Physio | Somatic Nervous System | Exam 2

What is the difference between sensation and perception?

• Sensation occurs when sensory receptors (e.g., in the skin, tongue, nose, eyes, ears) detect a stimulus and send signals to the brain or spinal cord.

• Perception is the interpretation of these signals—where the brain processes multiple incoming sensations, compares them with past experiences, and creates a meaningful pattern or conscious recognition.

What are the three main structural categories of sensory receptors?

1. Free Nerve Endings: Bare dendrites embedded in tissue, detect pain, temperature, etc.

2. Encapsulated Nerve Endings: Nerve endings surrounded by a connective tissue capsule (e.g., Meissner corpuscles), which enhances sensitivity.

3. Specialized Receptor Cells: Non-neural cells (e.g., photoreceptors in the eye) that release neurotransmitters onto sensory neurons.

What are free nerve endings, and what do they detect?

Free nerve endings are bare dendrites that penetrate tissues such as skin or mucous membranes.

They are commonly involved in detecting pain (nociception), temperature, and some light touches.

What is the purpose of the connective tissue capsule surrounding some nerve endings?

The capsule modifies the mechanical properties of the receptor, enhancing sensitivity and selectivity to certain types of stimuli—like pressure or fine touch.

Examples include Pacinian corpuscles and Meissner corpuscles.

How do specialized receptor cells differ from free or encapsulated nerve endings?

Specialized receptor cells (e.g., photoreceptors in the retina) are often non-neural structures that release neurotransmitters onto a nearby sensory neuron when stimulated

They are especially important in the special senses such as vision, taste, and hearing.

What are some common functional (physiological) classifications of sensory receptors?

1. Chemoreceptors: Detect chemical stimuli (e.g., odorants, tastants).

2. Thermoreceptors: Detect temperature changes (hot or cold).

3. Osmoreceptors: Monitor osmotic pressure in body fluids.

4. Mechanoreceptors: Detect physical deformation (pressure, vibration, stretch, hearing, balance).

Which senses commonly use chemoreceptors?

Taste (gustation) and smell (olfaction) rely heavily on chemoreceptors to detect dissolved chemicals in food or airborne particles in the environment.

What do thermoreceptors respond to?

Thermoreceptors respond to temperature changes—some specialize in detecting heat, while others detect cold, helping maintain core body temperature.

What role do osmoreceptors play in the body?

Osmoreceptors primarily detect the osmotic pressure (concentration of solutes) in body fluids, which helps regulate fluid balance and hydration levels

What types of stimuli do mechanoreceptors detect?

Mechanoreceptors respond to mechanical stress or strain—such as touch, pressure, vibration, stretch, hearing (sound waves), and balance (head movement).

Beyond the “classic five” senses, what are some additional senses recognized today?

Beyond taste, smell, sight, hearing, and touch, modern understanding includes:

1. Balance (equilibrium)

2. Proprioception (sense of body position)

3. Temperature (thermoreception)

4. Pain (nociception)

5. Visceral senses (e.g., fullness, blood pressure changes)

What primary taste categories can the human tongue detect?

Sweet, salty, sour, bitter, and umami (savory).

Each category corresponds to specific chemoreceptors in taste buds, primarily located on the papillae of the tongue

Where are taste buds located, and how do they function?

Taste buds reside on the papillae of the tongue (e.g., fungiform, circumvallate, foliate papillae).

Each taste bud contains gustatory cells (chemoreceptors) that bind tastants and send signals via cranial nerves to the brain for flavor perception

What is “umami,” and which compounds commonly trigger it?

Umami” is a savory taste triggered by certain amino acids, notably glutamate (as in monosodium glutamate) and nucleotides.

It is often described in protein-rich or savory foods (e.g., meats, broths, aged cheeses).

How do olfactory receptors work, and why are they replaced often?

Olfactory receptors are chemoreceptors located in the nasal epithelium; odor molecules dissolve in mucus and bind to receptor cells (often bipolar neurons)

Because of their direct exposure to the environment—and potential damage—olfactory neurons regenerate more frequently than most other neurons

Why is hearing considered a form of mechanoreception?

Sound waves cause vibrations that stimulate hair cells (mechanoreceptors) in the cochlea of the inner ear

These hair cells convert mechanical vibrations into electrical signals perceived as sound.

