Properties of nerve fiber by Pandian M, Dept Physiology DYPMCKOP, this ppt for MBBS, BPTH and all other Pre and Para medicos and biomedicos

Describe the types, functions & properties of nerve fibres
3.2.1 Classify nerve fibres
3.2.2 Classify nerve fibres based on the diameter & conduction velocity
3.2.3 Describe the salient features of Erlanger & Gasser
classification of nerve fibres
3.2.4 State the functions of type A, B & C nerve fibres
3.2.5 Compare & contrast the numerical classification with the
Erlanger & Gasser classification in the sensory nerve fibres

1. Pandian M Dept. of Physiology DYPMCKOP PROPERTIES OF NERVE FIBRES - 1

2. PY 3.2 Describe the types, functions & 3.2.1 Classify nerve fibres properties of nerve fibres 3.2.2 Classify nerve fibres based on the diameter & conduction velocity 3.2.3 Describe the salient features of Erlanger & Gasser classification of nerve fibres 3.2.4 State the functions of type A, B & C nerve fibres 3.2.5 Compare & contrast the numerical classification with the Erlanger & Gasser classification in the sensory nerve fibres

3. Structure of nerve fibre

4. NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM PERIPHERAL NERVOUS SYSTEM Brain Spinal Cord Visceral or Autonomic nervous system Somatic nervous system Sympathetic nervous system Parasympathetic nervous system Classification of nerve (supply skeletal muscles) (supply internal organs)

5. Depending upon functions: Sensory nerve fibers (afferent nerve fiber) Motor nerve fibers (efferent nerve fibers) Depending upon secretion of neurotransmitter Adrenergic nerve fibers Cholinergic nerve fibers Depending upon diameter and conductions of impulse (Erlanger- gasser classification) Classified into three major groups: Type A nerve fibers Type B nerve fibers Type C nerve fibers

6. Erlanger and Gasser Classification Fiber types Fiber Diameter (µm) Conduction velocity (m/s) Function A  12-20 70-120 Somatic motor and Proprioception A  5-12 30-70 Touch-Pressure A  3-6 15-30 Motor to muscle spindle A  2-5 12-30 Pain, Cold and Touch B <3 3-15 Autonomic preganglionic fibers C-Dorsal root Fibers 0.4-1.2 0.5-2 Somatic sensations C- sympathe tic fibers 0.3-1.3 0.7-2.3 Postganglionic sympathetic fibers

7. Among these fibers, type A nerve fibers are the thickest fibers and type C nerve fibers are the thinnest fibers. Type C fibers are also known as Type IV fibers. Except type C fibers, all the nerve fibers are myelinated. Type A nerve fibers are divided into four types: a. Type A alpha or Type I nerve fibers b. Type A beta or Type II nerve fibers c. Type A gamma nerve fibers d. Type A delta or Type III nerve fibers.

8. Numerical Classification Types Origin Fiber Types Ia Primary spindle afferent from from annulospiral endings A  Ib From Golgi tendon organ A  II Secondary spindle afferent from flower spray endings also from Touch and pressure receptors A  III Pain and Cold receptors A  IV Pain and Temperature C fibers in Dorsal root

9. PROPERTIES OF NERVE FIBRE 1. Excitability Sub threshold stimulus Threshold stimulus Suprathreshold 2. Conductivity 3.  Unfatigability 4. Refractory period 5. All or none response 6. Summation 7. Accommodation

10. Excitability Excitability, i.e. the capability of generating electrical impulses (action potential). Excitability is that property of the nerve fibre by virtue of which it responds to changes in the external or internal environments. (electrical impulses or the so-called action potentials) when it is stimulated by a suitable stimulus which may be mechanical, thermal, chemical or electrical.

11. Skeletal muscle cannot contract without a signal from the somatic nervous system Signal arrives through a motor nerve, communicated through a synapse called myoneural or neuromuscular junction

12. ELECTROTONIC POTENTIAL & LOCAL RESPONSE When nerve fibre is stimulated by subthreshold stimulus the A.P. is not produced but there do occur some changes in the RMP. These local, non-propagated response called E.P. Two types depending upon the nature- 1. Catelectrotonic potential- localised depolarizing changes in RMP when subthreshold stimulus applied with cathode. 2. Anelectrotonic potential- localised hyper-polarizing changes in RMP when subthreshold stimulus applied with anode.

