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Evolution of Nervous System

� History of evolution goes millions & millions & millions of years back

✁ From an unicellular organism to the most complex form

✁ All systems evolved

✁ Nervous System evolved very rapidly and is perhaps still evolving.

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Brain Body Wight Ratio

Animal Brain Wt. Body Wt. Ratio

Whale 9 kg. 90,000 kg. 1: 10,000

Elephant 6 kg. 6,000 kg. 1: 1,000

Man 1.5 kg. 70 kg. 1: 50

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Organization of Nervous System

Two parts

� Central N.S. Occupies the central axis of body

✁ Peripheral N.S.

✂ Somatic N.S.

✂ Autonomic N.S.

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Central Nervous System

Includes :

� Brain

1 Fore brain

2 Mid brain

3 Hind brain

� Spinal cord

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1 Fore brain

� Telencephalon

✁ Cerebral hemispheres

✂ Diencephalon

✄ Thalamus

✁ Hypothalamus

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2 Mid brain

� Ventral part

✁ Tegmentum, substantia nigra & basis peduncle

✂ Dorsal part

✄ Tectum constituting superior & inferior colliculi

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3 Hind brain

� Includes

✁ Pons

✁ Medulla Oblongata

✂ Cerebellum

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� Brain stem

✁ A commonly used term includes

✂ Midbrain

✂ Pons

✄ Medulla

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Spinal Cord

� Begins from foramen magnum & terminates at lower border of I lumbar vertebra. Below this it forms Cauda Equina

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Cross Section of spinal cord

� In the brain stem & spinal cord

✁ White matter surrounds the grey matter

✂ In the cerebrum & cerebellum

✄ Grey matter surrounds the white matter

✂ Fatty myelin gives white appearance

� Cell bodies of neurons provide a grey colour (ash colour) to grey matter

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✄ �✁ Post. Horn---- Sensory

✂ Anterior Horn-Motor (☎ ,✆ motor neurons & Renshaw cells)

✂ Intermediate Horn-Sympathetic fibers

BELL- MAGENDIE LAW :

In spinal cord Dorsal roots are sensory & Ventral roots are motor

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Peripheral Nervous System

� Somatic N.S.

✁ Cranial Nerves

✁ Spinal Nerves

� Autonomic N.S.

✁ Symp.

✁ Parasymp.

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Cranial nerves

12 pairs, cell bodies are in the brain

� Sensory✄ I, II & VIII

� Motor-----XI & XII

� Mixed-----III, IV, V, VI,VII, IX & X

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Spinal Nerves

31 pairs

� Cervical-----8 pairs

� Thoracic----12 ,,

� Lumbar-----5 ,,

� Sacral-------5 ,,

✁ Coccygeal✄ 1 ,,

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Cells in Nervous System

� Two types

✁ Glial cells & Neurons

Glial cells :

✂ 10 to 50 times more than neurons

✁ Supporting cells

✁ Do not generate Action Potential

✂ Retain the ability to divide & multiply throughout the life

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Glial cells--- Types

1 Macroglial cells: Ectodermal in origin

� Oligodendrocytes

� Astrocytes

✁ Fibrous astrocytes, in white matter

✂ Protoplasmic astrocytes, in gray matter

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✄ �

2 Microglial cells: Mesodermal in origin

✁ Microglia

3 Ependymal cells

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Oligodendrocytes

� Round or pear shaped

✁ Function : formation of myelin sheath

✁ Give multiple processes to form myelin on many neurons, about 20.

✁ Their counterpart in peripheral nervous system are Schwann cells

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✄ � �

✁ Nerve fibers in CNS have no neurilemma. So once degenerated, never regenerates

✂ Multiple sclerosis: an autoimmune crippling disorder shows patchy destruction of myelin in CNS.

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Astrocytes

� Star like in shape

✁ Functions:

✂ Form Blood Brain Barrier

✄ Produce substances trophic to Neurons

✂ Maintain concentration of ions and Neurotransmitters.

