Lecture 16: Sensory Systems

What are the 6 major sensory systems in mammals?

1. Vision (sight) - detection of light and visual information via the eyes.

2. Audition (hearing) - detection of sound waves via the ears.

3. Somatosensation (touch/body sensations) - includes pressure, temperature, pain, proprioception (body position).

4. Olfaction (smell) - detection of airborne chemical molecules via olfactory receptors in the nasal cavity.

5. Gustation (taste) - detection of dissolved chemicals via taste buds on the tongue and oral cavity. 

6. Vestibular system (balance and spatial orientation) - detection of head motion and equilibrium via semicircular canals in the inner ear. 

What does the visual system do?

Detect a subset of the energy waves (wavelengths) that enter our atmosphere from space.

How many classes of photoreceptor do mammals have?

Mammals have 4 classes of photoreceptor, which sample different parts of the visual spectrum. 

3 types of cone - detect blue, green or red wavelengths.

1 type of rod - converts light to a grey scale, sensitive to dim light levels.

What does the auditory system do?

Detects pressure waves (sound) as they travel through the air, water, and solid objects.

What are the parts of the ear?

Outer ear = funnels sound into the ear canal.

Middle ear (eardrum & tiny bones) = amplifies the vibrations. Mammals are the only vertebrates with 3 ear bones (malleus, incus, stapes) which increase sensitivity.

Inner ear (cochlea) = filled with fluid and lined with hair cells – specialized sensory cells that bend when the fluid moves.

Hair cells = triggers neurons in the auditory nerve, sending signals to the brain where sound is interpreted (pitch, volume, location, etc.).

What is mechanotransduction?

When the auditory system turns mechanical energy (vibrations) into neural signals. We don’t actually hear with our ears, we hear with our brain. The ears just collect and convert the signal.

What is the evolution of hearing?

In early reptile ancestors jawbones transmitted sound. Over millions of years two jawbones shrunk and migrated into the skull to become the malleus and incus. 

What functions has hearing evolved for?

Predator avoidance - detecting rustling, wingbeats, approaching footsteps.

Prey detection - owls and foxes locate prey purely by sound.

Communication - social mammals (wolves, dolphins, primates) rely on vocal communication.

Navigation - bats and some marine mammals use echolocation to “see” with sound.

What are examples of hearing sensitivity matching habitat?

Desert rodents hear low-frequency ground vibrations.

Bats and toothed whales evolved ultrasonic detection.

Elephants communicate in infrasound that travels miles.

What does the vestibular system do?

Tells the brain where the body is in space – balance, posture, and movement. 

These sensations are mediated by structures in the semicircular canals, utricle, and vestibule.

Semicircular canals - detect rotational movement (spinning, turning your head side to side).

Utricle & saccule (otolith organs) → detect linear acceleration and gravity (tilting, going up in an elevator).

How did the vestibular system evolve?

Mammals evolved a refined vestibular system that supports:

• Agile movement (running, climbing, jumping)

• Complex locomotion (bats flying, dolphins swimming)

• Quick head/eye coordination (keeping vision stable while moving).

The vestibular system stabilizes your gaze so the world doesn’t blur when you move. 

What does the taste system do?

Detects dissolved chemicals in food and fluids, helping animals identify nutrients and avoid toxins.

Consists of taste cells (in taste cells) on the tongue and palate.

Chemical stimuli in food dissolve in saliva and interact with receptors on the microvilli of taste cells. Many taste cells respond selectively to particular chemicals (e.g. sugars vs salts)

What does taste do?

1. Prepares the body for the physiological crisis that follows each meal.

2. Motivates (or inhibits) feeding.

3. Helps identify and discriminate the different chemicals in foods.

What is flavor made up of?

Taste + smell + texture + temperature = “flavor”

Up to 80% of flavor is actually olfaction. 

• Molecules dissolve in saliva.

• They bind to receptors on taste receptor cells within taste buds.

• Those receptors convert chemical signals into electrical signals.

• Signals travel via cranial nerves → gustatory cortex in the brain.

Sensory input from foods elicits a series of pre-emptive responses that prepare the body for the digestion and post-absorptive processing of foods.

How did the taste system evolve?

Taste evolved as a survival filter:

• Sweet/umami = seek nutrients

• Bitter/sour = avoid toxins or spoiled foods

Taste is a molecular early-warning system. 

What do the somatic senses do?

Mediated by sensory cells in the skin, mucus membranes, muscles, organs, arteries, tendons, and joints.

Our skin and mucus membranes contain a diversity of somatic sensory cells, which detect changes in temperature, mechanical pressure, vibrations, and pain.

