sensory system Flashcards
sensory system
Information about changes in the external and
internal environment of an organism is conveyed to
the CNS and endocrine by the sense organ.
sense organs
Based on stimulus modality, sense organs
are organized into the following types:
1. CHEMORECEPTORS that respond to
chemicals including odour, taste, etc.
2. THERMORECEPTORS responding to heat
and cold.
3. NOCICEPTORS or Pain receptors
4. MECHANORECEPTORS that respond to
touch, pressure, stretch etc
5. PHOTORECEPTORS enabling vision.
based on origin of stumuli divided into
Interoceptors detect stimuli orginating in
the internal organs and parts of the body,
e.g. pain, nausea, pressure etc.
* Proprioceptor sense the position and
movements of the body.
* Exteroceptors sense stimuli of external
origin.
sensory system contains
Sensory receptors: Receive stimuli from the external or internal environment,
The neural pathways: Conduct information from the receptors to the brain or
spinal cord, and
those parts of the brain that deal primarily with processing the information.
sensation
The Information that a sensory system processes lead to conscious
awareness of the stimulus
sensory information
The Information that a sensory system processes but does not lead to
conscious awareness of the stimulus
perception
A person’s awareness of the sensation (and, typically, understanding of its
meaning)
sensory receptors
Sensory receptors at the peripheral ends of afferent
neurons change this information into graded potentials that
can initiate action potentials, which travel into the central
nervous system.
The receptors are either specialized endings of the primary
afferent neurons themselves ( Figure 7.1a ) or separate
receptor cells (some of which are actually specialized
neurons) that signal the primary afferent neurons by
releasing neurotransmitters
Most sensory receptors are exquisitely sensitive to their
specific adequate stimulus but all sensory receptors can
be activated by different types of stimuli if the intensity
is sufficient
receptor potential magnitude varies
Receptor potential magnitude varies with
Stimulus strength,
Rate of change of stimulus application,
Temporal summation of successive receptor potentials,
Adaptation: decrease in receptor sensitivity
coding
Coding is the conversion of stimulus energy into a signal that conveys the relevant sensory information to the central nervous system
charactersistics of stimulus depends on
Type of input it represents,
Its intensity, and
The location of the body it affects
sensory unit
A single afferent neuron with all its receptor
endings
receptive field
The area of the body that leads to activity
in a particular afferent neuron when stimulated
Receptive fields of neighboring afferent neurons usually overlap
stimulus types/ stimulus modality
Stimulus Type/ Stimulus modality: for example heat,
cold, sound, or pressure
Modalities can be divided into submodalities. for
example
Cold and warm are submodalities of temperature,
salty, sweet, bitter, and sour are submodalities of taste
All the receptors of a single afferent neuron are
preferentially sensitive to the same type of stimulus
for example, they are all sensitive to cold or all to
pressure
stimulus intensty
Stimulus intensity is coded by the rate of firing of
individual sensory units (frequency) and by the number
of sensory units activated.
Sensory receptor potential amplitude tends to be
graded according to the size of the stimulus applied,
but action potential amplitude does not change with
stimulus intensity.
Increasing stimulus intensity is encoded by the
activation of increasing numbers of sensory neurons
(recruitment)
Resultant, increase in the frequency of action potentials
propagated along sensory pathways.
stimulus location
Stimuli of a given modality from a particular region of the body
generally travel along dedicated, specific neural pathways to
the brain, referred to as labeled lines.
The acuity with which a stimulus can be localized depends on
the size and density of receptive fields in each body region.
The greater the convergence, the less the acuity.
Other factors affecting acuity are
the size of the receptive field covered by a single sensory unit ( Figure
7.6a ), the density of sensory units,
And the amount of overlap in nearby receptive fields.
A synaptic processing mechanism called lateral inhibition
enhances localization as sensory signals travel through the CNS.