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Microglia

� Are scavenger cells

✁ Function as phagocytes

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Ependymal cells

Are epithelial cells that line

� CSF filled ventricles in the brain &

Central canal of the spinal cord

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Neurons

Important Facts:

� Total No. of Neurons in CNS about (1011) 100 billion

✁ Neurons can not multiply as they have no centrosome

✁ All neurons present at the time of death were present since birth

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✄ � �

✁ Regeneration in neurons: some evidences of regeneration are present at few sites

✂ Hippocampus

☎ Some areas in cerebral cortex

✂ Dopaminergic neuron in Substantia Nigra

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✄ � �

✁ Each neuron forms about 2000 synaptic endings

✂ Total no. of synapse about 2x1014

✂ New synapse formation occurs throughout the life & is the basis of learning & memory

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Neuron Classification

Classified

1 Depending upon number of poles from

which processes arise

2 ,, on functions

3 ,, upon length of axons

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1---- no. of poles

� Unipolar: in embryonic stage

✁ Pseudo unipolar: primary sensory neurons conveying impulses from sensory receptors to spinal cord

� Bipolar: in vestibular & cochlear ganglia, bipolar cells of retina

✁ Multipolar: most neurons

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✄ ✄� 2 Depending on functions

✁ Sensory

✁ Motor

� 3 Depending on length of axon

✁ Golgi type I : have long axons eg. neurons forming peripheral nerves & long tracts of brain and spinal cord.

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✄ � �✁ Golgi type II : short axons , eg. Numerous neurons in cerebral cortex &

cerebellar cortex

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Structure of Neuron

� Has three parts

✁ Dendrites

✁ Cell body

✂ Axon

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Dendrites

� Extends out of cell body

✁ 5 to 7 in no.

✁ Arborize extensively

✁ Have dendritic spines

✁ Form receptive end of neuron

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Cell Body

� Nissl Granules : rough endoplasmic reticulum, synthesize proteins, required for production of neurotransmitters & for repair. They are not seen in axon

� Centrosome not present : it shows loss of ability for division. Nerves once destroyed are replaced by glial cells

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Axon

� Long fiber

✁ Initial part is Axon Hillock

✁ First portion forms Initial Segment

✁ At terminal end synaptic knobs are present also called Terminal buttons or Axon telodendria

� Knobs contain synaptic Transmitters

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Synapse

� Definition : means Clasping of hands as in hand shaking. First described by Sherrington as the site of contiguity (no anatomical continuity)

� At these sites neurons make functional junctions with each other

✁ Impulses are passed from one neuron to other neuron

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✄ �✁ Total no. of synapse about 2x1014

✂ New synapse formation occurs throughout the life & is the basis of learning & memory

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Classification

Two types

� Anatomical &

� physiological

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Classification :

� Anatomical

✁ Axo dendritic: most common eg. Motor neurons in spinal cord, neurons in cerebral cortex & climbing fibers in cerebellum

✂ Axo somatic: eg. Motor neuron in spinal cord, basket cells of cerebellum, autonomic ganglia

✁ Axoaxonal: Spinal cord- presynaptic inhibition

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✄ ✄� Dendro dendritic : eg. Between mitral & granular cells in olfactory bulb

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Dendro- dendritic

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Physiological classification

� Electrical

✁ Conjoint

✁ Chemical

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Electrical Synapse

� Transmission through gap junctions

✁ Seen in Lateral Vestibular Nucleus, hippocampus & cerebral cortex

✁ Transmission is in both directions

✁ No delay

Conjoint Synapse both electrical & chemical transmission present.

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Chemical / Electrical Synapse

S N Property Chemical Sy. Electrical Sy.

1 Transmission Chemical Gap junctions

2 Direction One direction Either dir.

3 Cleft size 25 to 30 nm. 3nm.

4 Synaptic delay Present Absent

5 Hypoxia Sensitive Insensitive

6 No. of synapses on one neuron

Many, Help in learn. & mem.

