BIOL1711- Special Senses

General function of sensory receptors

To detect and respond to a stimulus, and then convert the stimulus energy into an electrical signal, and then initiate sensory input to the CNS

Why are sensory receptors called transducers?

They transduce stimulus energy (which is specific to the type of sensory receptor, e.g., light energy detected by the eye, sound energy by the ear, mechanical energy by blood vessels, etc.) to electrical energy, and it is sent along a neuron. Then the sensory information is sent to the CNS for interpretation

Features that allows sensory receptors to function as transducers

1) They establish a resting membrane potential across their plasma membrane

2) They contain modality gated channels within their plasma membranes that opens in response to a stimulus other than a neurotransmitter or voltage change

Receptive field

Area within which the dendritic endings of a single sensory neuron are distributed. The size determines the ability of the CNS to identify the exact location of a stimulus.

Small receptive field provides the ability to identify the stimulus location more specifically, whereas a large receptive field allows us to determine only the general region of the stimulus

Sensation

A stimulus that we are consciously aware of (has reached the cerebral cortex of the brain)

What filers our sensory input

Filtered by thalamus (except for olfaction) and does not reach cerebral cortex or is relayed to other areas of the CNS (e.g., hypothalamus, spinal cord, brain stem) where a response is initiated without our awareness.

E.g., information regarding our blood pressure, blood CO2 levels and chemical composition of materials within the small intestine are detected and responded on a subconscious level

What characteristics must a sensory receptor be able to provide the CNS with regarding a stimulus

Whether it is consciously perceived or not, it must include characteristics like its modality, location, intensity and duration

Modality of a stimulus

Provided by a given type of sensory receptor relaying sensory input along designated sensory neuron’s to specific regions of the CNS. The brain is like a switch board and interprets the source based on which “line” the signal arrived.

E.g., sensory receptors of the retina initiate sensory input along optic nerve to the primary visual cortex of the occipital lobe

E.g., sensory receptors of the ear initiate sensory input along the cochlear branch of the vestibulocochlear nerve to the auditory cortex of the temporal lobe

Location of a stimulus

Determined by the CNS because sensory information is relayed either from different regions of a sensory receptor or from different locations within the body along designated sensory neurons within a given nerve that reaches specific regions of the CNS.

E.g., optic nerve is composed of the axons of many sensory neurons that extend from different portions of the retina of the eye to communicate with designated regions within the visual cortex of the occipital lobe

Intensity of a stimulus

Can be interpreted because of the change in number of nerve signals that are arriving along a designated nerve. A greater or more intense stimulus results in both the most sensitive sensory receptors initiating nerve signals more frequently and the less sensitive sensory receptors (which are not typically active) initiating nerve signals

Duration of a stimulus

Detected by CNA as all sensory receptors become less sensitive to a constant stimulus and initiate a progressive decrease in nerve signals. This is called adaptation. However, the rate of decrease is different for various types of sensory receptors

Tonic receptors

Demonstrate limited adaptation. In response to a constant stimulus, these receptors continuously generate nerve signals and only slowly decrease the number relayed to the CNS.

E.g., sensory receptors within the inner ear that determine head position and proprioceptors in the muscles and joints that provide information of where your body is in space.

E.g., all pain receptors so that it provides motivation to address the cause of the pain and hopefully eliminate it so it will stop

Phasic receptors

Exhibit rapid adaptation to a constant stimulus. They generate nerve signals in response to a new (or changing) stimulus and quickly decrease the number of nerve signals relayed to the CNS.

E.g., deep pressure receptors that sense the increase pressure when we sit on chair

Somatic sensory receptors

Tactile receptors in the skin and mucus membranes. Proprioceptors in the muscles, tendons, and joints

Visual sensory receptors

Receptors within the walls of internal organs and blood vessels

Special senses

Complex sense organs located only in the head (e.g., sensory receptors responsible for smell, taste, vision, hearing and equilibrium

Exteroceptors

Detect stimuli in the external environment

Interoceptors

Detect stimuli within the body

Propriceptors

Detect stimuli within skeletal muscles, tendons, and joints that monitor body movement