Most specific ascending pathways synapse in the thalamus on the
way to the cerebral cortex after crossing the midline, such that
sensory information from the right side of the body is generally
processed on the left side of the brain, and vice versa.
lateral inhibito
In lateral inhibition, information from afferent neurons
whose receptors are at the edge of a stimulus is strongly
inhibited compared to information from the stimulus’s
center
Central Control of Afferent
Information
Information coming into the nervous system is subject
to modification by both ascending and descending
pathways.
Inhibition from collaterals from other ascending neurons
(e.g., lateral inhibition)
Inhibitory pathways descending from higher centers in the
brain (reticular formation and cerebral Cortex)
ascending pathway
Sensory pathways are also called ascending pathways
because they project “up” to the brain.
The central processes of the afferent neurons enter the
brain or spinal cord and synapse upon interneurons, where
they either converge or diverge
sensory pathways
Sensory pathways are generally formed by chains of three
or more neurons connected by synapses
Afferent neurons
Second-order neurons
Third-order neurons
types of acending pathway
Types:
specific ascending pathways
Non-specific ascending pathways
specific ascending pathways and there processing center
Somatic receptors (skin,
skeletal muscle, bones,
tendons, and joints)= somatosensory cortex in
the parietal lobe
the eyes= visual cortex , in the
occipital lobe.
the ears= auditory cortex , in the
temporal lobe
from the taste buds= gustatory cortex adjacent
to the region of the
somatosensory cortex
non specific ascending pathways
Nonspecific ascending pathways convey information
from more than one type of sensory unit to the
brainstem reticular formation and regions of the
thalamus that are not part of the specific ascending
pathways.
They indicate that something is happening, without
specifying just what or where
For example, to input from several afferent neurons,
each activated by a different stimulus, such as
maintained skin pressure, heating, and cooling. Such
pathway neurons are called polymodal neurons
Association Cortex and
Perceptual Processing
Information from the primary sensory cortical areas is
elaborated after it is relayed to a cortical association
area
The primary sensory cortical area and the region of
association cortex closest to it process the information
in fairly simple ways and serve basic sensory-related
functions.
Regions of association cortex farther from the primary
sensory areas process the sensory information in more
complicated ways.
Processing in the association cortex includes input from
areas of the brain serving other sensory modalities,
arousal, attention, memory, language, and emotions.
factors that affect perception
Sensory receptor mechanisms (e.g., adaptation) and
processing of the information along afferent pathways
Factors such as emotions, personality, experience, and social
background
Not all information entering the central nervous system gives
rise to conscious sensation e.g., the ear can detect
vibrations having a smaller amplitude
We lack suitable receptors for many types of potential
stimuli. E.g., we cannot directly detect ionizing radiation or
radio waves
Damaged neural networks may give faulty perceptions as in
the phenomenon known as phantom limb
Some drugs alter perceptions
Various types of mental illness
somatic sensation
A variety of receptors sensitive to one or a few stimulus
types provide sensory function of the skin and
underlying tissues.
Information about somatic sensation enters both
specific and nonspecific ascending pathways. The
specific pathways cross to the opposite side of the
brain.
The somatic sensations include touch, pressure, the
senses of posture and movement, temperature, and
pain.
touch and pressure
pressure: Rapidly adapting
mechanoreceptors of the skin give rise to sensations
such as vibration, touch, and movement, whereas
slowly adapting ones give rise to the sensation of
pressure.
Skin receptors with small receptive fields are involved
in fine spatial discrimination, whereas receptors with
larger receptive fields signal less spatially precise touch
or pressure sensations(fig. 7.15)
A major receptor type responsible for the senses of
posture and kinesthesia (sense of movement at a joint)
is the muscle-spindle stretch receptor.