Few, so rapid transmission

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Chemical synapse

� Structure

✁ Changes in post synaptic membrane

✂ Electrical,(development of EPSP & IPSP)

✄ Ionic

✁ Development of Action potential in post. Synaptic neuron

� Removal of transmitter

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Structure

� Includes

✁ Synaptic knob of pre synaptic neuron

✁ Synaptic cleft

✂ Post synaptic membrane

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Synaptic Knob

� No. of knobs✄

✁ One per synapse in mid brain

✁ 10,000 ,, spinal cord

� Knobs contain Synaptic vesicles

✁ Small clear-----Acetylcholine, glycine, GABA glutamate

✁ Small dense core------Catecholamines

✂ Large dense core -----Neuropeptides

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Synaptic cleft

� Is a gap of 20 to 30 nm

✁ Filled with some fluid & glycoproteins

✁ Separates pre & post synaptic membranes

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Postsynaptic Mem.

� Sub synaptic mem.

✁ Is membrane involved in the synapse

✁ Receptors are usually present here

� Post Syn. mem.

✁ Remaining part of mem.

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Changes at synapse

� Release of neurotransmitter from presynaptic terminal

✁ Electrical changes in subsynaptic & postsynaptic membrane, means development of EPSP or IPSP

� Ionic changes

✁ Removal of neurotransmitter

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� Nerve signal comes to terminal ending

☎

✁ Ca++ enters in the ending & causes

☎

✁ Release of neurotransmitter by exocytosis

✂

✁ Transmitter acts over post synaptic mem.

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Electrical changes

� Depending upon the type of neurotransmitter EPSP or IPSP is generated on sub synaptic membrane

✁ Action of neurotransmitter occurs with in ms.

� Usually all endings of one neuron contain only one type of transmitter, but there are examples where neurons contain & secret two or even three types of transmitters

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Co-transmitters / neuromodulators

Eg.

� Many cholinergic neurons contain VIP

� Many adrenergic & N. A. neurons contain ATP & neuropeptide Y.

� Physiological significance is not clear but can potentiate the action

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Excitatory post synaptic potential (EPSP)

� Transmitter usually glutamate

✁ Depolarization of Postsynaptic mem.

✁ Starts with a latency of 0.5 msec.

✁ Peaks in 1 to 1.5 msec

✁ Decline with a half life of 4.0 msec. (Declines exponentially)

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Ionic changes in Gen. of EPSP

� Excitatory transmitter acts over sub synaptic mem.

☎

✁ Na+ or Ca++ channels open

☎

✁ Only EPSP is produced as the current generated is small

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✄ ✄ ✄

� EPSP can also be produced by agents that close K+ channels

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Inhibitory post synaptic potential (IPSP)

� Transmitter, Glycine or Gama Amino Butyric Acid (GABA)

✁ Post Synaptic mem. is hyperpolarized

✁ Latency 1 to 1.5 ms

✁ Peaks 1 to 2 ms

✁ Decreases exponentially with a time constant of 3 ms

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Ionic changes

� Increase in Cl- transport

✁ IPSP can also be produced by

✂ Opening of K+ channels

✄ Closure of Na+ and Ca++ channels

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

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Slow EPSPs and IPSPs

� Are seen in autonomic ganglia, cardiac & smooth muscles and cortical neurons

✁ Latency of 100 to 500 ms

� Lasts for several seconds

✁ They are due to change in conductance of K+

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Genesis of action potential in post synaptic neuron

FACTS

� Electrical activity in one synapse is not adequate enough to stimulate post synaptic neuron

✁ Threshold is lowest at initial segment of neuron

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✄ � ✁ ✂ ☎ ✆ ✝

✞ Synaptic integration: all EPSPs & IPSPs are summated, net algebraically summed potential determines whether transmission of impulse will take place or not.

✞ If Summated potential is 10 to 15 mv spike potential is generated at Initial segment of neuron

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✄ ✄� IS spike leads to further Depolarization of 30 to 40 mv. & Action

potential is produced

✁ Action Potential is only produced at Initial segment

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✄ � �

✁ Action Potential travels in both directions

✂ Peripherally as nerve impulse

✂ Centrally towards soma & dendrites so as to clear existing EPSPs & IPSPs & Cell is once again ready to react to another set of stimuli

