexes

21.  Neuraxial blockade can impact the
cardiovascular system by causing the
following changes:
1.Decrease in blood pressure (33%
incidence of hypotension in non-
obstetric populations)
2.Decrease in heart rate (13% incidence of
bradycardia in non-obstetric
populations)
3. Decrease in stroke volume
4. Decrease in cardiac contractility

22.  Cardiac receptors are liked to CNS through
vagus and glossopharyngeal nerve
 Cardiac receptors are in atria , ventricle
,pericardium , coronary arteries, great vessels
and carotid arteries

23.  Baroreceptor reflex (carotid sinus reflex )
 Chemoreceptor reflex
 Bainbridge reflex
 Bezold-jarisch reflex
 Cushing reflex
 Oculocardiac reflex

24.  Its responsible for acute maintenance of blood pressure
 It’s a negative feedback mechanism
 Receptors are located in carotid sinus and aortic arch
 Afferents are glossopharyngeal and vagus nerves
 Central control by nucleus tractus solitarius located in medulla
 Cardiovascular center in medulla consists of two areas, areas for
increasing BP and decreasing BP
 When SBP is high , decreased sympathetic activity and thus
decrease HR ,contractility and vascular tone and also activation
of PNS

25.  Chemosensitive cells are located in the carotid bodies and
aortic bodies
 They are sensitive to PH , PaO2 , pCO2
 paO2 less than 50 mmhg or in acidosis send signal to medulla
through 9th or 10 th CN
 Chemosensitive area in medulla stimulating respiratory
centers and increase ventilatory drive

26.  Its elicited by stretch receptors in right atria
and cavoatrial junction
 Increase in right sided filling pressure send
vagal afferent signal to cvs centre in medulla
 Which inhibit parasympathetic activty so HR
increase and increase CO

27. Indirect effects:-
 Bain bridge reflex
predominates during spinal
anaesthesia: venous pooling in
the periphery reduces
stimulation of volume receptors
- diminishes the action of
cardiac sympathetic nerves-
vagal preponderance -
bradycardia
 Oxygen consumption is
reduced due to hypotension
and muscle relaxation

28.  Its responds to noxious ventricular stimuli
 It’s a cardioprotective reflex
 Receptors: Chemoreceptors and
mechanoreceptors on the left ventricular wall
 in severe hypotension receptors are activated
send signal through vagus to stimulate
parasymapthetic activity and mainatain cardiac
blood flow by bradycardia ,coronary artery
dilatation and hypotension

31. Triad of
hypertension ,
bradycardia,
irregular breathing
 Marey's law :
baroreceptors in the carotid sinus and the aortic arch
normally respond to a fall in blood pressure by
producing a compensatory tachycardia

32.  Sympathectomy results in Venous dilatation and arteriolar
dilatation
 The venous system contains about 75% of the total blood
volume while the arterial system contains about 25%.and
the arterial system is able to maintain much of its vascular
tone. So venodilation is predominate
 Reduce preload and afterload
 Reduce stroke volume
 Result in hypotension
 Cardiac output is slightly reduced or maintained

33.  Vasodilatory changes depends on patient baseline
sympathetic tone
 High sympathetic tone in elderly so affects more
 Total peripheral vascular resistance in the normal patient
(normal cardiac output and normovolemic) will decrease
15-18%.
 In the elderly the systemic vascular resistance may
decrease as much as 25% with a 10% decrease in cardiac
output.

34.  Heart rate may decrease in high neuraxial block due to
blockade of cardioaccelerator fibers arising from T1-T4
 Extensive sympathectomy in T5-L2 and reduction in
venous return and right atrial filling causes increase in
parasympathetic activity(revers bainbridge reflex) will
reduce HR but hypotension causes baroreceptor
sympathtic response above the level of blockade
 So two opposing responses are result in minimal change
in HR in T5-L2 blockade

35.  If it extends to T1 level ,result in blockade of
cardioaccelerator fibers , in addition to marked
hypotension (bezold-jarisch reflex) and unopposed
parasympathetic stimulation are all causes severe
bradycardia and asystole
 So spinal anesthesia in presence of hypovolemia may
leads to circulatory collapse

36.  Baseline HR <60
 High ASA grade
 Use of beta blockers
 Sensory level above T6
 Age <50yrs
 Prolonged PR interval

37.  Decrease in coronary blood flow from 153 to
74ml/100gm/mnt
 Decrease in MAP 119 to 62 mmhg
 Percent of extraction of myocardial oxygen
remain unchanged

38.  Hypotension may decreases regional cerebral
blood flow (CBF)especially in elderly and
hypertensive patients

39.  Neuraxial blockade plays a very minor role in
altering pulmonary function
 Even with high thoracic levels of blockade, tidal
volume is unchanged
 Vital capacity decreases by small amount due
to paralysis of abdominal muscle required for
forced exhalation

40.  phrenic nerve is innervated by C3-C5 and is
responsible for the diaphragm. phrenic nerve is
extremely hard to block, even with a high spinal
 Blockade of abdominal and intercostal muscle is
compensated by diapragm and accessory muscle
 Precaution should taken in respiratory diseased
patients , pregnancy and obese
 Respiratory arrest may happen if hypoperfusion of
brainstem

41.  Changes are due to sympathetic blockade
and compensatory vagal parasympathetic
activation
 GI hyper peristalsis
 Contracted gut –it helps excellent surgical
condition
 Hepatic blood flow decreased

42.  Neuraxial blockade has little effect on the
blood flow to the renal system.
 Autoregulation maintains adequate blood
flow to the kidneys as long as perfusion
pressure is maintained.
 Neuraxial blockade effectively blocks
sympathetic and parasympathetic control
of the bladder at the lumbar and sacral
levels.