temperature
Cold receptors are sensitive to decreasing temperature;
warmth receptors signal information about increasing
temperature
Afferent neurons with little or no myelination
They lack the elaborate capsular endings
Temperature sensors are ion channels in the plasma membranes
of the axon terminals that belong to a family of proteins called
transient receptor potential ( TRP ) proteins
Different isoforms of TRP channels have gates that open in
different temperature ranges, resulting Na influx and receptor
potential generation
TRP proteins can be opened by chemical ligands. For example,
capsaicin and ethanol are perceived as hot and cold
respectively
pain
Stimuli that cause tissue damage elicit a sensation of
pain, Receptors for such stimuli are known as
nociceptors
Nociceptors (like termoreceptor) respond to intense
mechanical deformation, extremes of temperature, and
many chemicals (H+ ,neuropeptide transmitters,
bradykinin, histamine, cytokines, and prostaglandins,
several of which are released by damaged cells)
Afferents having nociceptor endings synapse on
ascending neurons after entering the central nervous
system which secret glutamate and the neuropeptide
and substance P as neurotransmiter
refered pain
When incoming nociceptive afferents activate
interneurons, the sensation of pain is experienced at a site other than
the injured or diseased tissue e.g., during a heart attack feeling of
pain in left arm
Series of changes can occur in components of the pain pathway—
including the ion channels in the nociceptors themselves—that alters
the way these components respond to subsequent stimuli
hyperaglesia
An increased sensitivity to painful stimuli
aglesia
Analgesia is the selective suppression of pain without effects on
consciousness or other sensations.
stimulation produced aglesia
Electrical stimulation of
specific areas of the central nervous system can produce a
profound reduction in pain because descending pathways that
originate in these brain areas selectively inhibit the transmission
of information originating in nociceptors (Figure 7.16b) e.g.,
morphine like endogenous opioids and acupuncture
Transcutaneous electrical nerve stimulation
the
painful site itself or the nerves leading from it are stimulated by
electrodes placed on the surface of then
normal pathway of somatic sensory system
Specific ascending pathways projecting primarily to the
somatosensory cortex via the brainstem and thalamus.
They also synapse on interneurons that give rise to the
nonspecific ascending pathways
There are two major types of somatosensory pathways
from the body;
Ascending anterolateral pathway
Dorsal column pathway
Both pathways cross from the side where the afferent
neurons enter the central nervous system to the
opposite side either in the spinal cord (anterolateral
system) or in the brainstem (dorsal column system)
vision -eyes
Eyes: Perceiving a visual signal—capable of focusing and
responding to light, and the appropriate neural
pathways and structures to interpret the signal
light and vision
The color of light is defined by its wavelength or
frequency.
The light that falls on the retina is focused by the
cornea and lens.
Lens shape changes (accommodation) to permit viewing
near or distant images so that they are focused on the
retina.
Stiffening of the lens with aging interferes with
accommodation. Cataracts decrease the amount of light
transmitted through the lens.
An eyeball too long or too short relative to the focusing
power of the lens and cornea causes nearsighted (myopic)
or farsighted (hyperopic) vision, respectively.
presbyopia
Presbyopia: decline in the ability to accommodate for
near vision
cataracts
Cataracts: an opacity (clouding) of the lens associated
with smoking and diseases such as diabetes
astigmatism
Astigmatism: the lens or cornea does not have a
smoothly spherical surface
galucoma
Glaucoma: increased pressure within the eye
pupil size
Pupil size:
Miosis: narrow pupil size
Mydriasis: wide pupil size
retina
Retina: consist of photoreceptors and several other cell
types that function in the transduction of light waves
into visual information
photoreceptor
Photoreceptor: outer segment + inner segment
Types: rods and cones
photoreceptors contain molecules called photopigments
e.g., Rhodopsin for the rods, and distinct photopigments
for 3 types of cons
Photopigments: membrane-bound proteins called opsins
bound to a chromophore molecule(retinal).