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Inactivation of neuron

� Chemical transmitter is removed

✁ Diffusion out of cleft

✁ Enzymatic degradation

✂ Reuptake by pre synaptic knob

✄ ✄ ✄ ☎ ✆ ✝ ✞ ✟ ✠ ✞ ✡ ✆ ☎ ✡ ☛ ✟

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Properties of synapse

� Law of forward conduction

✁ Synaptic Delay

✁ Convergence & Divergence

✁ Summation

✁ Occlusion

� Subliminal fringe

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

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✄ ✄� Fatigue

✁ Synaptic plasticity & learning

✁ Response to hypoxia

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Law of forward conduction

� Because of the specific structure, neurons permit conduction of impulse from pre to post synaptic neuron only

✁ ✁ ✁ ✂ ✄ ☎ ✆ ✝ ✞ ✆ ✟ ✄ ✂ ✟ ✠ ✝

90

Synaptic delay

Minimum time required for transmission at one synapse. It is required for

� Release of neurotransmitter

✁ Diffusion up to post synaptic membrane

� Activation of receptors

✁ Ionic changes to generate EPSP or IPSP

� Minimum time is 0.5 ms at one synapse

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

91

Convergence & Divergence

� � � ✁ ✂ ✄ ☎ ✆ ✝ ☎ ✞ ✂ ✁ ✞ ✟ ✆

92

Summation

� Two types:

✁ Spatial : if two afferents are stimulated simultaneously with sub threshold stimuli, their EPSPs get summated and spike is generated

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

93

�✁✂✄☎✂✆�✝✞✞✂✄☎✟✠

✡✡✡☛☞✌✍✎✏✍✑☞☛✑✒✎

94

�✁✂✄☎✂✆�✝✞✞✂✄☎✟✠

✡☛

☞✌✍

✎✎✎✏✑✒✓✔✕✓✖✑✏✖✗✔

95

✄ � �

✁ Temporal: if an afferent is stimulated in quick succession (before the decay of first stimulation) again a response may be there.

✂ ✂ ✂ ☎ ✆ ✝ ✞ ✟ ✠ ✞ ✡ ✆ ☎ ✡ ☛ ✟

96

�✁✂✄☎✆✝✞✟✠✂✂✡

☛☞✌☞✌

✍✎✏✎

✑✑✑✒✓✔✕✖✗✕✘✓✒✘✙✖

97

���✁✂✄☎✆✝☎✞✂✁✞✟✆

98

Occlusion: due to overlapping

� � � ✁ ✂ ✄ ☎ ✆ ✝ ☎ ✞ ✂ ✁ ✞ ✟ ✆

99

Su

blim

ina

l fr

ing

e

�✁✂✄☎✆✝✄✞✄☎✟

✠✡☛☞✌✍✎✏✑✒

✓✔✕✖✗✖✘✙

✚✚✚✛✜✢✣✤✥✣✦✜✛✦✧✤

100

Fatigue

� Repeated stimulation of pre synaptic neuron finally leads to disappearance of the post synaptic response

✁ Causes:

✂ Exhaustion of chemical transmitter, as synthesis is not as rapid as release

✄ Inactivation of Ca++ channels

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

101

Synaptic Plasticity

� Plasticity is capability of being easily modulated

✁ Means synaptic transmission can be increased or decreased on the basis of past experiences

� It makes the basis of learning

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

102

✄ �✁ Changes at synapse are

✂ Post Tetanic Potentiation

✂ Long Term Potentiation

☎ Habituation

✂ Sensitization

✆ ✆ ✆ ✝ ✞ ✟ ✠ ✡ ☛ ✠ ☞ ✞ ✝ ☞ ✌ ✡

103

Hypoxia

� Synapses are more susceptible to hypoxia than the nerve fibers

✁ ✁ ✁ ✂ ✄ ☎ ✆ ✝ ✞ ✆ ✟ ✄ ✂ ✟ ✠ ✝

104

Synaptic Inhibition

Types:

� Post synaptic inhibition

✁ Direct

✂ Indirect caused by some previous activity

� Pre synaptic inhibition

✄ Negative feedback inhibition

� Feed-forward inhibition

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

105

Post Synaptic ✄ Direct Inhi.

� IPSP is generated on post synaptic neuron

✁ Bi-synaptic path way eg.

