The document discusses the classification and structure of neurons in the nervous system. It describes three main types of neurons based on the number of poles: unipolar, bipolar, and multipolar neurons. It also discusses the classification of neurons based on function into motor/efferent and sensory/afferent neurons. Additionally, it summarizes the structure of neurons including the nerve cell body, dendrites, and axon. The key roles and components of each part are defined.

1. Physiology-3
Department of physiotherapy
Central Nervous System
BY
Dr.Laraib jameel Rph
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2. Nervous system
• The nervous system is a complex network of nerves and cells that carry
messages to and from the brain and spinal cord to various parts of the
body.
• The nervous system includes both the Central nervous system and
Peripheral nervous system
• Difference between nerve & neuron:
• Nerve: bundle of neurons with fascia and blood supply. it is whitish fiber of
neuron cell which carry impulses to CNS & from CNS to effector organ.
• (fiber+vessel+ lymphatic) (in CNS+PNS)(act as conducting zone)
• Neuron: it is single nerve cell. it is specialized cell involved in transmitting
nerve impulses (chemical & electrical signal generated)

3. Nervous System
• Neuron or nerve cell :
• Neuron or nerve cell is defined as the structural and functional unit of nervous system. Neuron is similar to
any other cell in the body, having nucleus and all the organelles in cytoplasm.
• However, it is different from other cells by two ways:
• 1. Neuron has branches or processes called axon and dendrites
• 2. Neuron does not have centrosome. (help to replicate the chromosome into daughters) So, it cannot
undergo division.
• „CLASSIFICATION OF NEURON
• Neurons are classified by three different methods.
• A. Depending upon the number of poles
• B. Depending upon the function
• C. Depending upon the length of axon.
• „DEPENDING UPON THE NUMBER OF POLES
• Based on the number of poles from which the nerve fibers arise, neurons are divided into three types:

4. • 1. Unipolar neurons 2. Bipolar neurons 3. Multipolar neurons.
• 1. Unipolar Neurons:
• Unipolar neurons are the neurons that have only one pole. From a single
pole, both axon and dendrite arise. This type of nerve cells is present only
in embryonic stage in human beings.
• 2. Bipolar Neurons:
• Neurons with two poles are known as bipolar neurons. Axon arises from
one pole and dendrites arise from the other pole.
• 3. Multipolar Neurons:
• Multipolar neurons are the neurons which have many poles. One of the
poles gives rise to axon and all other poles give rise to dendrite

6. • DEPENDING UPON THE FUNCTION
• On the basis of function, nerve cells are classified into two types:
• 1. Motor or efferent neurons 2. Sensory or afferent neurons.
• 1. Motor or Efferent Neurons
• Motor or efferent neurons are the neurons which carry the motor impulses
from central nervous system to peripheral effector organs like muscles,
glands, blood vessels, etc. Generally, each motor neuron has a long axon
and short dendrites.
• 2. Sensory or Afferent Neurons
• Sensory or afferent neurons are the neurons which carry the sensory
impulses from periphery to central nervous system.
• Generally, each sensory neuron has a short axon and long dendrites.

7. • DEPENDING UPON THE LENGTH OF AXON
• Depending upon the length of axon, neurons are divided into two types:
• 1. Golgi type I neurons 2. Golgi type II neurons.
• 1. Golgi Type I Neurons: Golgi type I neurons have long axons. Cell body of
these neurons is in different parts of central nervous system (begins from
grey matter of CNS and extend from there) and their axons reach the
peripheral organs.
• 2. Golgi Type II Neurons: Neurons of this type have short axons. These
neurons are present in cerebral cortex and spinal cord. It has star like
appearance.
• These neuron distinction was introduced by Camillo Golgi he used golgi
stain under microscope that he had invented

9. Structure of neuron
• STRUCTURE OF NEURON: Neuron is made up of three parts: 1. Nerve cell
body 2. Dendrite 3. Axon. Dendrite and axon form the processes of neuron.
• Dendrites are short processes and the axons are long processes.
• Dendrites and axons are usually called nerve fibers.