What main structures does sound pass through to be detected by the inner ear?

1. Outer ear (auricle/pinna, ear canal) → collects sound waves.

2. Tympanic membrane (eardrum) → vibrates with incoming sound.

3. Middle ear ossicles (malleus, incus, stapes) → amplify vibrations.

4. Cochlea (inner ear) → hair cells convert vibrations to nerve impulses.

How do vibrations translate into nerve signals?

Eardrum vibrates in response to sound waves.

Ossicles amplify and transmit vibrations to the oval window of the cochlea.

Hair cells in the cochlear fluid bend, opening ion channels that initiate electrical impulses in the vestibulocochlear nerve (Cranial Nerve VIII).

Which structures detect linear acceleration and motion in the inner ear?

The maculae located in the utricle and saccule of the vestibule detect linear acceleration (e.g., moving forward in a car) and the pull of gravity

These mechanoreceptors (hair cells) bend in response to changes in head position.

Where is angular (rotational) motion detected, and how?

Angular motion is detected by hair cells in the semicircular canals, each containing a crista ampullaris.

When the head rotates, fluid moves within the canals, bending hair cells and sending signals about angular acceleration

What is the macula’s role in overall equilibrium?

The macula (in the utricle or saccule) contains otoliths (“ear stones”) that shift with gravity or linear acceleration, bending hair cells.

This provides crucial information for maintaining posture and balance.

Why is touch considered a “general” sense?

Touch (somatosensation) is distributed throughout the body via different receptors (mechanoreceptors, thermoreceptors, nociceptors) rather than being localized to a specific organ like the eye or ear.

What kind of receptor is responsible for vision, and where are these receptors located?

Vision depends on photoreceptors (rods and cones) found in the retina of the eye. Rods detect low light levels, while cones detect color.

How do photoreceptors convert light into signals?

When light hits photopigments in rods or cones, chemical changes occur, altering the receptor’s membrane potential.

This triggers neurotransmitter release to bipolar cells, then to ganglion cells, and eventually through the optic nerve to the brain.

What happens to sensory input once it enters the CNS?

Sensory signals travel via ascending pathways in the spinal cord and/or brainstem to higher centers (like the diencephalon or cerebral cortex).

Along the way, signals can be processed or modulated, and they often synapse in specialized nuclei (e.g., the thalamus).

Where do ascending somatosensory pathways typically begin and end?

They usually begin at the dorsal root ganglia (where sensory neurons enter the spinal cord) and ultimately project to the postcentral gyrus in the parietal lobe (primary somatosensory cortex)

How does the brain localize the source of a sound?

By comparing interaural time differences (the time at which a sound arrives at each ear) and interaural intensity differences (difference in loudness between ears), the brain can triangulate the direction of the sound source.

Why are Interaural Time Difference (ITD) and Interaural Intensity Difference (IID) important?

They are critical cues for sound localization:

• ITD: If a sound arrives at the right ear fractionally sooner than the left ear, the brain interprets the sound as coming from the right side.

• IID: A sound may be louder in one ear, helping locate its position.

Which cranial nerve is responsible for transmitting auditory information?

The vestibulocochlear nerve (Cranial Nerve VIII) carries signals from the cochlea (hearing) and the vestibular apparatus (balance) to the brain.

What is the vestibulo-ocular reflex, and why is it essential?

The VOR stabilizes images on the retina during head movement.

When the head moves, the inner ear detects this motion, and the reflex triggers compensatory eye movements so that vision remains clear.

How do our eyes and brain use binocular vision to create depth perception?

Each eye has a slightly different visual field.

The brain combines these fields in the visual cortex, and the discrepancies between them provide binocular cues, allowing us to perceive depth accurately.

What does the sensory homunculus represent?

It is a cortical map of the body’s sensory receptors in the primary somatosensory cortex (postcentral gyrus)

Body areas requiring finer sensory discrimination (e.g., fingertips, lips) occupy proportionally larger cortical regions.

Where in the cortex does somatosensory information ultimately get processed, and how?

After ascending through the spinal cord and brainstem, most somatosensory signals reach the thalamus and then project to the primary somatosensory cortex (postcentral gyrus) in the parietal lobe, where the brain constructs a detailed map (homunculus) of the body’s sensory input.