13. GRADED POTENTIAL when potential changes with the subthreshold stimulus it is called generator potential/ Graded potential. Properties of G.P.- It is local phenomenon. Increase in magnitude observed in stepwise with increase in strength of stimuli. Decremental conduction- it decay progressively with time & distance Depolarizing or Hyperpolarizing Summation .

14. Factors affecting the excitability Strength and duration (SD Curve) of the stimulus Effect of extracellular Ca 2+ Decrease in ECF Ca 2+ ↑es excitability of neuron by ↓ ing the Tp. ↑se in ECF Ca 2+ stabilizes the membrane by ↓ ing excitability.

15. What are threshold, subthreshold and suprathreshold stimuli? a) Threshold (minimal) stimulus : Minimum strength of stimulus required to elicit a response is called as threshold stimulus. b) Subthreshold (subminimal) stimulus : A stimulus whose strength is less than that of threshold stimulus is called subthreshold stimulus. c) Suprathreshold stimulus : A stimulus whose strength is more than that of threshold stimulus is called suprathreshold stimulus.

16. STRENGTH DURATION CURVE. Wednesday, December 7, 2016

17. Define rheobase, utilization time and chronaxie Rheobase : Strength of current just sufficient to excite a tissue when applied for indefinite period of time is called Rheobase. Utilization time : The minimum time required to elicit a response with strength equal to Rheobase is known as utilization time. Chronaxie : The minimum time required to excite a tissue when the current strength is twice a rheobase is called chronaxie.

18. ALL or NONE RESPONSE. Wednesday, December 7, 2016    When a stimulus of sub threshold intensity is applied then no AP is produced. If threshold stimulus applied response in the form of spike AP. If we increase strength of stimulus more than threshold no increase in magnitude of AP is observed.

19. ALL OR NONE LAW

20. MEMBRANE EXCITABILITY DURING ACTION POTENTIAL Wednesday, December 7, 2016 Depending on response to stimulus, period of AP is divided into REFRACTORY PERIOD (RP) Absolute refractory period (ARP) Relative refractory period (RRP)

21. REFRACTORY PERIOD In this period at which the muscle or nerve does not show any response to a stimulus. It is because already one action potential is in progress in the muscle during this period. The muscle is unexcitable to further stimulation until it is repolarized. Refractory period is of two types. 1. Absolute refractory period 2. Relative refractory period

22. 1. Absolute Refractory Period (ARP) ARP is the period during which the nerve or muscle does not show any response at all, From firing levels to 1/3 rd of Repolarization. Whatever may be the ( 2 nd stimulus) strength of stimulus.

23. Refractory period

24. IONIC BASIS Wednesday, December 7, 2016   During upstroke, m gates of Na + channels are opened rapidly & D uring Repolarization channels are closed by closure of inactivation gates (h) gates of Na + channels. These channels will not reopens until potential comes back to resting levels

25. 2. Relative Refractory Period Relative refractory period is the period, during which the muscle shows some response if the strength of stimulus is suprathershold .

26. Ionic basis Wednesday, December 7, 2016 During this Na + channels are coming out of inactivation stage & K + channels are still opened. Stronger stimulus open more Na + channels through m gates & produce response.

27. Refractory Period in Skeletal Muscle In skeletal muscle, whole of the latent period is refractory period. The absolute refractory period falls during first half of latent period (0.005 sec). And, relative refractory period extends during second half of latent period (0.005 sec). Totally, it is 0.01 sec.

28. Long Refractory Period in Cardiac Muscle In cardiac muscle, absolute refractory period extends throughout contraction period (0.27 sec). And, Relative refractory period extends during diastole period (about 0.26 sec). Totally it is about 0.53 sec. Thus, the refractory period in cardiac muscle is very long compared to that of skeletal muscle.

29. Significance of long refractory period in cardiac muscle Because of the long refractory period, cardiac muscle does not show: i . Complete summation of contractions ii. Fatigue iii. Tetanus.

