Here are the key types of mechanoreceptors and their properties: - Cutaneous mechanoreceptors: - Meissner's corpuscles - detect light touch and pressure on fingertips and lips. Found in dermal papillae. - Merkel's discs - detect sustained light touch. Found just below the epidermis. - Pacinian corpuscles - detect deep pressure and vibration. Found in dermis and connective tissue. - Ruffini endings - detect skin stretch and joint movement. Found in dermis and connective tissue. - Free nerve endings - detect pain. Found throughout the dermis and epidermis. - Proprioceptors: - Muscle spind

1. SYNAPSE

2. BY THE END OF THE LECTURE
YOU SHOULD BE ABLE TO
 Define and classify synapse
 Discuss steps of synaptic transmission
 Describe intracellular second messenger systems for
synaptic transmission
 Classify neurotransmitters, and know about the main
excitatory and inhibitory ones

3. WHAT IS A SYNAPSE?
DEFINITION:
It is the anatomic site of electrical communication betweens
neurons or neurons and muscles or glands.

4. SYNAPSE
 Information is transmitted in the nervous system
mainly in the form of nerve action potentials,
called simply “nerve impulses,”
 Where two neurons come into close proximity and
functional inter neuronal communication occurs,
the site of such communication is referred to as a
synapse.

5.  The central nervous system contains more than 100 billion neurons.
 Incoming signals enter this neuron through synapses located mostly on
the neuronal dendrites, but also on the cell body.
 The output signal travels by way of a single axon leaving the neuron.
 A special feature of most synapses is that the signal normally passes
only in the forward direction

6. Classification:
 Anatomical
 Functional

7. ANATOMICAL CLASSIFICATION

8. ANATOMICAL CLASSIFICATION
 Axoaxonic synapse
 Axodendritic synapse
 Axosomatic synapse

10. •As many as 100,000 (1 lac)
presynatic terminals on soma
and dendrites combined
•80-95% on dendrites
•5-20% on soma

11. PHYSIOLOGICAL CLASSIFICATION

12. PHYSIOLOGICAL CLASSIFICATION
 On the basis of mode of impulse transmission.
 Chemical
 Electrical

13. CHEMICAL SYNAPSES
Almost all the synapses used for signal transmission in the central
nervous system of the human being are chemical synapses.
In these, the first neuron or presynaptic neuron secretes at its nerve
ending a chemical substance called a Neurotransmitter.
This transmitter in turn acts on receptor proteins in the membrane of
the next neuron or post synaptic neuron to excite the neuron, inhibit
it, or modify its sensitivity in some other way.
Transmission is one-way.

17. CHEMICAL SYNAPSE:
 Presynaptic membrane, cleft, post synaptic
membrane
 One way transmission
 Neurotransmittors
 Excitatory---
 Inhibitory----
 Synapse labelled excitatory or inhibitory

18. CHEMCIAL
SYNAPSES
CAN
FURTHER BE
EXCITATORY
AND
INHIBITORY

19. SEQUENCE OF EVENTS

20. ELECTRICAL SYNAPSES
 Are characterized by direct open fluid channels that conduct
electricity from one cell to the next.
 Most of these consist of small protein tubular structures called
gap junctions that allow free movement of ions from the interior
of one cell to the interior of the next.
 Only a few examples of gap junctions have been found in the
central nervous system

22. PHYSIOLOGIC ANATOMY OFTHE
CHEMICAL SYNAPSE

24. ACTION OF THE TRANSMITTER
SUBSTANCE
ON THE POSTSYNAPTIC NEURON—
FUNCTION OF “RECEPTOR
PROTEINS”
 The membrane of the postsynaptic neuron contains large
numbers of receptor proteins,
 The molecules of these receptors have two important
components
(1) a binding component
(2) an ionophore component (ion channel or G protein linked)

25. ION CHANNELS
The ion channels in the postsynaptic neuronal
membrane are usually of two types:
1. cation channels that most often allow sodium ions to pass
when opened, but sometimes allow potassium and/or
calcium ions as well,
2. anion channels that allow mainly chloride ions to pass but
also minute quantities of other anions.

