biol 2410 - unit 6

The sources discuss sensory physiology, which is how the nervous system gathers information about the internal and external environment to maintain homeostasis1. Humans have 9 to 21 senses, more than the traditional five1. The senses can be categorised based on where the sensory signal originates and whether it is consciously or unconsciously perceived by the brain23.

Types of Senses

Conscious senses are perceived and processed in the conscious areas of the brain (i.e., in the sensory areas of the cerebral cortex)34. These include:

●

Somatic Senses: touch, temperature, pain, itch, proprioception (position of the body)4

●

Special Senses: vision, hearing, taste (gustation), smell (olfaction), and balance (equilibrium)3

Unconscious senses are processed in areas of the brain outside of the cerebral cortex. The stimuli associated with these senses include:

●

Somatic stimuli: muscle length and tension, proprioception5

●

Visceral stimuli: blood pressure, osmolarity of extracellular fluids, blood glucose concentration, internal (core) body temperature, lung inflation, pH of plasma and CSF, O₂ and CO₂ content of blood5

General Functions of Senses

The somatic, visceral, and special senses provide information about:

●

The external environment: helping to identify food sources, temperature changes, weather changes; potential mates, potential hazards, shelter, etc.5

●

Ourselves: providing information about energy stores, body temperature; water/ion balance (thirst/hunger); the position of our body, etc.6

General Properties of Sensory Systems

●

Sensory receptors are most sensitive to a certain stimulus or form of energy6. The specific form of the stimulus is called the sensory modality6.

●

Signal transduction in sensory receptors converts a stimulus into graded potentials6. The receptor acts as the transducer – converting the energy of the stimulus into electrical energy (a change in membrane potential)6.

●

Sensory neurons have receptive fields7.

●

The CNS integrates sensory information7.

●

Coding and processing distinguish stimulus properties7.

Types of Receptors

Sensory receptors can be classified by their structure, location, type of adequate stimulus, and the rate of adaptation78910.

Structure (General Senses Only)

●

Free nerve endings: dendrites of unipolar sensory neurons7. For example, pain receptors (nociceptors)11.

●

Encapsulated receptors: terminal dendrites wrapped in connective tissue8. For example, some touch receptors, like Meissner’s (tactile) corpuscles that detect light touch11.

Location

●

Interoceptors: detect stimuli in the internal environment (in viscera, blood vessels, etc.)8.

●

Exteroceptors: detect stimuli in the external environment, so receptors are at the body’s surface (skin)9.

Type of Adequate Stimulus

●

Adequate stimulus = modality or form of energy to which the receptor is most responsive9. Receptors can sometimes respond to other modalities but only if the stimulus strength has a high enough intensity9. For example, temperature receptors can respond to the chemical found in hot peppers (capsaicin) and cause you to feel heat9.

●

Includes:

○

Mechanoreceptors: detect physical forces/stimuli (touch, pressure, stretch, vibrations, gravity, sound waves, etc.). Includes baroreceptors (detect pressure/stretch in blood vessels, lungs, etc.) and some transducer cells like the hair cells in the inner ear12.

○

Thermoreceptors: detect hot and cold temperatures12.

○

Chemoreceptors: detect chemicals – for example, olfactory neurons; interoceptors for O₂ or pH12.

○

Photoreceptors: detect light. Includes the rods and cones (transducer cells) in the retina of the eye12.

○

Nociceptors: detect pain10.

○

Proprioceptors: detect body position and movement10.

Rate of Adaptation

●

In the presence of a continuous (sustained) stimulus, some receptors show adaptation = a reduction in the frequency of action potentials sent though sensory neurons to the brain10.

●

Types of receptors:

○

Tonic receptors: slowly adapting receptors. Fire rapidly upon stimulus, then decrease firing rate, but maintain firing rate under a constant stimulus10. Often used with sensations that are continuously monitored, like proprioceptors, baroreceptors in blood vessels, etc.13.