Chemoreceptors

Detect chemicals (molecules or ions) dissolved in fluid

Thermoreceptors

Detect changes in temperature

Photoreceptors

Detect changes in light intensity, colour and movement

Mechanoreceptors

Detect physical deformation of the plasma membrane due to touch, pressure, vibration, and stretch. Subtypes include baroreceptors, proprioceptors, tactile receptors, and other specialized cells such as hair cells in the cochlea of the ear

Nociceptors

Detect painful stimuli

General sense recepots

Somatic and visceral sensory receptors

Baroreceptors

A type of mechanoreceptor that is stimulated by changes in stretch or distention within a blood vessel. Monitor this as part of blood pressure regulation

Somatic nociceptors

Detect chemical, heat or mechanical changes to the body surfaces, joints, or skeletal muscles.

E.g., touching a hot pan or sprained ankle

Visceral nociceptors

Detect internal changes within the viscera. Often occurs due to either 1) deprivation of oxygen, 2) smooth muscle being stretched so much we are uncomfortable or 3) the tissue has suffered trauma or damaged cells have released chemicals that stimulate specific nociceptors

E.g., feeling discomfort in our internal organs

Referred pain

When sensory input from certain internal organs is perceived by the brain as originating not from the organ, but from somatic sensory receptors within the skin and skeletal muscle.

This is possible because general sense receptors (tactile receptors, proprioceptors and visceral sensory receptors) use common sense pathways with ascending tracts of the spinal cord to relay sensory input to the brain

Pain along medial side of the upper limb where T1 spinal nerve innervates

Referred pain, associated with myocardial infarction

Pain in inferior abdomen wall in the groin and loin regions (areas of lower trunk)

Referred pain of kidney and ureters along T10-L2 spinal nerves

Pain in medial buttocks

Referred pain from the urinary bladder from neurons that originate in S2-S4 region of the spinal cord

How do olfaction and gustation occur?

Through sensory receptors in the head that detect dissolved chemicals in the air and in our food.

Olfaction

Sense of smell, whereby volatile molecules (odorants) must be dissolved in the mucus in our nasal cavity to be detected by chemoreceptors.

Olfactory epithelium

Sensory receptor organ for smell. Lines the superior region of the nasal cavity, covering both the interior surface of the cribriform plate and the superior nasal conchae of the ethmoid bone.

Distinct cell types of the olfactory epithelium

Composed of pseudostratified columnar epithelium such as:

• Olfactory receptor cells: also called olfactory neurons, which detect odour-producing molecules

• Supporting cell: also called sustentacular cells, which sustain the olfactory receptor cells

• Basal cells: function as neural stem cells to continually replace olfactory receptor cells

Lamina propria

Areolar connective tissue layer internal to the olfactory epithelium

Contains mucus secreting olfactory glans (also called Bowans glands), blood vessels, and nerves

Supporting cells and olfactory epithelium

Secretions form the mucus that covers the exposed surface of the olfactory epithelium

Olfactory receptor cells

Bipolar neurons that undergone extensive differentiation and modification, and serve as the primary neuron in the sensory pathway for smell. Projecting from the dendrites are numerous thin, non-motile cilia called olfactory hairs that extend into the layer of mucus, which contain chemoreceptors that detect one specific odorant molecule

How do olfactory receptor cells enter the olfactory bulb?

They form bundles (fascicles) of the olfactory nerve (CN I) and project through the foramina in the cribriform plate of the ethmoid bone to enter

How often are olfactory receptor cells replaced through cell division?

Regenerated every 40 to 60 days by basal cells within the olfactory epithelium. Thus process decreases with age, and as such lose their sensitivity to odors (decreased ability to recognize odor molecules with age)

Olfactory bulbs

Terminal ends of olfactory tracts located inferior to the frontal lobes of the brain

Do olfactory pathways project the thalamus?

Unlike other sensory information, olfactory pathways do not project to the thalamus and therefore do not undergo any thalamic processing and filtering prior to reaching the cerebrum

How we detect different smells

During relaxed breathing, most inhaled air does not pass the olfactory epithelium. To be sure to detect different smells, we must sniff repeatedly or breathe deeply which causes inhaled air to mix and swirl in the superior region of the nasal cavity so that odor molecules diffuse into the mucus layer covering the olfactory receptor cells.