10. • NERVE CELL BODY
• Nerve cell body is also known as soma or perikaryon.
• It is irregular in shape.
• Like any other cell, it is constituted by a mass of cytoplasm called neuroplasm, which is
covered by a cell membrane.
• The cytoplasm contains a large nucleus, Nissl bodies, neurofibrils, mitochondria and
Golgi apparatus.
• Nissl bodies and neurofibrils (transport of protein & other substances) are found only in
nerve cell and not in other cells.
• Nucleus
• Each neuron has one nucleus, which is centrally placed in the nerve cell body.
• Nucleus has one or two prominent nucleoli.
• Nucleus does not contain centrosome. So, the nerve cell cannot multiply like other cells.

11. • Nissl Bodies:
• Nissl bodies or Nissl granules are small basophilic granules found in cytoplasm of neurons and
are named after the discoverer.
• These bodies are present in soma and dendrite but not in axon.
• Nissl bodies are called tigroid (granular E-R) substances, since these bodies are responsible for
tigroid or spotted appearance of soma after suitable staining.
• Dendrites are distinguished from axons by the presence of Nissl granules under microscope.
• Nissl bodies are membranous organelles containing ribosomes.
• So, these bodies are concerned with synthesis of proteins in the neurons. Proteins formed in
soma are transported to the axon by axonal flow.
• Number of Nissl bodies varies with the condition of the nerve. During fatigue or injury of the
neuron, these bodies fragment and disappear by a process called chromatolysis. Granules
reappear after recovery from fatigue or after regeneration of nerve fibers.
• Chromatolysis is dissolution of nissl body in cell body)

12. • Neurofibrils:
• Neurofibrils are thread-like structures present in the form of network in
the soma and the nerve processes.
• The neurofibrils consist of microfilaments and microtubules (provides
support & strength).
• Mitochondria:
• Mitochondria are present in soma and in axon. As in other cells, here also
mitochondria form the powerhouse of the nerve cell, where ATP is
produced.
• Golgi Apparatus:
• Golgi apparatus of nerve cell body is similar to that of other cells. It is
concerned with processing and packing of proteins into granules.

13. • DENDRITE:
• Dendrite is the branched process of neuron and it is branched repeatedly.
Dendrite may be present or absent. If present, it may be one or many in number.
• Dendrite has Nissl granules and neurofibrils. Dendrite transmits impulses towards
the nerve cell body.
• Usually, the dendrite is shorter than axon.
• AXON:
• Axon is the longer process of nerve cell.
• Each neuron has only one axon.
• Axon extends for a long distance away from the nerve cell body. Length of
longest axon is about 1 meter. Axon transmits impulses away from the nerve cell
body.

15. General organization of nervous system
• The nervous system consists of two parts
1. The central nervous system (CNS)
• The brain and spinal cord are the organs of the central nervous system. Because they are so vitally
important, the brain and spinal cord, located in the dorsal body cavity, are encased in bone for
protection.
• The brain is in the cranial vault, and the spinal cord is in the vertebral canal of the vertebral
column. (provide protection, shape to trunk)
2. The Peripheral Nervous System
• The organs of the peripheral nervous system are the nerves and ganglia.
• Nerves are bundles of nerve fibers, much like muscles are bundles of muscle fibers. Cranial nerves
and spinal nerves extend from the CNS to peripheral organs such as muscles and glands.
• Ganglia are collections, or small knots, of nerve cell bodies outside the CNS.
• Cranial- relating to cranium & skull

18. General organization of nervous system
• The autonomic nervous system has two divisions:
• The sympathetic nervous system: (FIGHT & FLIGHT) is involved in the stimulation of activities
that prepare the body for action, such as increasing the heart rate, increasing the release of sugar
from the liver into the blood, and other activities generally considered as fight‐or‐flight responses
(responses that serve to fight off or retreat from danger).
• The parasympathetic nervous system (PEACE) activates tranquil functions, such as stimulating
the secretion of saliva or digestive enzymes into the stomach and small intestine.
• Generally, both sympathetic and parasympathetic systems target the same organs, but often
work antagonistically. For example, the sympathetic system accelerates the heartbeat, while the
parasympathetic system slows the heartbeat. Each system is stimulated as is appropriate to
maintain homeostasis.
• The somatic nervous system, also called the somatomotor or somatic efferent nervous system,
supplies motor impulses to the skeletal muscles. Because these nerves permit conscious control
of the skeletal muscles, it is sometimes called the voluntary nervous system.