Reciprocal Innervation in spinal cord

✁ Means if flexors are contracting the extensor group will relax

✂ Present in spinal cord the inter neuron is Golgi Bottle Neuron it liberates Glycine

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

107

� If muscle is stretched strongly, it relaxes

✁ Receptor --- Golgi Tendon Organ

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

108

Indirect inhibition

May be because:

� Post synaptic neuron has just fired, so refractory

� During after hyper polarization neurons are less sensitive

✁ ✁ ✁ ✂ ✄ ☎ ✆ ✝ ✞ ✆ ✟ ✄ ✂ ✟ ✠ ✝

109

Presynaptic Inhibition

� It occurs at the presynaptic terminal before the signals reach at the synapse

✁ Synapse is Axo-axonal

� Transmitter secreted is (GABA) gamma-aminobutyric acid

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

110

Pre

�✁✂

✄☎✆✝✞✟✠✡

☛✆☞✠✌✠✟✠✍✆

✎✎✎✏✑✒✓✔✕✓✖✑✏✖✗✔

111

mechanisms

� Receptors are GABAA

✁ 1 Activation of presy. neuron increases Cl- conductance & decreases size of A.P. reaching the excitatory ending

� This reduces Ca++ entry & at the same time decreases amount of transmitter released

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

112

✄ � �

✁ 2 K+ channels open & K+ efflux also decreases the Ca++ influx

✂ 3 direct inhibition of transmitter release independent of Ca++ influx

✂ GABAB receptors are also present & they act by G protein- mediated effects on potassium channels

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

113

Negative feedback inhibition

� Seen in spinal cord

✁ Also called Renshaw cell inhibition

✁ Here neurons inhibit themselves in a negative feedback fashion

✁ Neurons check their own discharge

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

114

���✁✂✄☎✆✝☎✞✂✁✞✟✆

115

Feed forward inhibition

� Seen in cerebellum

✁ ✁ ✁ ✂ ✄ ☎ ✆ ✝ ✞ ✆ ✟ ✄ ✂ ✟ ✠ ✝

116

Applied aspects of Synaptic In.

� Myelinated cutaneous afferents affect pain sensation by pre synaptic inhibition

✁ Pre synaptic inhibition is antagonized by the convulsant drug Picrotoxin

✁ Barbiturates greatly pronounce pre synaptic inhibition

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

117

✄ ✄� Convulsant effect of strychnine is due to depression of post synaptic

inhibition

✁ ✁ ✁ ✂ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✂ ✠ ✡ ✞

118

Receptors

� Are like transducers that convert various types of energies into electric energy.

✁ These are modified nerve endings

� They have punctate character

✁ All sensory information do not necessarily reach the level of consciousness.

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

119

Classification

A According to the type of receptors

� ✁ ✂ ✄ ✄ ☎ ✆ ✝ ✞ ✟ ✆ ✠ ✡ ☛ ☞ ✌ ✡ ✡ ☎ ✍ ☎ ☛ ✌ ✞ ☎ ✟ ✆

✎ Telereceptors: hearing, vision,smell.

✎ Exteroceptors: touch, temp.,pain

✎ Interoceptors: proprioceptors, chemoreceptors, visceroceptors

✏ ✏ ✏ ✑ ✒ ✓ ✔ ✕ ✖ ✔ ✗ ✒ ✑ ✗ ✘ ✕

120

✄ ✄

B General or Anatomical classification

� Special senses: smell, vision, hearing

� Superficial or cutaneous senses: touch, temperature, pain, pressure

� Deep senses: sensation from joints, muscles, tendons

� Visceral senses: visceral pain

✁ ✁ ✁ ✂ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✂ ✠ ✡ ✞

121

According to type of stimulusReceptor Stimulus

Mechanoreceotors

Chemoreceptors

Thermoreceptors

Nociceptors

Photoreceptors

Mechanical, eg.Touch, pressure

Chemical changes eg. Taste, smell, osmoreceptors, glucoreceptors

Thermal (degree of warmth)

Noxious, damaging like pain

Electromagnetic: Light eg. Rods and Cones in retina

� � � ✁ ✂ ✄ ☎ ✆ ✝ ☎ ✞ ✂ ✁ ✞ ✟ ✆

122

According to degree of Adaptation

� Tonic: slow adaptation, receptors continue to send information for hours or even for days eg.