28. “Second Messenger” System in the Postsynaptic Neuron
 There are several types of second messenger
 systems.
 One of the most common types uses a group of proteins called G-
proteins
 prolonged postsynaptic neuronal excitation or inhibition is achieved
by activating a “second messenger” chemical system inside the
postsynaptic neuronal cell itself, and then it is the second messenger
that causes the prolonged effect.

29. Post synaptic potential
Excitatory
Inhibitory
Depends on the presence of
Receptors in the Postsynaptic Membrane

30. Excitation
 Opening of sodium channels to allow large
numbers of positive electrical charges to flow to the interior of the
postsynaptic cell.
 Depressed conduction through chloride or
potassium channels, or both.
Various changes in the internal metabolism of the postsynaptic
neuron to excite cell activity

31. Inhibition
 Opening of chloride ion channels through the postsynaptic neuronal
membrane
Increase in conductance of potassium ions out of the neuron
Activation of receptor enzymes that inhibit cellular metabolic
functions that increase the number of inhibitory synaptic receptors
or decrease the number of excitatory receptors

33. G PROTEINS AS SECOND
MESSENGERS

34. CHEMICAL SYNAPTIC
TRANSMITTERS
 2 types:
 Small-molecule, rapidly acting neurotransmitters
 cause most acute responses of the CNS
 Larger molecular size neuropeptides
 cause more prolonged actions, such as long-term changes in numbers of
neuronal receptors, long-term opening or closure of certain ion channels

35. TYPES OF
NEUROTRANSMITTERS

36. Small-Molecule, Rapidly ActingTransmitters
Class I
Acetylcholine
Class II:The Amines
Norepinephrine
Epinephrine
Dopamine
Serotonin
Histamine
Class III: Amino Acids
Gamma-aminobutyric acid (GABA)
Glycine
Glutamate
Aspartate
Class IV
Nitric oxide (NO)

37. Neuropeptide, Slowly ActingTransmitters or
Growth Factors
Hypothalamic-releasing hormones
Thyrotropin-releasing hormone
Luteinizing hormone–releasing hormone
Somatostatin (growth hormone inhibitory factor)
Pituitary peptides
Adrenocorticotropic hormone (ACTH)
Luteinizing hormone
Thyrotropin
Growth hormone
Vasopressin
Oxytocin
Peptides that act on gut and brain
Leucine , enkephalin

38. SYNAPTICTRANSMITTERS
Small molecules
 Acute response
 Short action
 Synthesized in cytosol of
nerve terminal
 Stored in small vesicles that
are reused
Neuropeptides
 Slow to act
 Prolonged action
 Synthesized in cell body
 Stored in large vesicles thar
are autolyzed after release
of neuropeptide

39. FEATURES OF SYNAPTIC TRANSMISSION

40. Students should be able to
 Understand features of synaptic transmission
 Apply or relate the concepts of excitation and inhibition of
synapse with certain clinical abnormalities

41. FEATURES/PROPERTIES OF
SYNAPSE
 EPSP/IPSP
 Fatigue of SynapticTransmission
 Synaptic delay
 Role of Synapses in Processing Information
 Effect of Acidosis or Alkalosis on SynapticTransmission
 Effect of Hypoxia on SynapticTransmission.
 Effect of Drugs on SynapticTransmission

42. EFFECT OF SYNAPTIC EXCITATION ONTHE
POSTSYNAPTIC MEMBRANE—
EXCITATORY POSTSYNAPTIC POTENTIAL.
 shows a presynaptic terminal
 that has secreted a transmitter
into the cleft
 This transmitter acts on the
membrane excitatory receptor
to increase the membrane’s
permeability to Na+.
 sodium ions diffuse rapidly to
the inside of the membrane.

43. EPSP
 This positive increase in voltage above the
normal resting neuronal potential-that is, to a
less negative value-is called the excitatory
postsynaptic potential (or EPSP)
 if this potential rises high enough in the positive
direction, it will elicit an action potential
 Discharge of a single presynaptic terminal can
never increase the neuronal potential from -65
millivolts all the way up to -45 millivolts.