21. Classification of nerve fibers
• Nerve fibers are classified by six different methods. The basis of classification differs in each method.
1. DEPENDING UPON STRUCTURE Based on structure, nerve fibers are classified into two types:
i. Myelinated Nerve Fibers Myelinated nerve fibers are the nerve fibers that are covered by myelin sheath.
• Function: rapid conduction of action potential.
• Location: central nervous system (white matter-Deeper tissue of brain-contains nerve fibers), peripheral
nervous system
• Composition: rich in lipids
ii. Non-myelinated Nerve Fibers Nonmyelinated nerve fibers are the nerve fibers which are not covered by
myelin sheath.
• Function: slow conduction of action potential.
• Location: central nervous system (gray matter), peripheral nervous system (autonomic, visceral nervous
system)

22. • MYELIN SHEATH
• Myelin sheath is a thick lipoprotein sheath. It is formed by Schwann cells. Schwann cells wrap up and rotate
around the axis cylinder in many concentric layers. The concentric layers fuse to produce myelin sheath.
• Myelin sheath is not a continuous sheath. It is absent at regular intervals. The area where myelin sheath is
absent is called node of Ranvier. Segment of the nerve fiber between two nodes is called internode.
• Myelin sheath is responsible for white color of nerve fibers.
• Functions of Myelin Sheath
• 1. Faster conduction: Myelin sheath is responsible for faster conduction of impulse through the nerve
fibers. In myelinated nerve fibers, the impulses jump from one node to another node. This type of
transmission of impulses is called saltatory conduction
• 2. Insulating capacity: Myelin sheath has a high insulating capacity. Because of this quality, myelin sheath
restricts the nerve impulse within single nerve fiber and prevents the stimulation of neighboring nerve
fibers.
• Insulate- to separate
• Schwann cell- glia of peripheral system

25. Classification of nerve fibers
2. DEPENDING UPON DISTRIBUTION
Nerve fibers are classified into two types
i. Somatic Nerve Fibers
• Somatic nerve fibers supply the skeletal muscles of the body.
• They are commonly referred to as motor neurons due to their termination
in skeletal muscle.
• ii. Visceral or Autonomic Nerve Fibers
• The visceral motor fibers (those supplying smooth muscle, cardiac muscle,
& glands) make up the Autonomic Nervous System.
• Viscera-internal organs of body(located in chest and abdomen)

26. Classification of nerve fibers
3. DEPENDING UPON ORIGIN
• On the basis of origin, nerve fibers are divided into two types:
• i. Cranial Nerve Fibers Nerve fibers arising from brain are called cranial nerve fibers.
• ii. Spinal Nerve Fibers Nerve fibers arising from spinal cord are called spinal nerve fibers.
4. DEPENDING UPON FUNCTION
• Functionally, nerve fibers are classified into two types
• i. Sensory Nerve Fibers Sensory nerve fibers carry sensory impulses from different parts of the
body to the central nervous system. These nerve fibers are also known as afferent nerve fibers.
• ii. Motor Nerve Fibers Motor nerve fibers carry motor impulses from central nervous system to
different parts of the body. These nerve fibers are also called efferent nerve fibers.
• Cranial- relating to skull & cranium

27. Classification of nerve fibers
5. DEPENDING UPON SECRETION OF NEUROTRANSMITTER
• Depending upon the neurotransmitter substance secreted, nerve fibers are
divided into two types:
• i. Adrenergic Nerve Fibers Adrenergic nerve fibers secrete noradrenaline.
• ii. Cholinergic Nerve Fibers Cholinergic nerve fibers secrete acetylcholine.
6. DEPENDING UPON DIAMETER AND CONDUCTION OF IMPULSE (ERLANGER-
GASSER CLASSIFICATION)
• Erlanger and Gasser classified the nerve fibers into three major types, on the
basis of diameter (thickness) of the fibers and velocity of conduction of
impulses:
• i. Type A nerve fibers ii. Type B nerve fibers iii. Type C nerve fibers.
• These groups include both sensory fibers and motor fibers

28. • Among these fibers, type A nerve fibers are the thickest fibers and type C
nerve fibers are the thinnest fibers.
• Except type C fibers, all the nerve fibers are myelinated.
• Group B nerve fibers are axons, which are moderately myelinated, which
means less myelinated than group A nerve fibers, and more myelinated
than group C nerve fibers.
• There are four subdivisions of group A nerve fibers:
• alpha (ɑ), beta (β), gamma (ɣ), and delta (δ).
• These subdivisions have different amounts of myelination and axon
thickness and therefore transmit signals at different speeds.
• Larger diameter axons and more myelin insulation lead to faster signal
propagation.