✁ Muscle spindle

✂ Pain

✁ Cold

✄ ✄ ✄ ☎ ✆ ✝ ✞ ✟ ✠ ✞ ✡ ✆ ☎ ✡ ☛ ✟

123

✄ � �

✁ Phasic: Show Rapid adaptation eg.

✂ Touch

✂ Pressure

✁ No adaptation

✂ Pain

✂ Vestibular receptors

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

124

Epicratic /protopathic

� Epicratic Sensation: Mild or light sensation perceived more accurately eg. Fine touch, tactile localization, tactile discrimination,temp.

between 25✄ 40✁ C

� Protopathic: Primitive or crude sensation eg. Pressure, pain,

temperature below 25 & above 40✁ C

✂ ✂ ✂ ☎ ✆ ✝ ✞ ✟ ✠ ✞ ✡ ✆ ☎ ✡ ☛ ✟

125

CUTANEOUS RECEPTORS

� Touch✄ ✁ ✂ ☎ ✆ ✆ ✝ ✂ ✞ ✟ ✆ ✠ ✡ ✞ ☛ ☞ ✆ ✠ ✌ ✂ ✍ ✁ ✂ ✞ ✎ ✂ ✌ ✟ ✆ ✏ ☎ ✆ ✠ ✑ ☞ ✒ ✒ ☎ ✝ ☎ ✟ ✆ ✂ ✝ ✏ ✡ ✞ ✓ ✔ ✝ ✆

✕ ✖ ✗ ✘ ✙ ✚ ✛ ✙ ✚ ✜ ✢ ✣ ✘ ✤ ✣ ✥ ✦ ✖ ✚ ✚ ✜ ✚ ✖ ✧ ✚ ✚ ✜ ✢ ★ ✜ ✩ ✙ ✗ ✖ ✪ ✘ ✜ ✢ ✫ ✗ ★ ✖ ✖ ✪ ✪ ✬ ✙

✭ Temperature:

✮ Cold------Nerve Endings of type A ✯ fibers (myel.)

✰ Warm----Nerve Endings of Type C fibers (unmye.)

✭ Pain: Free nerve endings

✱ ✱ ✱ ✲ ✳ ✴ ✵ ✶ ✷ ✵ ✸ ✳ ✲ ✸ ✹ ✶

126

✄ ✄� Deep pressure: Pacinian Corpuscle

✁ Proprioception:

✂ Muscle Receptors

☎ Pacinian Corpuscles

✂ Uncapsulated nerve endings.

✁ Synthetic senses

✆ ✆ ✆ ✝ ✞ ✟ ✠ ✡ ☛ ✠ ☞ ✞ ✝ ☞ ✌ ✡

127

���✁✂✄☎✆✝☎✞✂✁✞✟✆

128

���✁✂✄☎✆✝☎✞✂✁✞✟✆

129

Touch sensation

Two types:

� Crude touch✄ perception of touch

� Fine touch

✁ Tactile localization

✁ Two point discrimination

✂ ✂ ✂ ☎ ✆ ✝ ✞ ✟ ✠ ✞ ✡ ✆ ☎ ✡ ☛ ✟

130

Temperature sensation

Cold receptors are 4 to 10 times more.

� Types:

✁ A✂ myelinated for cold

✄ C unmyelinated for warm

� Present all over the body. Density-

✁ Greatest----in the lips

✄ Moderate---finger tips

✁ Least--------trunk

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

131

Diff. betwn. Cold & Warm receptors

S.No Cold receptors Warm Receptors

1

Fibers are thin myelinated A✁

dia. 2.5 µm.

C, unmyelinated dia. 0.4 to 1.2 µm

2

Fairly steady discharge between 10 to 35 � C

Discharge between 35 to 45✂ C

3

Maximum discharge at tissue temp. of 25 to 30� C

Max. at 38 to 43✂ C

✄ ✄ ✄ ☎ ✆ ✝ ✞ ✟ ✠ ✞ ✡ ✆ ☎ ✡ ☛ ✟

132

���✁✂✄☎✆✝☎✞✂✁✞✟✆

133

Paradoxical cold response

� If tissue temp. is beyond 45✁ C it leads to tissue damage & damage to cold receptors sometimes stimulate them & their discharge produce a sensation of cold

� Probably above 45✁ C tissue damage begins

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

134

Other / synthetic senses

� 2 point discrimination:depends on density of rec.

