• Pain is associated with nociception stimulation (nociception is the perception of
physiological pain) and is measurable physiological event.
• Nociception is a critical component of the body’s defense system, because it is
part of rapidwarning system relaying instructions to the CNS to initiate an
efferent motor response, triggering cognitive processes to reduce or remove
Stimulation of nociceptors:
Categories of Pain receptors:
• Mechanical nociceptors respond to mechanical damage, such as cutting,
crushing, or pinching
• Thermal nociceptors respond to temperature extremes, especially heat
• Chemical noceciptors respond equally to many kinds of irritating chemicals
released from damaged tissues
Fast and Slow afferent Pain fibers:
• Categorize pain as either fast pain or slow pain
• Pain impulses from nociceptors are transmitted to the CNS via on of two types of
afferent fibres, which range in size and speed: Adelta fibres which are largest and
fastest and C fibres which are the smallest and slowest.
• Signals resulting from cold, warmth and mechanical stimuli are transmitted via
fast myelinated Adelta fibres at rates of 630 m/sec
• Impulses carried via unmyelinated slow C fibres carry slow pain, heat, cold and
mechanical stimuli at a much slower rate <12 m/sec
• Slow pain pathway is activated by chemicals – bradykinin a normally inactive
substance that is activated by enzymes released into the ECF from damaged
tissue. It works by stimulating the polymodal nociceptors they respond to more
than one stimuli.
Higherlevel processing of pain input
• The primary afferent pain fibres synapse with specific secondorder interneurons
in the dorsal horm of the spinal cord. In response to stimulusinduced action
potentials, afferent pain fibres release neurotransmitters that influence these next
neurons in line –substance P and glumate.
• Substance P activates ascending pathways that transmit nociceptive signals to
higher level for further processing.
• Cortical somatosensory processing area localize the pain • Interconnections from the thalamus and reticular formation to the hypothalamus
and limbic system elicit the behavioral and emotional responses accompanying
the painful experience
• Glutamate ia s major excitatory neuron transmitter. Acts as two different plasma
membrane receptors on the dorsal horn neurons with two different outcomes.
First, binding of glutamate with its AMPA receptor leads to permeability changes
that ultimately result in the generation of action potentials in the dorsal horn cell.
These action potential transmit the pain message to higher centers. Second,
binding of glutamate with its NMDA receptors leads to CA+ entry into the dorsal
horn cell. Ca+ initiates second massager system that make the dorsal horn neuron
more excitable than usual. The hyper excitability contributes in part to the
exaggerated sensitivity of an injured area to subsequent exposure to painful or
normal stimuli such as light touch. i.e sunburn to clothing touch
• Chronic pain serves as a normal protective mechanism to warn of impending or
actual damage t the body.
The brain’s builtin analgesic system
• Suppresses transmission in the pain pathways as they enter the spinal cord. These
regions are known to be a part of the descending analgesic pathway.
• Electric stimulations of the periaqueductal grey matter results in profound
analgesia, as does stimulation of the reticular formation within the brain stem.
• This analgesic system suppresses pain by blocking the release of substance P from
afferent pain fibre terminals.
• The system depends on the presence of opiate receptors i.e morphinedulling pain
• Endorphins, enkephalins and dynorphins are a part of the body’s natural analgesic
system. They are released from descending analgesic pathways and bind with
opiate receptors on the afferent pain fibre termina. This binding suppresses the
release of substance P via presynaptic inhibition, thereby blocking further
transmission of pain signals
• *Substance P pain pathway and analgesic pathways. A) When activated by a
nonious stimulus, some afferent pain pathways release substance P, which
activates ascending pain pathways that provide brain regions with input for
processing different aspects of the painful experience. B) Endogenous opiates
released from descending analgesic (pain relieving) pathways bind with opiate
receptors at the synaptic knob of the afferent pain fibre. This binding inhibits the
release of substance P, thereby blocking transmission of pain impulse along the
ascending pain pathways.
• For vision, the eyes capture the patterns of illumination in the environment as an
optical picture on a layer of lightsensitive cells, the retina, much as a nondigital
camera captures an image on film Protective mechanisms and eye injuries:
• Eyelids act like shutters to protect the anterior portion of the eye from the
• Tears are produced by the lacrimal gland in the upper lateral corner under the
• Eyelashes trap fine, airborne debris such as dusts before it can fall into the eye.
The eye: a fluid sphere
• Eye is a spherical, fluidfilled structure enclosed by 3 layers: 1. The scleral/cornea
2. The choroid/ciliary body/iris and 3. The retina.
• Sclera forms the visible white part of the eye. The outer layer consists of cornea,
through which light rays pass into the interior of the eye.
• The middle layer underneath is the highly pigmented choroid which contains
many blood vessels that nourish the retina. It become specialized to form the
ciliary body and iris.
• Retina consists of an outerpigmented layer and an inner nervous tissue layers.
The latter contains the rods and cones, the photoreceptors that convert light
energy into nerve impulses.
• The interior of the eye consist of two fluidfilled cavities separated by an elliptical
lens, all of which are transparent to permit light to pass through the eye from the
cornea to the retina
• Vitreous humour semifluid, jellylike substance found between the lens and
• Aqueous humour a clear watery fluid found between the cornea and lens. It
contains nutrients for the cornea and leans. It is produced within cilary body
• If the aqueous humour is not drained as rapidly as it forms the excess will
accumulate in the anterior cavity causing the pressure to rise within the eye
condition known as glaucoma.
• The pigment in the iris is responsible for the eye colour.
• The round opening in the centre of the iris through which light eneters the interior
portions of the eyes is the pupil.
• It contains two sets of smooth muscle networks, one circular and other radial.
• Because muscle fibres shorten when they contract, the pupil gets smaller when the
circular muscle contracts and forms a smaller rings. This reflex pupillary
constriction occurs in bright light to decrease the amount of light entering the eye.
• When the radial muscle shortens the size of the pupil increases i.e dim light
• Iris muscles are controlled by the autonomic nervous system (ANS)
• Lights is a form of electromagnetic radiation composed of particlelike individual
packets of energy called photons that travel wavelike fashion. The distance
between peak is know an wave length • The visibl