Regions of the brain that integrate sensory input from sensory receptors

• Primary olfactory cortex: within temporal lobe, allows us to consciously perceive and identify smell

• Hypothalamus: controls visceral reactions to smell, such as salivation, sneezing, or gaggig

• Amygdala: centre for recognition of odors and often associated those odors to particular emotion

Adaptation of smell

An initially strong smell may seem to dissipate as olfactory receptor smells adapt to that odour. Occurs rapidly, as once receptors are stimulated the changes to ion channels alters the flow of ions, and it results in interfering with the subsequent generation of local receptor potentials within the olfactory receptor cells.

Gustation

Our sense of taste. Occurs when we come into contact with the taste producing molecules and ions of what we eat and drink (called tastants).

Gustatory cells are chemoreceptors located within the taste buds on the tongue and soft palate. And the tongue and soft palate also house mechanoreceptors and thermoreceptors to tell us about the texture and temperature of food, respectively.

Papillae

Epithelial and connective tissue elevations located on the dorsal surface of the tongu.

Four types: filiform, fungiform, foliate, and vallate

Filiform papillae

Short and spiked, distributed on anterior 2/3 of tongue surface. Assist in detecting texture and manipulating food. Do not detect taste.

Fungiform papillae

Mushroom/block shaped. Primarily located on tips and sides of tongue. Each contain only a few taste buds

Foliate papillae

Not well developed on human tongue. Extend as ridges on the posterior lateral sides of the tongue and house only a few taste buds during infancy and early childhood

Vallate papillae

Also called circumvallate papillae. Are the least numerous (10-12) but largest papillae on the tongue. Arranged in an inverted V shape on the posterior dorsal surface of the tongue. Most of our taste buds are house within the wall of these papillae along the side facing the depression

Taste buds

Sensory receptor organs that have the appearance of an onion. Each is composed of three distinct cell types:

• Gustatory cells: also called gustatory receptors, which detect taste producing molecules and ions in our food and drink

• Supporting cells: sustain the gustatory cells

• Basal cells: function as neural stem cells to continually replace the relatively short lived gustatory cells

Gustatory cells

Within the taste buds, are specialized neuroepithelial cells and chemoreceptors. Has gustatory microvilli at their dendritic endings which extend through an opening in the taste bud called the “taste pore” to the surface of the tongue (which is the receptive portion of the cell)

What keeps the environment most in the oral cavity

Saliva. Chemicals in our food dissolve in the saliva and then they are able to contact and stimulate gustatory cells

Gustatory pathways

Sensory neurons that contact gustatory cells are primarily components of the facial nerve (CN VII) and innervates taste buds from the anterior two thirds of the tongue, and the glossopharangeal nerve (CN IX) which innervates the taste buds from the posterior 1/3 of the tongue

Axons within these nerves project the medulla oblongata to synapose with the secondary neurons (e.g., triggers reflexes that increase salivation and release stomach secretions in anticipation of food arrival). Secondary neurons project to the thalamus and axons of the tertiary neurons project to the primary gustatory cortex in the insula of the cerebrum for conscious perception of taste.

Taste sensations

5 basics sensations: sweet, salt, sour, bitter, umami

Sweet tastes

Produced by organic molecules such as glucose, fructose, sucrose, or other molecules

Salt taste

Produced by metal ions such as sodium and potassium

Sour tastes

Associated with acids in the ingested material, such as hydrogen ions

Bitter tastes

Produced primarily by alkaloids (organic, nitrogen-containing molecules) such as quinine, unsweetened chocolate, nicotine and caffeine

Umami

Also called savory flour. Is a taste related to amino acids, such as glutamate and aspartate, to product a savory or meaty flavour

What heavily affects our ability to taste

Our olfactory sense. If you have a severe cold or sinus infection, the aroma of food could not reach olfactory receptors, so food is bland and almost tasteless.

Together taste and small give our food its flavour. We then perceive the taste of food when the brain interprets sensory info from both gustatory receptors and olfactory receptors.