✁ Lips & finger tips---2 to 3 mm

✁ Back of trunk-------60 mm

� Vibration: Receptors are rapid adapt.

✁ Pacinian corpuscle

✁ ✂ ✄ ☎ ✆ ✆ ✝ ✄ ✞ ✟ ✆ ✠ ✡ ✞ ☛ ☞ ✆ ✠ ✌ ✄ ✆

✍ Krause bulb

✎ ✎ ✎ ✏ ✑ ✒ ✓ ✔ ✕ ✓ ✖ ✑ ✏ ✖ ✗ ✔

135

✄ �✁ Lost in

✂ Tabes Dorsalis

✂ Peripheral neuropathies (diabetes)

☎ Post. Column disorders

✆ ✆ ✆ ✝ ✞ ✟ ✠ ✡ ☛ ✠ ☞ ✞ ✝ ☞ ✌ ✡

136

✄� Stereognosis --- Ability of person to identify common objects with

closed eyes

Receptors✁ touch & pressure with their central connections and cerebral cortex

✂ Astereognosis: loss of this ability.It is an early sign of damage to Parietal Lobe but receptor & conn. should be normal

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

137

..

� Itch: irritating condition caused by mild stimulation of skin eg. flea crawling on the skin. Can also be produced by chemicals like Bile salts, histamine, kinins, Itch powder etc.

� Receptor- naked N.E. of type C fib.

✁ Scratching relieves it. by presynaptic Inhibition in Dorsal horn cells (like pain)

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

138

✄� Tickle : is another variable of touch. It is regarded as pleasurable

feeling

✁ ✁ ✁ ✂ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✂ ✠ ✡ ✞

139

Activation of receptors

� Receptor can be stimulated by specific stimulus

Stimulation of receptor produces

✁ Electrical changes: generation of receptor potential

✁ Ionic changes

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

140

Receptor /generator Potential

� Is widely studied in Pacinian Corpuscles as they are big in size.

✁ Structure: a straight unmyelinated sensory nerve ending is surrounded by concentric lamellas of connective tissue

� I node of Ranvier is inside the corpuscle

✁ II node is located outside

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

141

✄ �✁ P.C. can be stimulated mechanically by a glass rod

✂ With small pressure a non propagating potential is generated. This is Receptor or Generator potential

✁ Receptor potential is local potential produced in unmyelinated nerve terminal

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

142

✄ � �

✁ Receptor pot. of ~10 mv. is capable to produce Action Potential

✂ It depolarizes nerve at First Node of Ranvier

✂ Action potential is produced only at I node of Ranvier

✂ As pressure is increased Receptor Potential becomes larger & nerve fires repetitively (increase in rate of firing)

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

143

Conclusion :

� Generator /receptor potential originates from unmyelinated nerve ending

✁ Action potential is produced at I node of Ranvier

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

144

���✁✂✄☎✆✝☎✞✂✁✞✟✆

145

�✁✂✄

☎✆✝✄✞✟☎✆✠✡✂✁☎✞

☛☎✂✄✞✂✁✠☞

✌✌✌✍✎✏✑✒✓✑✔✎✍✔✕✒

146

Ionic changes

� Increase in permeability for sodium ions in unmyelinated terminal

✁ Degree of permeability is directly related to the intensity of stimulus

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

147

Properties

Graded potentials Receptor, End Plate or Postsynaptic Potentials.

Action Potential

Propagation Do not (local pote.) Yes

Magnitude of response

Graded ✁ with ✁ in

streng. of stimulusAll or none

Summation Seen Not seen

Refractory Period

Not present Present

Threshold Not present Present � � � ✂ ✄ ☎ ✆ ✝ ✞ ✆ ✟ ✄ ✂ ✟ ✠ ✝

148

Weber Fechner Law

� Gradations of stimulus strength are discriminated approximately in proportion to the logarithum of stimulus strength

✁ ✁ ✁ ✂ ✄ ☎ ✆ ✝ ✞ ✆ ✟ ✄ ✂ ✟ ✠ ✝

149

✄ ✄ � �

✁ Magnitude of the sensation felt is proportionate to the log of the intensity of the stimulus . However a power function describes it more accurately R=KSA

✁ R is sensation felt

✂ S intensity of the stimulus

✁ K & A are constants.