Where are the somatic receptors?

Skin & mucus membranes (external environment).

Muscles, tendons, & joints (movement, stretch, body position).

Internal organs & arteries (pain, pressure, stretch).

What do the somatic receptors sense?

Touch/pressure.

Temperature.

Pain (nociception).

Proprioception → sensing your own body position (where your limbs are without looking).

What do somatic sensory neurons in the mouth do?

They provide texture, temperature, pain (spice/heat sensitivity), and proprioception during feeding. These senses evolved because they allow mammals to evaluate food before swallowing it, increasing survival.

What does the olfactory system do?

Helps identify objects (animate and inanimate), chemicals, in the air, and the source (location) of the odors.

Many animals use olfaction to locate foods. Humans can detect 1000’s of molecules (natural or synthesized).

The olfactory epithelium (inside each nostril) contains olfactory sensory neurons.

Why is olfaction critical for survival?

It informs animals about their surroundings when vision or hearing fail.

• Find food (e.g. wolves smell prey from kilometres away)

• Detect predators (rodents smell fox urine and freeze/retreat)

• Navigation/homing (salmon follow odor maps to their natal streams)

• Communication (mammals use scent marking to defend territory and signal status)

What does the endocrine system do?

Long-distance communication network. Instead of sending electrical signals through neurons (like the nervous system), it releases hormones into the bloodstream.

Communicates with the body via hormones.

What are hormones?

Hormones are molecules produced by endocrine cells. They are transported to tissues via the blood or extracellular fluid (ECF). 

Each hormone can only influence its target cells (i.e. cells that express receptors for it). Cells respond if they have the correct receptor.

What does the pituitary gland (“master gland”) do?

Regulates other endocrine glands.

Releases LH and FSH in both sexes → controls gamete production.

What do the gonads (ovaries/testes) do?

Produce gametes (eggs or sperm) and sex hormones.

Females = ovaries → estrogen & progesterone.

Males = testes → testosterone.

What do the sex hormones do?

Influence secondary sex characteristics, reproduction, and behavior. 

Females = estrogen promotes menstrual cycle & bone density.

Males = testosterone stimulates muscle mass & sperm production.

What do feedback cycles do?

Hormone release works on negative feedback loops.

Females = monthly horomone cycling.

Males = relatively steady hormone levels.

How did the endocrine system evolve?

• Enables coordination of reproductive timing (e.g. seasonal breeding in some mammals).

  • Photoperiod (day length) is detected by the brain → signals the endocrine system (via melatonin and gonadotropin hormones).

  • Hormones like LH and FSH trigger production of sperm/eggs only during favorable season.

• Allows organisms to respond to environmental pressures like stress or resource availability.

  • Stress hormones (cortisol, adrenaline) help escape predators or survive short-term challenges.

  • Marine mammals store fat and reduce metabolism during migration or fasting.

• Supports specialization (lactation, parental care strategies)

  • Prolactin and oxytocin regulate lactation and maternal bonding.

  • In some species, males also release prolactin, leading to paternal care (penguins, seahorses)

  • Testosterone influences behaviors like territorial defense or mating competition.

How is organization different in the nervous system vs endocrine system?

Nervous system = wired system → specific anatomical connections (synapses) between each neuron and its target cell.

Endocrine system = wireless system → no direct anatomical connection between endocrine cells and target cells.

How is transport of chemical messengers different in the nervous system vs endocrine system?

Nervous system = neurotransmitters released into and diffused across very short distances (synaptic cleft).

Endocrine system = hormones released into bloodstream and transported over long distances.

How is response latency & duration different in the nervous system vs endocrine system?

Nervous system = very rapid onset and offset – milliseconds.

Endocrine system = slower onset and longer duration – hours to months.

How is action different in the nervous system vs endocrine system?

Nervous system = produces rapid, precise, localized responses (knee-jerk reflex, maintaining balance).

Endocrine system = Produces slower, more widespread, longer-term responses (growth, metabolism, stress response, reproduction).

How is sound converted into a neural signal?

Sound waves → tympanic membrane → ossicles amplify → hair cells bend → action potential.

The bending of hair cells triggers neurotransmitter release.

What mechanism does the vestibular system use to detect orientation?

Fluid movement in semicircular canals bending hair cells.

Why do taste and smell vary widely across species?

Taste and smell evolve depending on dietary niche and ecological pressures.

What is the difference between the somatic sense and the vestibular system?

Somatic sense detect stimuli from the body surface; vestibular sense detect motion and balance.