30. Properties of synaptic transmission
• Synapse is the junction between two neurons. it is only a
physiological continuity between two nerve cells.

31. CLASSIFICATION
classification of synapse is on the basis of mode of impulse transmission. According to this, synapse
is classified into two categories:
1. Electrical synapse 2. Chemical synapse.
2. 1-Electrical Synapse
• Electrical synapse is the synapse in which the physiological continuity between the presynaptic
and the post synaptic neurons is provided by gap junction between the two neurons.
• There is direct exchange of ions between the two neurons through the gap junction. Because of
this reason, the action potential reaching the terminal portion of presynaptic neuron directly
enters the postsynaptic neuron.
• Important feature of electrical synapse is that the synaptic delay is very less because of the
direct flow of current.
• Moreover, the impulse is transmitted in either direction through the electrical synapse. This type
of impulse transmission occurs in some tissues like the cardiac muscle fibers, smooth muscle
fibers of intestine and the epithelial cells of lens in the eye.

33. • 2. Chemical Synapse Chemical synapse is the junction between a
nerve fiber and a muscle fiber or between two nerve fibers, through
which the signals are transmitted by the release of chemical
transmitter.
• In the chemical synapse, there is no continuity between the two
neurons because of the presence of a space called synaptic cleft
between the two neurons.
• Mechanism: Action potential reaching the presynaptic terminal
causes release of neurotransmitter substance from the vesicles of this
terminal. Neurotransmitter reaches the postsynaptic neuron through
synaptic cleft and causes the production of potential change.

34. • FUNCTIONS OF SYNAPSE
• Main function of the synapse is to transmit the impulses, i.e. action
potential from one neuron to another.
• However, some of the synapses inhibit these impulses. So the
impulses are not transmitted to the postsynaptic neuron.
• On the basis of functions, synapses are divided into two types:
• 1. Excitatory synapses, which transmit the impulses (excitatory
function)
• 2. Inhibitory synapses, which inhibit the transmission of impulses
(inhibitory function).

35. • EXCITATORY FUNCTION
• Excitatory Postsynaptic Potential:
• Excitatory postsynaptic potential (EPSP) is the nonpropagated electrical potential that develops during the
process of synaptic transmission. When the action potential reaches the presynaptic axon terminal, the
voltage gated calcium channels at the presynaptic membrane are opened. Now, the calcium ions enter the
axon terminal from ECF. Calcium ions cause the release of neurotransmitter substance from the vesicles by
means of exocytosis. Neurotransmitter, which is excitatory in function (excitatory neurotransmitter) passes
through presynapticnaptic membrane and synaptic cleft and reaches the postsynaptic membrane. Now, the
neurotransmitter binds with receptor protein present in postsynaptic membrane to form neurotransmitter-
receptor complex. Neurotransmitterreceptor complex causes production of a nonpropagated EPSP.
• Common excitatory neurotransmitter in a synapse is acetylcholine.
• Mechanism:
• Neurotransmitterreceptor complex causes opening of ligand gated sodium channels. Now, the sodium ions
from ECF enter the cell body of postsynaptic neuron. As the sodium ions are positively charged, resting
membrane potential inside the cell body is altered and mild depolarization develops. This type of mild
depolarization is called EPSP. It is a local potential (response) in the synapse.

36. • INHIBITORY FUNCTION
• 1. Postsynaptic or Direct Inhibition
• Postsynaptic inhibition is the type of synaptic inhibition that occurs due to the
release of an inhibitory neurotransmitter from presynaptic terminal instead of an
excitatory neurotransmitter substance. It is also called direct inhibition. Inhibitory
neurotransmitters are gammaaminobutyric acid (GABA), dopamine and glycine.
• 2. Presynaptic inhibition
• occurs due to the failure of presynaptic axon terminal to release sufficient
quantity of excitatory neurotransmitter substance. It is also called indirect
inhibition.
• Significance of Synaptic Inhibition
• Synaptic inhibition in CNS limits the number of impulses going to muscles and
enables the muscles to act properly and appropriately.