44. ELECTRICAL EVENTS DURING
NEURONAL
INHIBITION
Inhibitory post synaptic
potential
The inhibitory synapses
Open mainly chloride
channels,
An increase in negativity
beyond the normal resting
membrane potential level is
called an inhibitory
postsynaptic potential (IPSP)

45. TYPES OF INHIBITION
 Post synaptic inhibition
 Presynaptic inhibition

47. WHAT MAKES A INHIBITORY/EXCITATORY
SYNAPSE
Excitatory synapse Inhibitory synapse
Opening of sodium channels to allow
large numbers of positive electrical
charges to flow to the interior of the
postsynaptic cell.
Opening of chloride ion channels
through the postsynaptic neuronal
membrane.
Depressed conduction through chloride
or potassium channels, or both.
Increase in conductance of potassium
ions out of the neuron.
Various changes in the internal
metabolism of the postsynaptic neuron to
increase excitatory membrane receptors
or decrease the number of inhibitory
membrane receptors.
Activation of receptor enzymes that
increase the number of inhibitory
synaptic receptors or decrease the
number of excitatory receptors.

48. SUMMATION
Spatial: at same time. Many presynaptic terminals
(EPSP of at least 10-20 mV is required to reach threshold. One EPSP is
usually 0.5 to 1 mV.
 Remember whatever membrane potential change occurs, it is spread
over the entire soma (high electrical conductivity). It will die in time
not over distance)
 Temporal: Same terminal. Many times
When impulse comes- channels open for a millisecond and close-
EPSP/IPSP lasts for 15 msec then dies.
Repeated impulse- channels open again and again- EPSPs summate
before they die-amplify-maybe threshold is threshold is reached.

49. SPATIAL SUMMATION
 “Spatial Summation” in
Neurons
 many pre synaptic
terminals are usually
stimulated at the same time.
 Even though these terminals
are spread over wide areas of
the neuron, their effects can
still summate;
 that is, they can add to one
another until neuronal
excitation does occur.

50. TEMPORAL SUMMATION
 Successive discharges from
a single presynaptic
terminal if they occur rapidly
enough, can add to one
another;
 that is, they can
“summate.”This type of
summation is called
temporal summation.

52. EPSP
 Summation
 Amplitude varies
 Dies off
 Ligand gated channels
Action potential
 All or none law
 Fix amplitude
 Length of nerve fiber
 Voltage gated channels
 Shows absolute and relative
refractory period

53. -CONCEPTS OFTHRESHHOLD,
FACILITATION

55. SYNAPTIC FATIGUE
 When excitatory synapses are repetitively stimulated at a rapid
rate, the response by the postsynaptic neuron is at first very
great, but the firing rate becomes progressively less in
succeeding milliseconds or seconds.
 This is called fatigue of synaptic transmission.
 The development of fatigue may be a protective mechanism
against excess neuronal activity

56. THE MECHANISM OF FATIGUE IS
MAINLY
 exhaustion or partial
exhaustion of the stores of
transmitter substance in the
presynaptic terminals.
 progressive inactivation of
many of the postsynaptic
membrane receptors
 slow development of
abnormal concentrations of
ions inside the postsynaptic
neuronal cell.

57. SYNAPTIC DELAY
During transmission of a neuronal signal from a presynaptic
neuron to a postsynaptic neuron, a certain amount of time is
consumed
 This is called the synaptic delay.
 Minimum delay time is 0.5 milliseconds
 From the measure of delay time, one can then estimate the
number of series neurons in the circuit.

58. REASONS FOR SYNAPTIC DELAY
 discharge of the transmitter substance by the presynaptic
terminal,
 diffusion of the transmitter to the postsynaptic neuronal
membrane,
 action of the transmitter on the membrane receptor,
 action of the receptor to increase the membrane permeability,
and
 inward diffusion of sodium to raise the excitatory postsynaptic
potential to a high enough level to elicit an action potential.

59. PROCESSING INFORMATION
AND MEMORY
 The storage of information --- memory, is a function of the
synapses.
 Each time certain types of sensory signals pass through
sequences of synapses, these synapses become more capable of
transmitting the same type of signal the next time, a process
called facilitation.
 The synapses become so facilitated that signals generated
within the brain itself can also cause transmission of impulses
even when the sensory input is not excited.
 This gives the person a perception of experiencing the original
sensations, although the perceptions are only memories of the
sensations.