The photopigments vary with respect of opsin (absorb
different wave length of light)
in absence of light
In the absence of light, action of the enzyme guanylyl
cyclase converts GTP into cGMP which maintain
ligand-gated cation channels in an open state, and allow
influx of Na+ and Ca 2+ results
in presence of light
In the presence of light, the opsin protein shape alters
and promotes an interaction between the opsin and a
protein called transducin which activates cGMP-
phosphodiesterase, which rapidly degrades cGMP
ligand-gated cation channels get closed
(hyperpolarization)
dark adaption
Dark adaptation: When someone entered from bright
sunlight into a darkened room, a temporary “blindness”
takes place.
light adaption
Light adaptation: When someone step from a dark place
into a bright one. Initially, the eye is extremely
sensitive to light as rods are overwhelmingly activated,
and the visual image is too bright and has poor contrast.
photopigments
The photopigments of the rods and cones are made up
of a protein component (opsin) and a chromophore
(retinal).
The rods and each of the three cone types have different
opsins, which make each of the four receptor types
sensitive to different ranges of light wavelengths.
When light strikes retinal, it changes shape, triggering a
cascade of events leading to hyperpolarization of
photoreceptors and decreased neurotransmitter release
from them. When exposed to darkness, the rods and cones
are depolarized and therefore release more
neurotransmitter than in light.
neural pathway of vision
The rods and cones synapse on bipolar cells, which
synapse on ganglion cells.
Ganglion cell axons form the optic nerves, which exit the
eyeballs.
The optic nerve fibers from the medial half of each retina
cross to the opposite side of the brain in the optic chiasm.
The fibers from the optic nerves terminate in the lateral
geniculate nuclei of the thalamus, which sends fibers to
the visual cortex.
Photoreceptors also send information to areas of the brain
dealing with biological rhythms.
Coding in the visual system occurs along parallel
pathways in which different aspects of visual
information, such as color, form, movement, and depth,
are kept separate from each other.
color vision
The colors we perceive are related to the wavelength of
light. The three cone photopigments vary in the
strength of their response to light over differing ranges
of wavelengths
Certain ganglion cells are excited by input from one type
of cone cell and inhibited by input from a different cone
type.
Our sensation of color depends on the output of the
various opponent color cells and the processing of this
output by brain areas involved in color vision.
Color blindness is due to abnormalities of the cone
pigments resulting from genetic mutations.
color blindness
There are several types of defects in color vision that
result from mutations in the cone pigments.
The most common form of color blindness , red–green
color blindness
Predominant in men (1 out of 12) and rare in women (1
out of 200)
Color blindness results from a recessive mutation in one
or more genes encoding the cone pigments.
eye movement
Six skeletal muscles control eye movement to scan the
visual field for objects of interest, keep the fixation
point focused on the fovea centralis despite movements
of the object or the head, and prevent adaptation of
the photoreceptors.
hearing
Sound energy is transmitted by movements of pressure
waves.
a. Sound wave frequency determines pitch.
b. Sound wave amplitude determines loudness.
Humans audible hearing limits (keen hearing) 1000-4000
Hz
Range of frequencies audible to human (20 to 20,000
Hz).
sequence of sound transmission
Sound waves enter the external auditory canal and press against the
tympanic membrane, causing it to vibrate.
The vibrating membrane causes movement of the three small middle
ear bones; the stapes vibrates against the oval window membrane.
Movements of the oval window membrane set up pressure waves in
the fluid-filled scala vestibuli, which cause vibrations in the cochlear
duct wall, setting up pressure waves in the fluid there.
These pressure waves cause vibrations in the basilar membrane,
which is located on one side of the cochlear duct.
As this membrane vibrates, the hair cells of the organ of Corti move
in relation to the tectorial membrane.
Movement of the hair cells’ stereocilia stimulates the hair cells to
release glutamate, which activates receptors on the peripheral ends
of the afferent nerve fibers.