37. • PROPERTIES OF SYNAPSE
1- ONE WAY CONDUCTION – BELL-MAGENDIE LAW
• According to BellMagendie law, the impulses are transmitted only in one
direction in synapse, i.e. from presynaptic neuron to postsynaptic neuron.
2- SYNAPTIC DELAY
• Synaptic delay is a short delay that occurs during the transmission of
impulses through the synapse. It is due to the time taken for: i. Release of
neurotransmitter ii. Passage of neurotransmitter from axon terminal to
postsynaptic membrane iii. Action of the neurotransmitter to open the
ionic channels in postsynaptic membrane.
• Normal duration of synaptic delay is 0.3 to 0.5 millisecond. Synaptic delay
is one of the causes for reaction time of reflex activity.

38. • 3- FATIGUE
• During continuous muscular activity, synapse becomes the seat of fatigue along with Betz
cells present in motor area of frontal lobe of cerebral cortex.
• Fatigue at synapse is due to the depletion of neurotransmitter substance, acetylcholine.
• Depletion of acetylcholine occurs because of two factors: i. Soon after the action,
acetylcholine is destroyed by acetylcholinesterase ii. Due to continuous action, new
acetylcholine is not synthesized.
• 4- SUMMATION
• Summation is the fusion of effects or progressive increase in the excitatory postsynaptic
potential in post synaptic neuron when many presynaptic excitatory terminals are
stimulated simultaneously or when single presynaptic terminal is stimulated repeatedly.
Increased EPSP triggers the axon potential in the initial segment of axon of postsynaptic
neuron. Summation is of two types:

39. • i. Spatial Summation Spatial summation occurs when many
presynaptic terminals are stimulated simultaneously.
• Temporal Summation Temporal summation occurs when one
presynaptic terminal is stimulated repeatedly.
• Thus, both spatial summation and temporal summation play an
important role in facilitation of response
• 5. ELECTRICAL PROPERTY
• Electrical properties of the synapse are the EPSP and IPSP

41. Function of neurotransmitters
• Neurotransmitter is a chemical substance that acts as a mediator for the transmission of
nerve impulse from one neuron to another neuron through a synapse
• DISCOVERY: Existence of neurotransmitter was first discovered by an Austrian scientist
named Otto Loewi in 1921. He dreamt of an experiment, which he did practically and
came out with the discovery.
• CRITERIA FOR NEUROTRANSMITTER
• Nowadays, many substances are categorized as neuro - transmitters. To consider a
substance as a neurotransmitter, it should fulfill certain criteria as given below:
• 1. It must be found in a neuron
• 2. It must be produced by a neuron
• 3. It must be released by a neuron
• 4. After release, it must act on a target area and produce some biological effect
• 5. After the action, it must be inactivated.

42. Neurotransmitter
• CLASSIFICATION OF NEUROTRANSMITTERS
• DEPENDING UPON CHEMICAL NATURE
• Many substances of different chemical nature are identified as
neurotransmitters. Depending upon their chemical nature, neurotransmitters
are classified into three groups.
• 1. Amino Acids: Neurotransmitters of this group are involved in fast synaptic
transmission and are inhibitory and excitatory in action. GABA, glycine, glutamate
(glutamic acid) and aspartate (aspartic acid) belong to this group.
• 2. Amines: Amines are the modified amino acids. These neurotransmitters
involve in slow synaptic transmission. These neurotransmitters are also inhibitory
and excitatory in action. Noradrenaline, adrenaline, dopamine, serotonin and
histamine belong to this group.
• 3. Other: One such substance is acetylcholine. It is formed from the choline and
acetyl coenzyme A in the presence of the enzyme called choline acetyltransferase

43. • DEPENDING UPON FUNCTION
• Some of the neurotransmitters cause excitation of postsynaptic
neuron while others cause inhibition.
• Thus, neurotransmitters are classified into two types:
• 1. Excitatory neurotransmitters (Depolarization)
• Ex- anxiety
• 2. Inhibitory neurotransmitters (Repolarization)
• Ex- anesthesia, depression