30. SUMMATION When one subliminal stimulus is applied, it does not produce any response in the nerve fiber because, the subliminal stimulus is very weak. However, if two or more subliminal stimuli are applied within a short interval of about 0.5 millisecond, the response is produced. It is because the subliminal stimuli are summed up together to become strong enough to produce the response. This phenomenon is known as summation.

31. DIFFERENCE BETWEEN GP & AP Graded potential Action potential Amplitude Proportion to strength of stimulus Once threshold stimulus is reached amplitude remain same Conduction Travel in decremental fashion, gradually decrese with time & distance Conducted in all or none manner Summation Can be summated Cannot be Nature Can be a depolarize & hyperpolarize Always a large depolarize Mechanism Due to opening of ligand gated or leaky ion channels Due to opening of voltage gated ion channels Properties Does not have threshold or refractory period Have both Example Receptor membrane potential in sensory nerve endings Motor End Plate potential Action potential of nerve fibre , cardiac and skeletal muscle

32. ACCOMMODATION Wednesday, December 7, 2016 While stimulating a nerve fiber continuously, the excitability of the nerve fiber is greater in the beginning. Later the response decreases slowly and finally the nerve fiber does not show any response at all. This phenomenon of adaptation to the stimuli is called accommodation

33. IONIC BASIS OF ACCOMMODATION. Cause for Adaptation When a nerve fiber is stimulated continuously, Depolarization occurs continuously. Continuous depolarization inactivates the sodium pump and increases the efflux of potassium ions. Wednesday, December 7, 2016

34. INFATIGUABILITY. Wednesday, December 7, 2016 Nerve fibre cannot fatigued due to its absolute refractory period. Nerve fiber cannot be fatigued, even if it is stimulated continuously for a long time. The reason is that nerve fiber can conduct only one action potential at a time. At that time, it is completely refractory and does not conduct another action potential.

35. Conductivity Conductivity, i.e. the ability of propagating the electrical impulses generated along the entire length of nerve fibres. In unmyelinated fibres In myelinated fibres Factor affecting velocity of conduction

36. AXOPLASMIC TRANSPORT

37. Functions of Neuron

38. ZONES OF THE NEURON From the functional point each neuron is divided into four Zones 1. Receptor zone (dendritic zone) 2. Site of origin of conducted impulse 3. Zone of all or none transmission 4. Zone of secretion of transmitter (nerve endings).

39. Receptor zone (dendritic zone) is the region, where local Potential changes are generated by integration of the Synaptic connections. 2. Site of origin of conducted impulse is the site, where Propagated AP are generated . In case of Spinal motor neuron , initial segment and in cutaneous Sensory neurons first node of ranvier is the site of origin of conducted impulses.

40. One-way propagation of an action potential. For simplicity, potentials are shown only on the upper membrane, local currents are shown only on the inside of the membrane, & repolarizing currents are not shown. Action potential is initiated in region 1, and local currents depolarize region 2.

41. Action potential in region 2 generates local currents; region 3 is depolarized toward threshold, but region 1 is refractory.

42. Action potential in region 3 generates local currents, but region 2 is refractory.

43. 3. Zone of all or none transmission in the neuron is the Axon. 4. Zone of secretion of transmitter (nerve endings). The Propagated impulses (action potential) to nerve endings Cause the release of neurotransmitter .

44. Functional organization of neurons

45. Factor affecting conductivity Temerature RMP Firing level or level of threshold potential Diameter of the nerve Myelination of nerve Resistance offered by extracellular fluid Concentration of ions

46. Recording Of A.P.

47. MONOPHASIC- when one electrode is inside & other is outside of cell

48. BIPHASIC- When both electrode out or intracellular

49. COMPOUND A.P.- in mixed nerve like a monophagic

50. With each AP very small difference in conc. of Na + & K + in ICF & ECF. Types of AP – Spike potential- in nerve and skeletal muscle Plateau potential – in cardiac muscle Slow potential – in smooth muscle

51. Referred :- Text book of Medical Physiology Guyton, 13 th edition, Text book of Medical Physiology Indu khurana , Text book of Medical Physiology Vander’s Text book of Medical Physiology Sembulingam & LPR

52. Thank You