60. DRUGS INCREASING NEURONAL
EXCITABILITY
 Caffeine
 theophylline
 theobromine
found in coffee, tea, and cocoa, respectively, all increase neuronal
excitability, presumably by reducing the threshold for excitation
of neurons.

61. IMPORTANT
NEUROTRANSMITTERS
 GABA
 Glycine
 Serotinin
 glutamate

62. SHOULD KNOW…
 Most common excitatory NT in CNS – glutamate
 Most common inhibitory NT in CNS – glycine, GABA
 NT can be inactivated via:
 Diffuses out of synaptic cleft
 Actively transported into pre-synpT
 Enzymatically degreaded (if the NT is acetycholine)

63. SUMMARY
•A synapse is the anatomic site of electrical communication
betweens neurons or neurons and muscles or glands. It can
be Chemical or electrical
•Steps include Spread of AP in presynaptic membrane Ca
influx Nt release post synaptic receptors IPSP or EPSP
•G proteins act as intracellular second messengers; their
alpha and beta/gamma subunits triggering different
intracellular events
•Neurotransmitters maybe classified as Rapidly acting small
molecules or slowly acting neuropeptides/ growth factors

64. SENSORY RECEPTORS
Dr. Sadia Nazir
Assistant Professor Physiology
LMDC

65.  Student should be able to
• Understand the types of sensory receptors
• Enumerate and understand the properties of receptors

66. SENSORY RECEPTORS
 Information about the internal and external environment
activates the CNS via a variety of sensory receptors.
 These receptors are transducers that convert various forms of
energy in the environment into action potentials in neurons.
 Stimulus is an event or particular form of energy that evokes a
specific functional reaction in an organ or receptor. (mechanical,
chemical, EMG, temp)

67. SENSE ORGANS
 Receptors are dendritic endings of afferent neurons that are
associated with non-neuronal cells forming sense organs.

68.  The term Sensory unit
means sensory axon and all
its peripheral branches.
 The receptive field of
a sensory neuron is the
particular part of the body
surface in which a stimulus
will trigger the firing of that
neuron.

69. CLASSIFICATION OF SENSORY
RECEPTORS
Type of sensation
 Mechanoreceptors
 Thermoreceptors
 Nociceptors
 Electromagnetic
 Chemoreceptors
Distance of perception
 Teleceptors
 Exteroceptors
 Interoceptors
 Proprioceptors

70. CLASSIFICATION OF RECEPTORS
 Mechanoreceptors
Epidermis & dermis
Joints, tendons, ligaments &
muscles
Cochlea
Vestibular apparatus
Baroreceptors in vessels
 Thermo receptors
Warmth & cold
 Nociceptors
Pain
 Electromagnetic
Rods & cones
 Chemoreceptors
Taste & smell
Carotid & aortic bodies
Hypothalamus
Osmoreceptors

71. TACTILE MECHANORECEPTORS
Encapsulated
 Meissner’s corpuscles in
dermal papilla
 Pacinian corpuscles in
dermis, ligaments, joint
capaules
 Ruffni’s end organs in
dermis and in deeper tissues
Non-Encapsulated
 Free nerve endings in dermis,
ligaments, cornea, bones
 Hair end organs in hairy skin
 Merkel’s discs in non hairy and
hairy skin

72. Mechanoreceptors
Skin (epidermis, dermis)
Deeper tissues

74. SENSORY CODING
Receptors encode four elements of stimuli
 Modality
 Location
 Intensity
 Duration

75. MODALITY OF SENSATION
 Differential Sensitivity of Receptors for particular stimulus or
specific energy for which it is designed
 When nerve fiber from the receptors is stimulated the
perception is that for which the receptor is specialized, no
matter where and how that nerve is stimulated.This is Muller’s
law of specific energy.
 The specificity of nerve fibers for transmitting only one modality
of sensation is called the labeled line principle.

76. EXPLANNATION
 Each nerve tract terminates at a specific point in
the central nervous system, and the type of
sensation felt when a nerve fiber is stimulated is
determined by the point in the nervous system to
which the fiber leads.
 For instance if a pain fiber is stimulated, the
person perceives pain regardless of what type of
stimulus excites the fiber.
 Likewise, if a touch fiber is stimulated by electrical
excitation of a touch receptor or in any other way,
the person perceives touch.