45. Neurotransmitter
• TRANSPORT AND RELEASE OF NEUROTRANSMITTER
• Neurotransmitter is produced in the cell body of the neuron and is transported through axon. At
the axon terminal, the neurotransmitter is stored in small packets called vesicles. Under the
influence of a stimulus, these vesicles open and release the neurotransmitter into synaptic cleft. It
binds to specific receptors on the surface of the postsynaptic cell.
• Receptors are G proteins, protein kinase or ligand-gated receptors.
• INACTIVATION OF NEUROTRANSMITTER
• After the execution of the action, neurotransmitter is inactivated by four different mechanisms:
• 1. It diffuses out of synaptic cleft to the area where it has no action
• 2. It is destroyed or disintegrated by specific enzymes
• 3. It is engulfed and removed by astrocytes (macrophages)
• 4. It is removed by means of reuptake into the axon terminal.
• Ex- epinephrine, dopamine, serotonin (involved in memory function)

50. Neuropeptides
• Neuropeptides are small proteins produced by neurons that act on G protein-coupled
receptors and are responsible for slow-onset, long-lasting modulation of synaptic
transmission.
• Neuropeptides often coexist with each other or with other neurotransmitters in single
neurons.
• According to their chemical nature, coexisting messengers are localized to different cell
compartments:
• neuropeptides are packaged in large granular vesicles (LGVs),
• whereas low-molecular weight neurotransmitters are stored in small synaptic vesicles.
• This compartmentalization allows selective release in response to stimuli.
• Individual LGVs may store several different neuropeptides, at times
with monoamines or neurotrophic factors.
• LGVs release their neuropeptide cargo by full-fusion exocytosis

52. Synthesis storage and release of
neuropeptides
• Neuropeptides are synthesized from larger inactive precursor proteins in
the lumen of the endoplasmic reticulum
• After a series of modification steps common to many proteins in the
endoplasmic reticulum, inactive pro-peptides are packaged at the trans-
Golgi network into immature secretory granules along with peptide-
specific biosynthetic processing enzymes.
• As the granules mature into large dense-core vesicles, processing to
smaller, further modified, active neuropeptides occurs.
• Neuropeptides are then stored, transported to distal parts of the neuron,
and eventually released in response to a stimulus.
• Different peptides can be sorted into separate granules, and granules are
trafficked to both axons and dendrites.

55. Sensory receptors
• A sensory receptor is a structure that reacts to a physical stimulus in
the environment, whether internal or external.
• It is a sensory nerve ending that receives information and conducts a
process of generating nerve impulses to be transmitted to the brain
for interpretation and perception.
• Stimuli in the environment activate specialized receptors or receptor
cells in the peripheral nervous system. Different types of stimuli are
sensed by different types of receptors.
• Sensory receptors vary in classifications but generally initiate the
same process of registering stimuli and creating nerve signals.

56. • Functions
• In a sensory system, sensory receptors serve as the front-liners because
they are in contact with the stimulus.
• Taste or gustatory receptors, odor or olfactory receptors have receptor
molecules which undergo a process of binding to chemicals in the stimuli.
• For instance, the chemicals in food interaction with the taste receptors of
the taste bud so that an action potential or a nerve signal can be created.
• Other sensory receptors function by means of transduction.
• Photoreceptors of the eye contain rhodopsin and other proteins that
transduce or transform light energy into electrical impulses. Without these
sensory receptors, both sensation and perception cannot occur.

58. Sensory receptors
• Classification by morphology:
• Sensory receptors that are classified according to morpohology or form are usually divided into two main
groups
• Free nerve endings are dendrites whose terminal ends have little or no physical specialization.
• Free nerve endings such as thermoreceptors and nociceptors have unmyelinated terminal neuronal
branches (i.e. no myelinated sheath or protection, thus they are bare).
• The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve
endings.
• Encapsulated nerve endings are dendrites whose terminal ends are enclosed in a capsule of connective
tissue. Encapsulated receptors such as Meissner's and Pacinian corpuscles are protected by layered
connective neurons with encapsulated nerve endings that respond to pressure and touch
• Sense organs (such as the eyes and ears) consist of sensory neurons with receptors for the special senses
(vision, hearing, smell, taste, and equilibrium) together with connective, epithelial, or other tissues.
• Pacinian+pressure &vibration)
• Meissner's = George = for light touch