○

Phasic receptors: rapidly adapting receptors. Fire at the onset of the stimulus, then stop if stimulus intensity remains constant (adaptation)13. They do not fire again until there is a change in the intensity of the stimulus13. For example, many touch receptors13. For example, putting glasses on in the morning – produces a burst of APs in mechanoreceptors for touch (and you feel the glasses) ⇒ over time the firing rate will decrease and eventually stop as the receptors adapt13.

Signal Transduction by Sensory Receptors

●

Stimulation leading to activation of the receptor triggers the signal transduction process in the receptor cell. The minimum stimulus required to activate a receptor is the receptor threshold14.

●

Signal transduction involves changes in membrane potentials of sensory neurons or non-neuronal transducer receptor cells14.

●

The graded potential produced on the receptor cell is called a receptor potential. It is triggered by an adequate stimulus (the preferred stimulus of the receptor cell type)14. The intensity of the stimulus is encoded by the magnitude (amplitude) of the graded potential14.

●

The amplitude of the graded potential is translated into the frequency of action potentials in the associated neurons15.

Sensory Pathways

Transduced signals then travel along sensory neurons to the brain15. Depending on the sense involved, the pathway may involve 2–3+ neurons15. For most general senses, except some proprioceptive pathways, the pathway is always three neurons:

●

Primary (first order) sensory neurons:

○

Unipolar neurons16

○

First neuron in the pathway16

○

Are the receptor in which transduction occurs (in the dendrites) OR they are the unipolar neurons that receive the transduced signal from the non-neuronal receptor cell16.

●

Secondary (second order) sensory neurons:

○

Multipolar neurons16

○

Cell bodies in the dorsal horn of the spinal cord or in the medulla depending on the sense involved16

●

Tertiary (third order) sensory neurons:

○

Multipolar neurons17

Receptive Fields of Sensory Receptors

●

Receptive field = physical area in which the presence of a stimulus will activate the receptor and alter the firing of a sensory neuron17. Often associated with cutaneous (skin) receptors and the retina (so would represent the area in a patch of skin that would activate the receptor in question or the area of the retina that would need to be stimulated by light to activate specific rods/cones)17.

●

Primary sensory neurons converge onto secondary neurons, allowing for spatial summation of signals on the secondary neuron18.

●

The more convergence of primary sensory neurons onto secondary sensory neurons, the larger the receptive field and the lower the sensory acuity18. Sensory acuity = how accurately a stimulus can be located18. When there is a lot of convergence, the receptive fields associated with each individual primary sensory neuron (primary receptive fields) overlap more and form one large secondary receptive field18. So instead of having sensation originating from each of the individual receptive fields, the sensation is assigned to the larger secondary receptive field, which makes it more difficult to determine the precise location of the stimulus18. Multiple stimuli occurring within the same secondary receptive field will be interpreted as one stimulus19.

●

The less convergence of primary sensory neurons onto secondary sensory neurons, the smaller the receptive fields and the higher the sensory acuity19. When there is less convergence the receptive fields associated with each individual primary sensory neuron do not overlap, and so each primary neuron/receptor can send signals to the brain, which makes it easier to determine the precise location of the stimulus (higher sensitivity)19.

Two-point Discrimination Tests

●

Determine the sensory acuity of touch receptors on the surface of the skin20.

●

Tested using an instrument called an aesthesiometer20.

●

In areas of the skin where there is less convergence and the secondary receptive fields are small (such as the skin on fingertips), the two points of the aesthesiometer activate separate pathways to the brain and both points are felt even if they are close together (positive two-point discrimination test) = High acuity20.

●

In areas where there is more convergence and secondary receptive fields are large (such as the skin on your back) the two points of the aesthesiometer activate only a single pathway to the brain and are perceived by the brain as a single point, even if the two points are far apart (i.e. negative two-point discrimination test) = Low acuity21.

Somatotopic Maps

●

Each part of the body can be mapped to a specific area of the primary somatosensory cortex (the part of the cortex that perceives general senses)21.

●

These maps were developed by electrically stimulating the different regions of the cortex in people undergoing brain surgery22. Patients were conscious – local anesthetics can be used to remove the cranium (skull cap) and the brain itself does not have any pain receptors. It can perceive pain from other areas, but has no ability to detect its own pain22. Canadian doctor Wilder Penfield was instrumental in developing these maps22.