77. LAW OF PROJECTION
 No matter which part of a pathway you stimulate between
the receptor and the brain center, your brain will locate the
stimulus where receptors are (e.g. phantom limb)
 reorganization of brain map in somatosensory cortex
 Some believe it can be blocked by local anesthetics in
spinal cord.

78.  A phantom limb is the sensation that an amputated or
missing limb is still attached to the body.
 Approximately 60 to 80% of individuals with an amputation
experience phantom sensations in their amputated limb, and
the majority of the sensations are painful.
 Phantom sensations may also occur after the removal of body
parts e.g. after amputation of the breast, extraction of a tooth
(phantom tooth pain) or removal of an eye (phantom eye
syndrome)

79. DALE’S LAW
 At one type of synapse only one type of neurotransmitter is
released

81. TRANSDUCTION OF SENSORY
STIMULI INTO RECEPTOR POTENTIAL
AND NERVE IMPULSES
Receptor Potentials
When pressure is
applied to pacinian
corpuscle , a small
non-propogated
depolarizing potential
develops called
receptor potential.

82. MECHANISMS OF RECEPTOR
POTENTIALS.
 Different receptors can be excited in one of several ways to cause receptor
potentials:
 (1) by mechanical deformation of the receptor, which stretches the receptor
membrane and opens ion channels;
 (2) by application of a chemical to the membrane, which also opens ion channels;
 (3) by change of the temperature of the membrane, which
alters the permeability of the membrane; or
 (4) by the effects of electromagnetic radiation,

84. RELATION OFTHE RECEPTOR
POTENTIALTO ACTION POTENTIALS
 When the receptor potential rises above the threshold for eliciting
action potentials in the nerve fiber attached to the receptor, then
action potentials occur.
 More the receptor potential rises above the threshold level, the
greater becomes the action potential frequency.

85. RELATION OFTHE RECEPTOR POTENTIAL
TO ACTION POTENTIALS
 More the receptor potential
rises above the threshold
level, the greater becomes
the action potential
frequency.

86. RELATION BETWEEN STIMULUS
INTENSITY ANDTHE RECEPTOR
POTENTIAL.
 Amplitude increases rapidly
and then less rapid rise at
high stimulus strength
 the frequency of repetitive
action potentials transmitted
from sensory receptors also
increase
 But very strong stimulation
decrease action potentials
as well

87. ADAPTATION OF RECEPTORS
 Another characteristic of all
sensory receptors is that they
adapt either partially or
completely to any constant
stimulus after a period of time.
 That is, when a continuous
sensory stimulus is applied, the
receptor responds at a high
impulse rate at first and then at
a progressively slower rate
until finally the rate of action
potentials decreases to very
few or often to none at all.

88. MECHANISMS BY WHICH RECEPTORS
ADAPT.
 part of the adaptation results from readjustments in the
structure of the receptor itself,
 and part from an electrical type of accommodation in the
terminal nerve fibril

89.  Slowly Adapting Receptors
Detect Continuous Stimulus
Strength—The “Tonic”
Receptors.
 impulses from the muscle
spindles and Golgi tendon
apparatuses allow the nervous
system to know the status of
muscle contraction
 Receptors of the macula in the
vestibular apparatuses
 pain receptors
 baroreceptors of the arterial tree
 chemoreceptors of the carotid
and aortic bodies.
 Rapidly Adapting Receptors
Detect Change in Stimulus
Strength—The “Rate
Receptors,” “Movement
Receptors,” or “Phasic
Receptors.”
 Pacinian and meissner’s
 Importance of the Rate
Receptors—Their Predictive
Function.

