BIO 202 ch 9

What is sensation?

The process by which sensory receptors detect internal or external stimuli and convert them into electrical signals that the brain interprets.

What is the purpose of sensation?

To detect changes in the internal and external environment, protect the body, maintain homeostasis, and allow interaction with the environment.

What are the four major events of sensation?

1. Stimulation of a sensory receptor 2. Transduction of the stimulus 3. Generation of action potentials 4. Integration of sensory input in the brain.

What is transduction?

The conversion of a physical or chemical stimulus into an electrical signal (receptor potential).

What happens when a receptor potential reaches threshold?

An action potential is generated and travels to the CNS.

How do sensory signals reach the brain?

Stimulus → Receptor → Sensory neuron → Spinal cord/Brainstem → Thalamus (most senses) → Cerebral cortex.

What are mechanoreceptors?

Receptors that detect touch, pressure, vibration, stretch, hearing, and balance.

What are thermoreceptors?

Receptors that detect changes in temperature.

What are photoreceptors?

Receptors in the retina that detect light.

What are chemoreceptors?

Receptors that detect chemicals responsible for taste, smell, and blood chemistry.

What are nociceptors?

Pain receptors that respond to tissue damage caused by mechanical, thermal, or chemical stimuli.

What is a receptive field?

The area monitored by a single sensory receptor.

How do small receptive fields affect sensation?

They provide excellent localization and fine discrimination.

How do large receptive fields affect sensation?

They provide poorer localization of stimuli.

What is sensory coding?

The process by which the nervous system identifies the modality, location, intensity, and duration of a stimulus.

How is stimulus modality determined?

By the type of receptor activated.

How is stimulus location determined?

By which receptors and neural pathways are activated.

How is stimulus intensity determined?

By the frequency of action potentials and the number of receptors activated.

How is stimulus duration determined?

By receptor adaptation.

What is the labeled-line principle?

Each sensory receptor sends information through a dedicated neural pathway to a specific area of the brain, allowing the brain to identify the stimulus.

What is sensory adaptation?

A decrease in receptor sensitivity during a constant stimulus.

What are rapidly adapting (phasic) receptors?

Receptors that stop responding quickly after stimulation begins (touch and vibration).

What are slowly adapting (tonic) receptors?

Receptors that continue responding while the stimulus is present (pain and proprioception).

What sensations are tactile sensations?

Touch, pressure, vibration, itch, and tickle.

Which receptors detect tactile sensations?

Mechanoreceptors.

How are tactile stimuli transduced?

Mechanical deformation opens mechanically gated ion channels, allowing Na⁺ to enter, producing a receptor potential.

Which receptors detect temperature?

Thermoreceptors.

What temperatures activate cold receptors?

10-35°C (50-95°F).

What temperatures activate warm receptors?

30-45°C (86-113°F).

What detects extreme temperatures?

Nociceptors.

What is the purpose of pain?

To protect tissues from injury.

What are the three types of nociceptors?

Mechanical, thermal, and polymodal.

What stimulates mechanical nociceptors?

Cutting, crushing, or pinching.

What stimulates thermal nociceptors?

Extreme heat or cold.

What stimulates polymodal nociceptors?

Mechanical, thermal, and chemical stimuli.

Trace the pain pathway.

Nociceptor → Sensory neuron → Spinal cord → Spinothalamic tract → Thalamus → Somatosensory cortex.

What is referred pain?

Pain perceived in an area different from the site of injury because visceral and somatic sensory neurons share spinal pathways.

Example of referred pain.

A heart attack causing pain in the left arm or shoulder.

What is proprioception?

The sense of body position and movement.

What information do proprioceptors detect?

Muscle length, muscle tension, joint position, and body position.

Where is the olfactory epithelium located?

The roof of the nasal cavity.

What cells are found in the olfactory epithelium?

Olfactory receptor neurons, supporting cells, and basal stem cells.

How does olfactory transduction occur?

Odor molecules dissolve in mucus → bind receptors → activate G proteins → ion channels open → receptor depolarizes → action potentials begin.

Trace the olfactory pathway.

Odor → Olfactory receptor → Olfactory nerve (CN I) → Olfactory bulb → Olfactory tract → Primary olfactory cortex.

Which sense bypasses the thalamus?

Smell (olfaction).

Where are taste buds located?

On the papillae of the tongue.

What are the five basic tastes?

Sweet, salty, sour, bitter, and umami.

How does taste transduction occur?

Food chemicals dissolve in saliva → bind taste receptors → receptor depolarizes → neurotransmitter released → action potential generated.

Trace the gustatory pathway.