43.  Urinary retention can occur due to the loss
of autonomic bladder control. Detrusor
function of the bladder is blocked by local
anesthetics.

44.  Peripheral vasodilatation results in fall in core
temperature results in nonbehavioral
shivering response
 Behavioral responses like try to cover body or
going to universal flexion are not seen due to
somatic blockade

45. Continuous spinal anesthesia
Unilateral spinal anesthesia
Selective spinal anesthesia

46.  It allows incremental dosing of local
anesthetic and predictable titration of block
to an appropriate level
 It have better hemodynamic stability than
single shot spinal
 Used in severe AS , pregnant women with
heart disease or morbid obese,

47.  Needle with laterally facing opening is
preferred (hustead or tuohy needle)
 Catheter is inserted and threaded 2-3 cm into
subarachnoid space

48. Unilateral spinal anesthesia & Selective spinal
anesthesia
 Both are small dose technique
 Depends on baricity and patient positioning
 U/L used in knee arthroscopy , unilateral
inguinal hernia
 In selective spinal anesthetizing only sensory
fibers to a specific area

49.  For assesing sympethetic blockade : use HR and
blood pressure
 For sensory: cold sensation(c-fibers) and pinprick
sensation (A delta fibers) are used more than
mechanical stimuli like touch , pressure(A beta
fibers )
 Loss of sensation to cold occurs first & verified
using ice, ethyl chloride spray ,or alcohol
 Followed by loss of pinprick &verified using a
needle

50.  Dermatomal level, most cephalad is cold
then pinprick,then touch
For motor block: modified bromage scale is
used
0 No motor block
1 inability to raise extended leg ; able to move knees
and feet
2 inability to raise extended leg and move knee; able to
move feet
3 complete block of motor limb

51.  Dermatomal level of
peritoneum is T4
bladder is T10
uterus is T10
Skin incision for these structures are more down as
compared to spinal segment

52.  Upper abdominal surgery : T4
 Cesarean delivery :T4
 TURP: T10
 Hip surgry :T10
 Foot and ankle surgery :L2

53.  Drug factors
 Patient factors
 Procedural factors

54.  Dose
 Volume
 Concentration
 Temperature
 Baricity

55.  It is the ratio of density of local anesthetic
solution to the density of CSF
 Density: is mass per unit volume (gm/ml )of
solution at specific temperature
 Density varies inversely with temperature
 specific gravity : ratio of density of a solution
to the density of water
 No units for baricity and s.gravity

56.  Density of csf : 1.00059g/L
 ISOBARIC: LA have same density as csf
 HYPERBARIC :LA have more denser than csf
 HYPOBARIC:LA have lower density than csf
 To make hyperbaric add dextrose to LA
 To make hypobaric add sterile water to LA

57.  Hyperbaric drug spread to dependent region
of spinal canal
 Hypobaric : nondependent part
 Isobaric not influenced by gravity and it settle
at the site of infiltration
 Hyperbaric drug in lateral decubitus position
settle on dependent side

58.  Patient in sitting position, hyperbaric drug inject
into L3-L4/L4-L5 spaces and immediately kept in
supine or lateral position drug will rapidly spread to
thoracic kyphosis and sacral kyphosis
 If patient kept in sitting position more sacral fibers
are involved so saddle anesthesia for perineal
surgeries
 If patient into lithotomy position lumarlordosis
disappear so drug spread cephalad
 If hypobaric drug injected in sitting position drug
will go cephalad
 Isobaric gives segmental block

59.  More reliable determinent is dose of drug

60.  CSF volume
 Advanced age
 Pregnancy
 Weight
 Height(length of lower limb bone more important
for height than vertebral column so less
important for spread of drug)
 Spinal anatomy
 Intra abdominal pressure
 Menopause
 Gender

61.  CSF volume and spread of LA is inversely related
 In obese and pregnancy increased abdominal mass
and epidural fat ,may decrease csf volume so
increase spread
 Csf density varies with sex ,menopausal
status,pregnancy
 In elderly csf volume decreased and nerve roots
more sensitive so more spread

62.  Gender
csf is denser in men so reducing baricity of LA
so less cephalad spread
in lateral position broader shoulders of males
relative to hip helps more headup
 Spinal anatomy :kyphosis more spread
scoliosis less changes
 Pregnancy loss of lumbar lordosis , less
volume of csf ,progestron mediated neuronal
sesitivity increases spread

63.  Patient position
 Epidural injection post spinal
 Level of injection
 Fluid currents
 Needle orifice direction
 Needle type

64.  Position should not be affect truly isobaric drug
 Spreading of drug stop within 20-25 mnts so
positioning is important
 10 degree headup reduce the spread
 Trendelenberg position increase spread (loss of
lordosis )
 Sitting position helps in saddle block

65.  Cephalad orifice of needle tip increase spread
 More Unilateral block achieved if orifice on one side
 Injection rate and barbotage (repeated aspiration and
reinjection) not affect
 Slower injection prevent leakage and also increase spread
 Injection of saline or LA into epidural space after spinal
increase the block height