61. • Classification by location:
Exteroceptors occur at or near the surface of the skin and are sensitive to
stimuli occurring outside or on the surface of the body. These receptors
include those for tactile sensations, such as touch, pain, and temperature,
as well as those for vision, hearing, smell, and taste.
• Interoceptors (visceroceptors) respond to stimuli occurring in the body
from visceral organs and blood vessels. These receptors are the sensory
neurons associated with the autonomic nervous system.
• Proprioceptors respond to stimuli occurring in skeletal muscles, tendons,
ligaments, and joints. These receptors collect information concerning body
position and the physical conditions of these locations.

62. • By Location
• The types of sensory receptors according to location include cutaneous
receptors and mechanoreceptors.
• Sensory receptors located in the dermis or epidermis of the skin are called
cutaneous receptors. These include nociceptors and thermoreceptors.
• Mechanoreceptors, on the other hand, are located in muscle spindles,
enabling them to detect muscle stretch. Other receptors are located inside
the body, such as the baroceptors in the blood vessels.
• Telereceptors:
• Telereceptors are the receptors that give response to stimuli arising away
from the body. These receptors are also called the distance receptors

63. • Classification by type of stimulus detected:
• Mechanoreceptors respond to physical force such as pressure (touch
or blood pressure) and stretch.
• Photoreceptors respond to light.
• Thermoreceptors respond to temperature changes.
• Chemoreceptors respond to dissolved chemicals during sensations of
taste and smell and to changes in internal body chemistry such as
variations of O 2, CO 2, or H + in the blood.
• Nociceptors respond to a variety of stimuli associated with tissue
damage. The brain interprets the pain.

65. • Pathway
• Graded potentials in free and encapsulated nerve endings are called generator
potentials.
• When strong enough to reach threshold they can directly trigger an action
potential along the axon of the sensory neuron.
• Action potentials triggered by receptor cells, however, are indirect.
• Graded potentials in receptor cells are called receptor potentials. These graded
potentials cause neurotransmitter to be released onto a sensory neuron causing a
graded post-synaptic potential. If this graded post-synaptic potential is strong
enough to reach threshold it will trigger an action potential along the axon of the
sensory neuron.
• (graded potential- generate potential--------- reach the threshold level-------trigger
action potential by receptor cells------ neurotransmitter release by sensory
neuron------ graded post synaptic potential------ reach on threshold level)

66. PROPERTIES OF RECEPTORS
• PROPERTIES OF RECEPTORS
• 1. SPECIFICITY OF RESPONSE – MÜLLER LAW Specificity of response
or Müller law refers to the response given by a particular type of
receptor to a specific sensation. For example:
• pain receptors give response only to pain sensation.
• Similarly, temperature receptors give response only to temperature
sensation. In addition, each type of sensation depends upon the part
of the brain in which its fibers terminate. Specificity of response is
also called Müller’s doctrine of specific nerve energies.

67. PROPERTIES OF RECEPTORS
• 2. ADAPTATION – SENSORY ADAPTATION
• Adaptation is the decline in discharge of sensory impulses when a receptor is stimulated
continuously with constant strength. It is also called sensory adaptation or desensitization.
• Depending upon adaptation time, receptors are divided into two types:
• i. Phasic receptors – rapidly adapt, useful in situations where it is important to signal a change in
stimulus – tactile (touch) receptors. Touch and pressure receptors are the phasic receptors
• ii. Tonic receptors, which adapt slowly. Muscle spindle, pain receptors and cold receptors are the
tonic receptors. Tonic receptors do not adapt at all or very slowly, important when maintaining
information about a stimulus is valuable – stretch, pain receptors
• 3. RESPONSE TO INCREASE IN STRENGTH OF STIMULUS – WEBERFECHNER LAW
• During the stimulation of a receptor, if the response given by the receptor is to be doubled, the
strength of stimulus must be increased 100 times. This phenomenon is called WeberFechner law,
which states that intensity of response (sensation) of a receptor is directly proportional to
logarithmic increase in the intensity of stimulus.

