The Five Senses

Senses and Receptors

• Sense organs are located throughout the human body and send signals to the brain.
• The brain perceives the signals and initiates a response to stimuli.
• Sense organs use receptors to trigger responses.
• A sensory receptor detects changes in the internal or external environment.
• There are many types of sensory receptors, each with a specific function and stimulus.
• Sensory receptors connect to neurons.

Types of Sensory Receptors

• There are five types of sensory receptors:
  • Mechanoreceptors: Transduce mechanical energy (e.g., touch and hearing).
  • Change shape when mechanically pushed or pulled.
  • Chemoreceptors: Transduce chemical compounds (e.g., taste and smell).
  • Allow animals to detect chemical substances in food, water, and air.
  • Photoreceptors: Transduce light energy (e.g., sight).
  • Use pigments to absorb light energy.
  • Thermoreceptors: Recognize differences in hot and cold.
  • Example: pit organ in some snakes, allowing them to sense warm-blooded prey.
  • Electroreceptors: Recognize differences in electrical potentials.
  • Example: Ampullae of Lorenzini in sharks, located along the nose.
  • Contain an electrically conductive gel that detects minute changes in electrical fields surrounding animals.
  • Muscle activity creates electrical potentials, enabling sharks to detect prey.

Human Senses

• Humans have 5 senses: sight, smell, touch, taste, and sound.

The Sense of Sight

• The eye is the organ of sight.

Important Structures of the Eye

• Sclera: The tough, opaque, curved, sheet of connective tissue that forms the outer coat of the eyeball.
• Choroid: A sheet of cells filled with a black pigment that absorbs extra light, preventing blurred images.
• Retina: Contains light-sensitive photoreceptors (rods and cones).
• Cornea: A thin, transparent sheet of connective tissue at the front of the eye, continuous with the sclera.
• Iris: Pigmented cells that give the 'colour' of an eye, regulating light entry.
• Pupil: Light opening, the size of which is regulated by the iris.
• Lens: A transparent, elastic ball behind the iris that bends light rays to focus them on the retina.
• Macula: Area in the back of the eye, at the center of the retina. It is the region of sharpest vision because it has the greatest density of receptor cells, and the retina is thinner there. The cones are most concentrated there.
• Optic nerve: Transmits images to the brain via complex signals.
• Optic chiasma: The area where the optic nerves cross about halfway to the back of the brain.
• Primary visual cortex: Located in the occipital lobe of the brain, receives signals from other neurons.

Photoreceptors: Rods and Cones

• The eye focuses light on the retina, which contains rods and cones.
• Rods: Approximately 125 million in the human eye.
  • Respond to a broad range of light wavelengths and are responsible for black and white vision.
  • Highly sensitive to light; function well in dim conditions.
• Cones: Approximately 6.4 million in the human eye.
  • Detect different wavelengths, enabling colour vision.
  • Three types of cones: red, blue, and green light.

Image Formation

• The pattern of colour and light intensity detected by rods and cones is converted into nerve impulses.
• These impulses are sent via the optic nerve to the brain.
• The image on the back of the eye is upside down due to the lens flipping the image.
• The brain processes the information to perceive the image as right-side up.

Accommodation

• Accommodation: The ability to change focus for near or far vision by changing the shape of the lens.
• Ciliary muscle: Part of the ciliary body surrounding the lens, responsible for accommodation.
  • To focus on distant objects, the ciliary muscle relaxes, and the lens becomes flattened.
  • To focus on close objects, the ciliary muscle contracts, making the lens rounder.

Vision Problems and Correction

• Myopia: Near-sightedness, where the image focuses before the retina.
  • Corrected with concave lenses.
• Hyperopia: Far-sightedness, where the image focuses after the retina.
  • Corrected with convex lenses.

The Sense of Smell (Olfaction)

• Uses chemoreceptors located in the olfactory epithelium in the roof of the nasal cavity.
• Approximately 100 million olfactory receptor cells.
• Ciliated tips extend into a layer of mucus, binding with dissolved compounds.
• Olfactory nerve (cranial nerve I) extends to the olfactory bulb in the brain.
• Information is transmitted to the olfactory area of the brain.

Pheromones

• Animals communicate via pheromones, small volatile molecules secreted into the environment.
• Mammals have a vomeronasal organ that detects pheromones, located separately from the olfactory epithelium.

The Sense of Taste

• Uses chemoreceptors located in the taste buds of the tongue.
• Sweet foods tend to have high caloric value, while poisons are generally bitter.
• Approximately 10,000 taste buds in humans, each containing 50-150 receptor cells.
• Receptor cells live about 1-2 weeks and are then replaced.

Basic Tastes

• Sweet, sour, salty, bitter, and umami.
• Umami: Response to salts of glutamic acid, such as monosodium glutamate (MSG).

Cranial Nerves Involved in Taste

• Glossopharyngeal nerve (cranial nerve IX): Innervates the posterior one-third of the tongue.
• Facial nerve (cranial nerve VII): Innervates the anterior two-thirds of the tongue.
• Trigeminal nerve (cranial nerve V): Carries information related to touch, pressure, pain, and temperature, but is not involved in taste.

Taste Disorders

• Aguesia: Complete inability to taste (rare).
• Hypoguesia: Diminished ability to taste.
• Causes: heavy smoking, radiation therapy, dehydration, use of drugs, Bell's palsy.

The Sense of Touch

• Uses mechanoreceptors located in the skin.
• Tactile hairs at the base of hair follicles sense body orientation or air and water vibrations.
• Receptor potential develops when the hair is bent or moved, generating action potentials.
• Receptors only respond when the hair is moving.

Touch Receptors in the Skin

• Merkel discs: Sense touch and pressure, adapt slowly.
• Meissner corpuscles: Sensitive to light touch.
• Nociceptors: Pain receptors, free nerve endings found in almost every tissue.

The Sense of Hearing

• Uses mechanoreceptors located in the ears.

Parts of the Ear

• Outer ear: Gathers sound and directs it to the eardrum. Consists of the pinna and the external (auditory) canal. The tympanic membrane (or ear drum) separates the outer ear from the middle ear.
• Middle ear: Transforms sound energy for the inner ear. Consists of three bones: the malleus, incus, and stapes. Important for sound transformation and reduction of harmful sounds.
• Inner ear: Location of auditory receptors. Includes the three semicircular canals and the cochlea.

Hearing Process

• Sound waves cause vibrations in the tympanic membrane (eardrum).
• Movement of the eardrum moves the three tiny bones of the middle ear.
• The malleus is in contact with the eardrum, and the stapes is in contact with the oval window of the cochlea.
• The bones amplify vibrations.
• Vibrations pass through the oval window to the cochlea.
• The cochlea contains fluid and is lined with hair cells.
• Fluid stimulates hair cells to move.
• Hair cells are connected to the nerve endings of the auditory nerve (cochlear nerve), which transmits information to the brain.

Balance and Equilibrium

• Parts of the ear function in maintaining balance and equilibrium.
• The three semicircular canals detect angular movement of the head.
• The utricle and saccule tell the brain which way is up, inform of body position, and detect acceleration.
• These organs also move hair cells, which stimulate the vestibular nerve.