Taste receptor → Cranial nerves VII, IX, X → Brainstem → Thalamus → Gustatory cortex.

What are the accessory structures of the eye?

Eyelids, eyelashes, conjunctiva, lacrimal glands, and extraocular muscles.

What are the functions of the accessory structures?

Protect, lubricate, and move the eye.

What are the major retinal cells?

Photoreceptors, bipolar cells, and ganglion cells.

What forms the optic nerve?

Ganglion cell axons.

What are rods?

Photoreceptors specialized for dim light and black-and-white vision.

Characteristics of rods.

Highly sensitive to light, poor visual acuity, no color vision.

What are cones?

Photoreceptors specialized for color vision and sharp vision in bright light.

Characteristics of cones.

Require bright light, provide color vision, and high visual acuity.

How does phototransduction occur?

Light activates photopigments → Na⁺ channels close → photoreceptor hyperpolarizes → neurotransmitter release decreases → bipolar cells respond.

Trace light through the eye.

Cornea → Aqueous humor → Pupil → Lens → Vitreous humor → Retina.

What is accommodation?

Changing the curvature of the lens to focus images.

What happens when viewing nearby objects?

Ciliary muscles contract and the lens becomes thicker.

What happens when viewing distant objects?

The lens flattens.

Trace the visual pathway.

Photoreceptors → Bipolar cells → Ganglion cells → Optic nerve → Optic chiasm → Optic tract → Thalamus → Primary visual cortex.

What happens at the optic chiasm?

Fibers from the nasal retina cross to the opposite side of the brain.

What structures make up the outer ear?

Auricle and external auditory canal.

What structures make up the middle ear?

Tympanic membrane, malleus, incus, and stapes.

What structures make up the inner ear?

Cochlea, vestibule, and semicircular ducts.

What determines pitch?

Frequency.

What determines intensity (loudness)?

Amplitude.

What unit measures sound intensity?

Decibels (dB).

Trace the pathway of sound.

Sound wave → Tympanic membrane → Malleus → Incus → Stapes → Oval window → Cochlea → Basilar membrane → Hair cells.

Which hair cells detect sound?

Inner hair cells.

What do outer hair cells do?

Increase the sensitivity of inner hair cells.

How do hair cells convert sound into nerve impulses?

Hair cells bend → K⁺ channels open → Depolarization → Ca²⁺ enters → Neurotransmitter released → Auditory nerve generates action potentials.

Trace the auditory pathway.

Hair cells → Vestibulocochlear nerve (CN VIII) → Brainstem → Thalamus → Primary auditory cortex.

What are the otolithic organs?

The utricle and saccule.

What do the utricle and saccule detect?

Linear acceleration, deceleration, and head tilt.

What do the semicircular ducts detect?

Rotational (angular) acceleration and deceleration.

Trace the equilibrium pathway.

Hair cells → Vestibulocochlear nerve (CN VIII) → Vestibular nuclei → Cerebellum → Motor pathways controlling posture and balance.

What is the role of the thalamus in sensation?

It is the major sensory relay station that sends sensory information to the cerebral cortex.

How does structure determine function in sensory systems?

Each receptor is specialized to detect one type of stimulus such as light, chemicals, pressure, or temperature.

How do electrochemical gradients create receptor potentials?

Stimuli open ion channels, allowing ions (usually Na⁺) to move down their electrochemical gradients, producing receptor potentials.

Examples of sensory disorders.

Peripheral neuropathy, hearing loss, retinal degeneration, cataracts, glaucoma, vertigo, anosmia, and ageusia.

How does the nervous system code a stimulus?

Modality = type of receptor activated; Location = receptor and pathway activated; Intensity = frequency of action potentials and number of receptors activated; Duration = receptor adaptation.

Trace light from the eye to the visual cortex.

Cornea → Aqueous humor → Pupil → Lens → Vitreous humor → Retina (rods/cones) → Bipolar cells → Ganglion cells → Optic nerve → Optic chiasm → Optic tract → Thalamus (LGN) → Primary visual cortex.

Trace sound from the ear to the auditory cortex.

Sound waves → Tympanic membrane → Malleus → Incus → Stapes → Oval window → Cochlea → Hair cells → Vestibulocochlear nerve (CN VIII) → Brainstem → Thalamus → Primary auditory cortex.

How do the semicircular ducts detect rotation while the utricle and saccule detect linear acceleration?

The semicircular ducts contain endolymph that moves during head rotation, bending hair cells in the ampulla to detect rotational (angular) acceleration. The utricle and saccule contain otoliths that shift with gravity and straight-line movement, bending hair cells to detect linear acceleration, deceleration, and head tilt.