Nerve Physiology

Dr. Niranjan Murthy H L Asst Prof of Physiology SSMC, Tumkur

NERVOUS SYSTEM 1) Central nervous system- Brain & Spinal cord 2) Peripheral nervous system

E) Neuron- functional unit F) Neuroglia

STRUCTURE OF NEURON •

Nerve cell with all it’s processes is neuron

Parts:I. Axoni) generally long ii) arises from axon hillock iii) axis cylinder has axoplasm, neurofibrils & mitochondria iv) axons end in terminal buttons v) carry impulses away from cell body II. Dendrite:i) multiple & short ii) contain nissl granules iii) carry impulses towards soma

III. Cell body:- Neurocyton or Soma i) Nucleus- pale, large, spherical, central ii) Neuroplasm- has neurofibrils, nissl granules, mitochondria, golgi apparatus, neurosecretory material

CLASSIFICATION OF NEURONS I.

(a) Golgi bottle type I (b) Golgi bottle type II II. Anatomic classificationa) Unipolar b) Pseudounipolar c) Bipolar d) Multipolar e) Apolar

III. Physio-anatomic classificationa) afferent i) somatic ii) visceral b) efferent i) somatic ii) visceral IV. Depending on myelination a) myelinated b) unmyelinated

V. Erlanger-Gasser’s Classification:Type Function diameter (μm) Aα

proprioception, somatic 12-20 motor Aβ touch, pressure 5-12 Aγ motor to muscle spindle 3-6 Aδ pain, temperature, 2-5 touch B preganglionic autonomic <3 C i) Dorsal root- pain, touch, 0.4-1.2 ii) postganglionic 0.3-1.3

conduction velocity (m/s) 70-120 30-70 15-30 12-30 3-15 0.5-2 0.7-2.3

VI. Numerical classification Number origin fiber type Ia Muscle spindle, Aα annulospiral ending Ib Golgi tendon organ Aα II Muscle spindle, flower-spray A β ending, touch, pressure III Pain, temperature, touch Aδ IV Pain dorsal root ‘C’ fibers

MYELINATION Nerve cells in grey matter are naked. As they enter white matter they acquire myelin sheath. As the nerve leaves CNS it acquires neurolemma (sheath of schwann) Myelin sheatha protein-lipid complex Envelops the axon except at its ending & at nodes of ranvier

MyelinogenesisInside CNS myelin is produced by oligodendroglia & outside CNS by schwann cells Schwann cell wraps around axon up to 100times. This is compacted by apposition of protein zero. Nodes of ranvier are periodic 1μm constrictions which are 1mm apart where there is no myelination

PROPERTIES OF NERVE •

EXCITABILITY- it’s the ability of a cell to produce action potential in response to a stimulus. action potential- it’s a self-propagating change in potential across a cell membrane.

LOCAL RESPONSE

ELECTROTONIC POTENTIAL ACTION POTENTIAL Produced due to application of subthreshold stimulus

Produced due application of threshold stimulus

It is a local response

Propagative type of response

It is a graded response

All or nothing response

It has no latent period

It has a latent period

It has no refractory period

It has a refractory period

Not affected by hypoxia, anaesthesia

Not produced during hypoxia, anaesthesia

Stimulus- it’s a change in environment which brings about a change in potential across a membrane in an excitable tissue Types of stimuliiv) Electrical v) Chemical vi) Thermal vii) Mechanical viii) Electromagnetic it can also be classified into subliminal, minimal (threshold), sub-maximal and maximal, depending on the strength of stimulus.

STRENGTH

STRENGTH-DURATION CURVE

2 X RHEOBASE

RHEOBASE

CHRONAXI E

UTILISATION TIME

TIME

RHEOBASE- minimum current required to produce action potential. UTILIZATION TIME- time taken for response when rheobase current is applied. CHRONAXIE- time taken for response when twice rheobase current is applied. It is a measure of excitability of tissues.

Factors affecting excitability • • • • • •

Temperature Mechanical pressure Blood supply Chemicals- CO2 & narcotics pH- increased excitability in alkaline and reduced excitability in acidic media. Ions- Na+, Mg++ and K+ are neuroexcitatory and Ca++ is neurosedative

II. CONDUCTIVITY Action potential is self-propagative Conduction may orthodromic or antedromic In axon, conduction is towards terminal buttons physiologically. In myelinated nerves, conduction is saltatory type.

STIMULUS

+ + + + + + + + + + + + + + + + + + + + + -+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + - - - - - + + + + + + + + + + - - - - - - - - - - + + + + + - - - - - - - - - - - - - - - - - - - - - - + + + + + - - - - - - - - - - + + + + + + + + - - - - - -

+ + + + + + + + +

Factors affecting conductivity i) ii) iii) iv) v) vi) vii) viii)

Temperature Mechanical pressure Blood supply Chemicals pH Ions Size of the nerve Myelination

IONIC BASIS OF EXCITATION & CONDUCTION Resting membrane potentialmainly due to leaky K+ channels( -70mv) Action potentialit has depolarization, repolarization, after-depolarization and afterhyperpolarization phases. It is mainly due to Na+ and K+ conductance.

Catelectrotonic current Surface becomes less positive Reduced potential difference b/w inside & outside Opening of voltage-gated Na+ channels Rapid influx of Na+ Potential increases towards Na+ equilibrium potential

Na+ channels enter inactivated state in few milliseconds Slow opening of voltage-gated K+ channel Efflux of K+ ions repolarization

III. ALL OR NONE RESPONSE The action potential doesn’t occur in a nerve if the stimulus is sub-threshold. If the stimulus is threshold and above, the action potential produced will be of same amplitude, regardless of intensity of stimulus. * The frequency of action potential increases with the increasing intensity of stimulus.

IV.REFRACTORY PERIOD 1) Absolute refractory periodit is the period during an action potential, during which a second stimulus can’t produce a second response. 4) Relative refractory periodit is the period during an action potential, during which a stimulus of higher intensity can produce a second response

V.ACCOMODATION • When a stimulus is applied very slowly, no matter however strong it might be, it fails to produce an action potential. • Cause: a slowly applied stimulus causes slower opening of Na+ channels with concomitant opening of K+ channels. The influx Na+ of is balanced by efflux of K+ .

COMPOUND ACTION POTENTIAL • Multi-peaked action potential recorded from a mixed nerve bundle is called a compound action potential.