68. PROPERTIES OF RECEPTORS
• 4. SENSORY TRANSDUCTION
• Biological Transducers: Actually receptors function like a transducer. Transducer is a device,
which converts one form of energy into another. So, receptors are often defined as the biological
transducers, which convert (transducer) various forms of energy (stimuli) in the environment into
action potentials in nerve fiber.
• Stimuli----- action potential
• Sensory transduction in a receptor is a process by which the energy (stimulus) in the environment
is converted into electrical impulses (action potentials) in nerve fiber (transduction = conversion
of one form of energy into another).
• When a receptor is stimulated, it gives response by sending information about the stimulus to
CNS. Series of events occur to carry out this function such as the development of receptor
potential in the receptor cell and development of action potential in the sensory nerve. Sensory
transduction varies depending upon the type of receptor. For example, the chemoreceptor
converts chemical energy into action potential in the sensory nerve fiber.
• Touch receptor converts mechanical energy into action potential in the sensory nerve fiber

69. PROPERTIES OF RECEPTORS
• 5. RECEPTOR POTENTIAL
• Definition: Receptor potential is a nonpropagated transmembrane potential difference that
develops when a receptor is stimulated. It is also called generator potential.
• Receptor potential is short lived and hence, it is called transient receptor potential.
• Receptor potential is not action potential. It is a graded potential.
• It is similar to excitatory postsynaptic potential (EPSP) in synapse, endplate potential in
neuromuscular junction and electric potential in the nerve fiber.
• Properties of Receptor Potential
• Receptor potential has two important properties.
• i. Receptor potential is nonpropagated; it is confined within the receptor itself ii. It does not obey
all or none law. (stimulus is not independent of strength of stimulus)
• Significance of Receptor Potential
• When receptor potential is sufficiently strong (when the magnitude is about 10 mV), it causes
development of action potential in the sensory nerve.

71. PROPERTIES OF RECEPTORS
• Mechanism of Development of Receptor Potential
• Pacinian corpuscles are generally used to study the receptor potential
because of their large size and anat mical configuration. These corpuscles
can be easily dissected from the mesentery of experimental animals. In the
pacinian corpuscle, the tip of the nerve fiber is unmyelinated. This
unmyelinated nerve tip extends through the corpuscle as center core fiber.
The concentric layers of the corpuscle surround the core fiber of the nerve.
Pacinian corpuscles give response to pressure stimulus. When pressure
stimulus is applied, the Pacinian corpuscle is compressed. This compression
causes elongation or change in shape of the corpuscle. The change in shape
of the corpuscle leads to the deformation of center core fiber of the
corpuscle. This results in the opening of mechanically gated sodium
channels. So, the positively charged sodium ions enter the interior of core
fiber. This produces a mild depolarization, i.e. receptor potentia

73. PROPERTIES OF RECEPTORS
• 6. LAW OF PROJECTION
• When a sensory pathway from receptor to cerebral cortex is
stimulated on any particular site along its course, the sensation
caused by stimulus is always felt at the location of receptor,
irrespective of site stimulated. This phenomenon is known as law of
projection.
• Examples of Law of Projection
• If sometic area in right cerebral cortex, which receives sensation from
left hand is stimulated, sensations are felt in left hand and not in
head.

74. Graded potentials Action potentials
Depending on the stimulus, graded potentials can
be depolarizing or hyperpolarizing.
Action potentials always lead to depolarization of
membrane and reversal of the membrane
potential.
Amplitude is proportional to the strength of the
stimulus.
Amplitude is all-or-none; strength of the stimulus is
coded in the frequency of all-or-none action
potentials generated.
Amplitude is generally small (a few mV to tens of
mV).
Large amplitude of ~100 mV.
Duration of graded potentials may be a few
milliseconds to seconds.
Action potential duration is relatively short; 3-5 ms.
Ion channels responsible for graded potentials
may be ligand-gated (extracellular ligands such as
neurotransmitters), mechanosensitive, or
temperature sensitive channels, or may be channels
that are gated by cytoplasmic signaling molecules.
Voltage-gated Na+
and voltage-gated K+
channels
are responsible for the neuronal action potential.
The ions involved are usually Na+
, K+
, or Cl−
.
The ions involved are Na+
and K+
(for neuronal
action potentials).