90. TYPES OF NERVE FIBERS

91.  To classify nerve fibers
 To understand the properties and differences of nerve fibers

92. NERVE FIBERSTHATTRANSMIT
DIFFERENTTYPES OF SIGNALS, AND
THEIR PHYSIOLOGIC CLASSIFICATION
nerve fibers come in all sizes between 0.5
and 20 micrometers in diameter—
the larger the diameter, the greater the conducting velocity
The range of conducting velocities is between 0.5 and 120 m/sec

93. GENERAL CLASSIFICATION OF
NERVE FIBERS
In the general classification, the fibers are divided into types
 A, B and C,
 A&B are myelinated. C fibers are not
 the type A fibers are further subdivided into
 A α (annulospiral endings)
 A β (touch and pressure)
 A ɣ (motor to muscle spindles)
 Aδ (Pain and temp)
 B fibers are pre-ganglionic fibers
 C fibers are post ganglionic and also transmit slow pain

94. ALTERNATIVE CLASSIFICATION USED
BY SENSORY PHYSIOLOGISTS
 Group Ia
Muscle spindle
 Group Ib
Golgi tendon organs
 Group II
Cutaneous tactile receptors
 Group III
Fibers carrying temperature, crude touch, and pricking pain sensations
 Group IV
Un-myelinated fibers carrying pain, itch, temperature, and crude touch
sensations

95. Diameter in
micrometers
Conduction
velocity in m/sec
Motor FiberType
Aα 0.12-20 0.72-120 Extrafusal skeletal
muscle fibers
Aγ 0.12-8.2 0.12-48 Intrafusal muscle
fibers
B 0.21-33 0.86-18 Preganglionic
autonomic fibers
C 0.2-2 0.5-2 Postganglionic
autonomic fibers

96. Peripheral nerve
fibers
Afferents Diameter of nerve
fibers in mm
Conduction velocity
in m/sec
Receptors
Sensory FiberType
Aα Ia and Ib 0.13-20 0.80-120 m/sec Primary muscle spindle
Golgi tendon organ
Aβ II 0.16-12 0.35-75 Secondary muscle
spindles, skin
mechanoreceptors
Aδ III 0.11-5 0.15-30 Skin mechanoreceptors
thermal receptors, fast
pain
C IV 0.2-1.5 0.5-2 Slow pain and temp

97.  Transmission of Signals of Different Intensity in NerveTracts-
 Spatial andTemporal Summation

98. Temporal summation
 A mean for transmitting
signals of increasing
strength is by increasing the
frequency of nerve impulses
in each fiber, which is called
temporal summation.
Spatial summation

99. RELAYING OF SIGNALS IN
NEURONAL POOL

101. DIVERGING PATHWAYS

102. Two types of divergence
 Amplifying type in same direction
 Divergence into multiple tracts to enter multiple areas of brain

103. CONVERGING PATHWAYS

104. CONVERGENCE OF SIGNALS
 Convergence means signals from multiple inputs uniting to
excite a single neuron.
 Convergence from a single source.That is, multiple terminals
from a single incoming fiber tract terminate on the same
neuron.
 Convergence can also result from input signals (excitatory or
inhibitory) from multiple sources

105. NEURONAL CIRCUITWITH BOTH
EXCITATORY AND INHIBITORY
OUTPUT SIGNALS
 RECIPROCAL INHIBITION CIRCUIT (Flexors & extensors of legs)

106. PROLONGATION OF A SIGNAL BY A
NEURONAL POOL-
"AFTERDISCHARGE”
A signal entering a pool causes a prolonged output discharge,
called after-discharge, after the incoming signal is over.
The input stimulus may last only 1 millisecond or so, and yet the
output can last for many milliseconds or even minutes.
The most important mechanisms by which after-discharge occurs
are the following.
 Synaptic after-discharge due to long acting neurotransmitter
 Reverberatory circuits

107. REVERBERATORY, OR
OSCILLATORY CIRCUIT
 The output neuron simply
sends a collateral nerve fiber
back to its own dendrites or
soma to re-stimulate itself.
 Once the neuron
discharges, the feedback
stimuli could keep the
neuron discharging for a
protracted time.

108. CONTINUOUS SIGNAL OUTPUT
FROM SOME
NEURONAL CIRCUITSWITHOUT
INPUT
(1) continuous intrinsic
neuronal discharge
(inter-neurons of spinal
cord & cerebellum)
(2) continuous reverberatory
circuits that do not fatigue

109. RHYTHMICAL SIGNAL OUTPUT

110. MECHANISMS OF STABILIZING
NERVOUS SYSTEM FUNCTION
 Inhibitory circuits
 Synaptic fatigue
Short term by synaptic fatigue
Long term by affecting no. of synaptic receptors