●

Areas of the body that appear larger on the map have greater acuity/sensitivity (less convergence)23. Areas of the body that appear smaller on the map have lower acuity/sensitivity (more convergence)23.

Integration of Sensory Signals by CNS

Sensation vs. Perception

●

Sensation occurs when sensory information is detected by receptors in the peripheral nervous system. For example, auditory receptors detect a loud, high-pitched ringing noise23.

●

Perception occurs in the brain and involves awareness, organization, and interpretation (assigning of meaning) of the sensation24. For example, the high pitched ringing noise is a fire alarm and you need to gather your things and leave the building24.

●

In the brain, perception is built from sensation – but not all sensations result in perception24.

Sensory Information Pathways

●

From spinal cord to brain via ascending (sensory) pathways comprised of primary, and secondary neurons and in many cases tertiary neurons24.

●

Directly to brainstem via cranial nerves (exception – pathway for olfaction – olfactory nerves)25.

Sensory Information Processing

●

Some sensory information may be processed at the level of the spinal cord or brainstem25. Because they do not reach the cortex of the brain, this sensory information is unconscious25. For example:

○

Most visceral reflexes, like changes in blood pressure, are not perceived by the brain because they are integrated in the brain stem or spinal cord25.

○

Some of the sensory pathways for proprioception and equilibrium project primarily to the cerebellum. As a result, they do not synapse in the thalamus and therefore do not reach the cortex – as such, they are unconscious26.

●

Some sensory information is processed in the cortex26. Recall that the cortex is the conscious area of the brain, so conscious sensation and perception occur in the sensory areas of the cortex. If you can physically feel a stimulus, then the sensory signal has ended up in the cortex26. Many pathways for sensory information synapse with neurons in the thalamus27. The thalamus then relays those signals to the appropriate area of the cortex27.

●

A stimulus may activate a primary sensory neuron (sensation), but may not have enough intensity to surpass the perceptual threshold27. Perceptual threshold = level of stimulus necessary for the conscious brain to become aware of the sensation28.

Properties of a Stimulus

All sensory stimuli are converted into action potentials (APs) via transduction28. So the properties associated with each stimulus must be encoded in the pathway of the APs or in the APs themselves28. The four main properties of a stimulus are:

●

Modality: interpretation of the type of stimulus28. For example, touch, temperature, pain, sound, light, taste etc.29.

○

Determined by the neuronal pathway that brings the sensory signals to the CNS = labeled line coding29. Each receptor has a specific adequate stimulus29. When stimulated by an adequate stimulus, the receptor initiates signaling in a hardwired pathway to the higher processing centres in the brain30. The processing centres in the brain (cortex) contain specific groups of neurons that are associated with specific modalities30. Any signals coming through this pathway will always be interpreted as the modality associated with the region of the brain stimulated30. Even if the receptor is removed, any signal in the pathway will still be perceived by the brain in the correct modality30. As a result, it is the pathway that determines how the brain perceives the modality30.

●

Location: interpretation of where on the body a stimulus has occurred29.

○

Determined by which receptive fields are activated31. The receptive fields for any part of the body are mapped onto the cortex of the brain (somatotopic maps) and stimuli from adjacent areas of the body are processed in adjacent areas of the brain32. Labeled line coding ensures that when a particular receptive field is stimulated, the pathway of neurons to the brain will connect to the specific region of the brain associated with the area stimulated32.

○

Can also be determined by lateral inhibition:

■

Mechanism to increase contrast between activated receptive fields and non-activated receptive fields33.

■

Steps:

1.

Stimulus activates receptive fields of primary neurons33.

2.

Primary neuron closest to the stimulus has a higher frequency of APs and releases more neurotransmitter33. Adjacent primary neurons that are further from the stimulus produce fewer APs33.

3.

Collaterals of the secondary neuron closest to the stimulus inhibit adjacent secondary neurons (= lateral inhibition, which is an example of presynaptic inhibition)34.

4.