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

150

Coding of Sensory Information

Questions to be considered

� All nerves carry electric signals than, how different sensation are produced ?

✁ How the site of stimulation is localized ?

� How the grading of sensation is done ?

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

151

✄�

Po

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le m

ech

an

ism

s

✁✂☎✆✆✝✞✟✠✡☛☞✌✞✍✎✝☛✏✠✑✝☞✍✏✍☞✎✝✞✒✝✝✎✝✞✓✍✝✠

✁

Law

of

Pro

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✁

Gra

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g o

f se

nsa

tio

n

✔✔✔✕✖✗✘✙✚✘✛✖✕✛✜✙

152

✄ �

Principle was first enunciated by Muller in 1835

✁ The specific sensory pathways are discrete from sense organs to brain. If this pathway is stimulated anywhere , the sense evoked is that for which the receptor is specialized.

✂ If a growth in spinal cord presses the pain pathway, the sensation felt is of pain.

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

153

� ✁ ✂ ✄ ☎ ✆ ✆ ✝ ✞ ✟ ✠ ✝ ✞ ✝ ☎ ✡ ☎ ✞ ✆ ☛ ☞ ✌ ✍ ☎ ✞ ✎ ✝ ✏ ✎ ✂ ✞ ✑ ✡ ☎ ✆ ✠ ✟ ☎ ✑ ☛ ✎ ✎ ✆ ☛ ✄ ✌ ✍ ☎ ✆ ✝ ✒ ☎ ✍ ✂ ✏ ✓ ☛ ✆ ✔ ✎

course from receptor to the cortex, the conscious sensation produced is referred to the site where receptor is located. eg.

✕ ✕ ✕ ✖ ✗ ✘ ✙ ✚ ✛ ✙ ✜ ✗ ✖ ✜ ✢ ✚

154

✄ � �

✁ Phantom Limb, a person who has lost his limb in an accident or amputation, usually experiences intolerable pain and other sensations in the limb that is no longer there. It is because of irritation of afferent nerves at the stump, where a neuroma is formed.

✂ ✂ ✂ ☎ ✆ ✝ ✞ ✟ ✠ ✞ ✡ ✆ ☎ ✡ ☛ ✟

155

� 1 By frequencies of Action Potentials generated in the sensory nerve fibers. Frequency directly depends on strength of stimulus

� 2 By number of recruitment of sensory units, as the strength is increased more and more units start firing

✁ ✁ ✁ ✂ ✄ ☎ ✆ ✝ ✞ ✆ ✟ ✄ ✂ ✟ ✠ ✝

156�

Inne

✁ Moto tes

✂ ✂ ✂ ✄ ☎ ✆ ✝ ✞ ✟ ✝ ✠ ☎ ✄ ✠ ✡ ✞

157

Properties of Receptors

Law of Adequate stimulus

� Receptors response maximally to a specific stimulus

✁ ✁ ✁ ✂ ✄ ☎ ✆ ✝ ✞ ✆ ✟ ✄ ✂ ✟ ✠ ✝

158

✄

Adaptation

� Tonic: slow adaptation, receptors continue to send information for hours or even for days eg.

✁ Muscle spindle

✂ Pain

✂ Cold

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

159

✄ � �

✁ Phasic: Show Rapid Adaptation eg.

✂ Touch

✂ pressure

☎ ☎ ☎ ✆ ✝ ✞ ✟ ✠ ✡ ✟ ☛ ✝ ✆ ☛ ☞ ✠

160

�

✁✂✄☎✆✝✞✟✠✡

✡✄☎☛✝✠

✡✠☎✡☞✌✍

✎✏☞☎

✎☎✟

✎✝✝✄✂✑✡

✒✠✌✄✒✓✠✌✎✝✔✌✎☎✆✓✠✡✕

Se

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rla

p

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