Separate parts of the basilar membrane vibrate maximally in
response to particular sound frequencies;
high frequency is detected near the oval window
Low frequency toward the far end of the cochlear duct.
vestibular system
Vestibular apparatus: connected series of endolymph-filled,
membranous tubes that also connect with the cochlear duct
The hair cells detect changes in the motion and position of the
head by a stereocilia transduction mechanism
The vestibular apparatus consists of three membranous
semicircular canals and two saclike swellings, the utricle and
saccule
Housed in tunnels in the temporal bone on each side of the head
The semicircular canals detect angular acceleration during
rotation of the head along three perpendicular axes
The utricle and saccule (see Figure 7.42 ) provide information
about linear acceleration of the head, and about changes in head
position relative to the forces of gravity
chemica senses
Chemoreceptors: The receptors sensitive to specific
chemicals, They respond to chemical changes in their
environment;
Internal: two examples are receptors that sense oxygen
and hydrogen ion concentration in the blood
External: the receptors for taste and smell, which affect a
person’s appetite, saliva flow, gastric secretions, and
avoidance of harmful substances
taste
Taste Buds:
10,000 or so taste buds found in the mouth and throat, the
vast majority on the upper surface and sides of the tongue
Taste buds are small groups of cells arranged like orange
slices around a hollow pore and are found in the walls of
visible structures called lingual papillae( Figure 7.47 ).
Some of the cells serve mainly as supporting cells, but others
are specialized epithelial cells that act as receptors for
various chemicals in the food we eat.
Hair-like microvilli: increase the surface area of taste
receptor cells and contain integral membrane proteins that
transduce the presence of a given chemical into a receptor
potential.
At the bottom of taste buds are basal cells, which
divide and differentiate to continually replace taste
receptor cells damaged in the occasionally harsh
environment of the mouth.
To enter the pores of the taste buds and come into
contact with taste receptor cells, food molecules must
be dissolved in liquid
taste receptors
Many different chemicals can generate the sensation of
taste by differentially activating a few basic types of
taste receptors
Taste sub-modalities generally fall into five different
categories according to the receptor type most strongly
activated; sweet, sour, salty, bitter, and umami (yet
others to be discovered)
Each group of tastes has a distinct signal transduction
mechanism
salt taste
is detected by entering sodium ions in the receptor cell
membrane channels, depolarizing the cell and stimulating the
production of action potentials in the associated sensory neuron.
sweet taste
Sweet receptors have integral membrane. Binding of sugars to
these receptors activates a G-protein-coupled second-messenger
pathway that ultimately blocks K+ channels and thus generates a
depolarizing receptor potential.
sour taste
Sour taste is stimulated by foods with high acid content.
Hydrogen ions block K+ channels in the sour receptors, and the
loss of the hyperpolarizing K+ leak current depolarizes the
receptor cell.
bitter
Bitter flavor there are many varieties of bitter receptors. All of
those types, however, generate receptor potentials via
G-protein-mediated second-messenger pathways and ultimately
evoke the negative sensation of bitter flavor.
umami
Umami receptor cells also depolarize via a G-protein-coupled
receptor mechanism
smell
Sense of smell (olfaction): The olfactory receptor(chemoreceptors)
perceive the odour of thousands of chemical with perfect accuracy
Location: a small patch of epithelium called the olfactory
epithelium in the upper part of the nasal cavity
Olfactory receptor neurons survive for only about 2 months
Specialized afferent neurons that have a single, enlarged dendrite that
extends to the surface of the epithelium
Several long, nonmotile cilia extend from the tip of the dendrite and lie
along the surface of the olfactory epithelium
The cilia contain the receptor proteins that provide the binding sites for
odor molecules
The axons of the neurons form the olfactory nerve, which is cranial
nerve I
factors affecting olfaction
Factor affecting Olfaction:
Hunger: Sensitivity is greater in hungry subjects
Gender: Women in general have keener olfactory sensitivities
than men
Smoking: Decreased sensitivity has been repeatedly associated
with smoking
Age: The ability to identify odors decreases with age
State of the olfactory mucosa: The sense of smell decreases
when the mucosa is congested, as in a head cold
Genetic: Genetic defects resulting in a total lack of the ability
to smell ( anosmia).
For example, defects in genes on the X chromosome, as well as in
chromosomes 8 and 20, can cause Kallmann syndrome. This is a
condition in which the olfactory bulbs fail to form, as do regions of
the brain associated with regulation of sex hormones.