Lateral inhibition turns off signaling in pathways that are further from the stimulus, which increases the contrast in AP frequency between the different receptive fields35.

5.

Increased contrast between the center and the sides of the receptive fields by the time the signals reach the brain - makes the stimulus easier to localize and as a result enhances perception of the stimulus35.

■

Lateral inhibition is also an example of population coding in which sensory information from several neurons/pathways are combined and compared to provide the CNS with more information than it could have received from an individual neuron/receptor35.

●

Intensity: interpretation of the strength of a stimulus (for example, how to tell if a feather has dropped on your foot vs. a brick)29.

○

Determined by:

1.

The number of receptors activated (population coding)36. The greater the number of receptors that are activated the greater the intensity36. For example, activating more than one receptor of the same type or activating multiple types of receptors (for example, touch, pressure, and pain at the same time)36.

2.

The frequency of action potentials reaching the brain36. Graded potentials in receptors vary in strength with the stimulus36. So… Stronger stimulus à stronger graded potential à increase in action potential frequency à releases more neurotransmitter in the brain (alerts brain to increased intensity)37.

○

Example: A feather touching the bottom of your foot may only activate a few touch receptors37. A brick falling on your foot, would activate, touch, pressure, and possibly pain receptors37. With more receptors activated, there would be an increase in the frequency of APs reaching the brain37.

●

Duration: interpretation of how long the stimulus is applied/present29.

○

Determined (coded for) by duration of action potentials in neurons38.

○

The longer a stimulus is present, the longer action potentials will continue to be sent to the brain38.

○

Exception is phasic receptors, which demonstrate adaptation to a stimulus38.

Somatosensory Pathways

●

Pathways for general senses (touch, temperature, nociception, pressure, proprioception) project to cortex or to cerebellum38.

●

Pathways to cortex have primary, secondary and tertiary neurons39. Secondary neurons decussate – cross over to the opposite side of body in either the:

1.

Spinal cord – for nociception, temperature and coarse touch39

2.

Medulla – for fine touch, proprioception and vibration39

●

All secondary neurons connect with tertiary neurons in the thalamus39.

●

Sensations are perceived in the primary somatosensory cortex39.

●

Somatosensory pathways use three types of neuronal axons (nerve fibers)40:

1.

Type Aβ (A-beta) fibers:

■

Large diameter myelinated neurons (fast conductance)40

■

Carry sensory information for cutaneous mechanical stimuli40.

2.

Type Aδ (A-delta) fibers:

■

Small myelinated neurons40

■

Carry sensory information for cold, fast pain (nociceptors)40

3.

Type C fibers:

■

Small diameter, unmyelinated neurons (slow conductance)40

■

Carry sensory information for slow pain (nociceptors)40

●

Most receptors for the general senses are located in the skin40.

Mechanoreception

●

Touch receptors (mechanoreceptors) = most abundant type of receptor in body41.

●

Mostly found in skin41. For example:

○

Pacinian corpuscles that detect generalized pressure and vibrations – deep in the dermis of the skin41.

●

Also found in some internal organs41. For example:

○

Walls of blood vessels have baroreceptors (pressure-stretch receptors) that respond to changes in blood pressure41.

○

Walls of the lungs have baroreceptors (pressure-stretch receptors) that respond to overinflation of the lungs41.

●

Stimulated by different types of physical contact/deformation:

○

Stretch42

○

Steady pressure42

○

Fluttering/stroking movements42

●

Signal transduction occurs through opening of mechanically gated ion channels42. Deformation of the receptor membrane in response to the stimulus opens the ion channel and allows ions (usually Na+) to move into or out of the receptor cell causing a graded potential42.

Thermoreception

●

Thermoreceptors are free nerve endings in the skin, liver, skeletal muscles, and hypothalamus43.

●

The membrane of the free nerve ending contains temperature receptor proteins that are cation channels called transient receptor potential (TRP) channels43. These can be classified as cold receptors and warm/heat receptors:

○

Cold receptors – are activated at temperatures below body temperature43.

○

Warm receptors are activated at temperatures between 37 and 45℃43.

