Dr. Niranjan Murthy H L Asst Prof of Physiology SSMC, Tumkur
NERVOUS SYSTEM 1) Cent Centra rall nerv nervou ous s syst system em-- Brai Brain n & Spin Spinal al cord 2) Per Periphe iphera rall nerv nervou ous s sys systtem
E) Neur Neuron on-- func functi tion onal al unit unit F) Neuroglia
STRUCTURE OF NEURON •
Nerve cell with all it’s processes is neuron
Parts:I. Axon Axon-i) 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
conduction velocity (m/s) 70-120 70- 120
motor Aβ
touch, pressure
5-12
30-70
Aγ
motor to muscle spindle
3-6
15-30 15- 30
Aδ
pain, temperature,
2-5
12-30
<3
3-15 3- 15
touch B
preganglionic autonomic
C
i) Dorsal root- pain, touch, 0.4-1.2 ii) postganglionic
0.3-1.3 0.3- 1.3
0.5-2 0.7-2.3
VI. Numerical classification Number Ia
origin
fiber type
Muscle spindle,
Aα
annulospiral ending Ib
Golgi tendon organ
Aα
II
Muscle spindle, flower-spray
Aβ
ending, touch, pressure III
Pain, temperature, touch
IV
Pain
root ‘C’ fibers
Aδ dorsal
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
Myelinogenesis-Myelinogenesis Inside 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 m yelination
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 viii)) Elec Electr trom omag agne netic tic it can also be classified into subliminal, minimal (threshold), sub-maximal and maximal, depending on the strength of stimulus.
STRENGTH-DURATION CURVE
T G N E R T S
2 X RHEOBASE
RHEOBASE
CHRONAXI
UTILISATION TIME
E
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) vii) viii) viii)
Temperature Mechanical pressure Blood supply Chemicals pH Ions Size Size of the the ner nerve ve Myelinatio Myelination n
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 otential
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, s lowly, 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.