○

At temperatures higher than 45℃, nociceptors (pain) are activated44.

●

Signal transduction in thermoreceptors is complex and depends on the exact TRP channels involved44. For example:

○

Stimulus of cold receptors opens TRP channels sensitive to cold, which allows Na+ and Ca++ to pass into the cell causing a graded potential (receptor potential)44. Cold temperatures may also close K+ leakage channels, which would also lead to a depolarizing graded potential44.

●

Thermoreceptors can also be stimulated by chemicals44. For example:

○

Menthol in the mint plant activates cold receptors (meaning that it opens the ion channels that are normally opened by cold temperatures)45.

○

Capsaicin from chili peppers activates warm receptors (meaning that it opens the ion channels that are normally opened by warm/hot receptors)45.

●

In these cases, signal transduction is likely to occur via GPCR45.

Nociception

●

Free nerve ending in the skin, muscles, joints, bones and some internal organs (viscera)45.

●

The CNS lacks nociceptors, so the brain can perceive pain from almost anywhere in the body except for itself45. Headaches result from pain originating in structures surrounding the brain – blood vessels, the meninges, muscle fibers, cranial nerves, etc46.

●

Activated by strong noxious (harmful) stimuli that may damage tissue46. For example:

○

Extreme hot or cold temperatures46

○

Extreme pressure (extreme mechanical stimulus)46

○

Chemical stimuli46

●

Two types of pain:

1.

Fast pain: transmits rapidly to CNS through type Aδ fibers46. For example, acute (sharp) pain that occurs at the moment you stub your toe46.

2.

Slow pain: transmits more slowly to CNS through type C fibers47. For example, dull throbbing pain that persists in the moments after you stub your toe47.

●

Pain can be modulated by input from other senses (for example, mechanoreceptors), or from the brain itself (descending pain control pathways), by stimulating inhibitory interneurons in the spinal cord/brain where neurons in the nociceptive pathway synapse with one another47. Inhibitory interneurons act like gates that open/close the nociceptive pathway to modulate pain47.

Special Senses

The sources provide detailed explanations for each special sense:

●

Olfaction (Smell): olfaction is a form of chemoreception in which molecules from matter produce odors that are detected by olfactory receptors (bipolar neurons)4748. Signal transduction involves the binding of an odorant molecule to a GPCR, activating the cAMP second messenger pathway, leading to depolarization of the olfactory receptor and the generation of an action potential49. Signals are relayed from the olfactory bulb to the olfactory tract to the olfactory cortex, where the odor is perceived and signals are sent to other areas of the cortex, like the limbic system50. Anosmia, or the loss of olfactory senses, as a result of COVID-19 is thought to be caused by infection of the olfactory supporting cells by SARS-CoV-2, which impairs their ability to clear odorant ligands, effectively blocking the receptor sites and leading to a loss of sensation/perception5152.

●

Gustation (Taste): taste is another form of chemoreception, involving five different taste sensations: sweet, bitter, umami, sour, and salt53. Taste buds contain taste receptor cells (TRCs) that each detect only one of the five taste sensations5354. Different taste sensations use different signal transduction pathways: sweet, bitter, and umami use GPCR second messenger pathways that activate a G-protein called gustducin, while salt and sour tastes use ion channels for Na+ and H+ respectively54. For sweet, umami, and bitter tastes, the binding of a ligand to a GPCR activates gustducin, which then activates second messenger pathways, leading to an increase in intracellular calcium and the release of ATP, which acts as a paracrine molecule to stimulate a graded potential in the primary gustatory neuron5455. For sour and salt tastes, H+ or Na+ ions enter the taste receptor cell through ion channels, activating intracellular signaling pathways that result in an increase in intracellular calcium, triggering the release of a neurotransmitter (serotonin) that stimulates graded potentials and action potentials in the primary sensory neuron5556.

●

Hearing: hearing is the body's ability to perceive sound waves, which have two components: frequency (perceived as pitch) and amplitude (perceived as loudness)57. The ear consists of three parts:

1.

The outer ear (pinna and ear canal) directs sound waves to the tympanic membrane (eardrum)5758.

2.

The middle ear ossicles (malleus, incus, stapes) transmit vibrations from the tympanic membrane to the inner ear58.

3.

The inner ear contains the vestibular apparatus (for equilibrium) and the cochlea (for hearing)59.

●

The cochlea has three fluid-filled channels: the vestibular duct, tympanic duct, and cochlear duct (filled with endolymph)59. Vibrations from the stapes on the oval window create waves in the fluids inside the ducts, causing the basilar membrane to vibrate59. The cochlear duct contains the tectorial membrane and hair cells, which are non-neural mechanoreceptor cells for hearing60. When the stereocilia (hair-like structures) on the hair cells are bent towards the klinocilium (tallest stereocilium) by movement of the tectorial or basilar membrane, signal transduction occurs60. This involves the opening of mechanically gated ion channels, leading to depolarization of the hair cell and the release of neurotransmitter, which stimulates graded potentials and action potentials in the primary auditory neurons6162. The frequency of action potentials to the brain determines sound intensity (loudness), while labeled line coding likely determines pitch6263. The auditory pathway involves four neurons, starting with the primary auditory neurons that form the cochlear branch of Cranial Nerve VIII (vestibulocochlear nerve)64. The primary auditory neurons synapse with secondary neurons in the cochlear nucleus in the medulla, which then transmit signals to the midbrain and thalamus, before finally reaching the auditory cortex in the temporal lobe, where perception takes place6465. Some secondary neurons decussate in the medulla, meaning that signals from both ears are processed in both hemispheres of the brain65. Hearing loss can be conductive, central, or sensorineural:

○

Conductive hearing loss occurs when sound cannot be transmitted through the middle ear due to blockages or damage66.

○

Central hearing loss results from damage to nerve pathways leading to the auditory cortex67.

○

Sensorineural hearing loss is caused by damage to hair cells, often due to aging or chronic exposure to loud noises6768.

●

Equilibrium (Balance): the vestibular apparatus in the inner ear provides information about movement and position of the body, consisting of the saccule, utricle, and semicircular canals (which sense rotational movements)68. These structures contain hair cells that are activated by mechanical stimuli, similar to those in the cochlea6970. Equilibrium pathways begin with the primary neurons in the vestibular branch of Cranial Nerve VIII and include the vestibulo-cerebellar pathway (for unconscious signaling about balance and coordination of movements), the vestibulo-oculomotor pathway (for eye movements), and pathways to the reticular formation, thalamus, and cortex (for conscious sensation of changes in body position)707172.

●

Vision: vision is the body's ability to detect and perceive light energy (photons). The eye is a three-layered sphere filled with fluid, consisting of the sclera/cornea (outer layer), choroid/ciliary body/iris (middle layer), and retina (inner layer)72737475. Light enters the eye through the cornea, and the pupil size is adjusted by the iris based on light intensity76. The lens bends light to focus it on the retina, changing shape depending on the distance of the object being viewed7677. Photoreceptors in the retina, rods (for grayscale/night vision) and cones (for colour/day vision), transduce light energy into electrical and chemical signals7578. Signal transduction in rods involves the photopigment rhodopsin, which is activated by light, leading to a decrease in cGMP levels and hyperpolarization of the rod cell, reducing neurotransmitter release onto bipolar cells7980818283. Vision pathways involve the relay of signals from photoreceptors to bipolar cells to ganglion cells, which form the optic nerve84. The optic nerve relays signals to the optic chiasm, where axons from the left visual field of the right eye and the right visual field of the left eye cross over, before continuing to the lateral geniculate nucleus in the thalamus and finally the visual cortex in the occipital lobe848586. Impaired vision can be caused by:

○

Myopia (near-sightedness): difficulty seeing distant objects due to the focal point falling in front of the retina87.

○

Hyperopia (far-sightedness): difficulty seeing near objects due to the focal point falling behind the retina8788.

○

Astigmatism: blurred vision due to flaws in the curvature of the cornea or lens, causing light rays to scatter88.

●

Visual processing and perception occur in the