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Overview of Sensory Systems

  • Sensory systems allow perception of sensory information available from the environment.
  • Examples of unique sensory capabilities:
    • Snakes can "see" infrared radiation.
    • Elephants communicate using infrasound, which is below the human hearing range.

Major Senses

  • Vision
  • Hearing
  • Taste
  • Smell (olfaction)
    • Olfactory pathways from the nose project to the olfactory cortex.
  • Touch (somatic senses)
    • Somatic senses project to the primary somatic sensory cortex.
  • Equilibrium: pathways project to the cerebellum.

Less Common Senses

  • Thermoception: sense of temperature.
  • Proprioception: orientation of the body in space.
  • Equilibrioception: sense of balance.
  • Interoception: sense of physiological condition. Example sensations include:
    • Thirst or dry mouth
    • Speed or strength of heartbeat
    • Hunger or nausea
    • Feeling hot or cold
    • Urgency to urinate
    • Breathing rate or depth
    • Tickly or itchy skin sensations
    • Muscle pain or tension

Sensory Receptors

  • Sensory receptor cells transform physical and chemical stimuli into neural signals.
  • Mechanism of receptor action:
    • Receptors open or close ion channels, altering the membrane potential.
    • Some sensory receptors release neurotransmitters to initiate action potentials (e.g., hair cells in the inner ear).
    • The intensity of a stimulus influences the amount of neurotransmitter released.

Types of Receptors:

  • Ionotropic receptors:

    • Function as ion channels (e.g., mechanoreceptors, thermoreceptors, electro-sensors).
    • Bind to neurotransmitters to cause direct changes in ion flow and generate action potentials in the receptor cell.
    • Responses are fast and short-lived.

  • Metabotropic receptors:

    • Affect ion channels indirectly through G proteins and second messengers.
    • Engage in signaling cascades that alter ion channels.
    • Responses are slow and long-lasting.

Touch and Tactile Perception

  • The skin detects multiple sensations: temperature, pain, itch, texture, and contact.
  • Touch is defined as the physical sensation of contact with an object.
  • Tactile perception: mental processing and interpretation of touch sensations.
  • Different mechanoreceptors facilitate sensation of touch and perception.

Mechanoreceptors

  • Rapidly and Slowly Adapting Mechanoreceptors:
    • Mechanoreceptors respond to mechanical changes.
    • Rapidly adapting mechanoreceptors:
    • Fire rapidly initially but fall silent if the stimulus persists (adapt rapidly).
    • Sensitivity to changes (dynamic).
    • Example:
    • A cockroach lands on your leg; rapidly adapting mechanoreceptors quickly alert you.
    • Slowly adapting mechanoreceptors:
    • Fire continuously as long as stimulus is applied.
    • Provide ongoing information about sustained mechanical stimulation.

Free Nerve Endings

  • Can adapt rapidly or slowly.
  • Respond to potential harm signals: pain, itch, and temperature.
  • In response to a cockroach landing, can sense irritation or abrasions.

Encapsulated Nerve Endings

  • Meissner’s corpuscles:
    • Adapt quickly; detect changes in pressure/touch (dynamic touch).
    • Example: initial touch of cockroach.
  • Merkel’s discs:
    • Adapt slowly; sense sustained pressure.
    • Example: presence of cockroach over time.
  • Ruffini’s corpuscles:
    • Adapt slowly; react to skin stretch.
    • Example: cockroach moving.
  • Pacinian corpuscles:
    • Adapt rapidly; detect deep pressure.
    • Example: cockroach jump or vibration sensation.

Somatosensory Homunculus

  • Represents the uneven distribution of body parts in the somatosensory cortex.
  • Body areas with highest tactile sensitivity occupy greater cortical tissue for processing.

Proprioception

  • Mechanoreceptors in muscles, tendons, and ligaments send constant information regarding:
    • Position of limbs in space.
    • Stresses on muscles and joints (length and tension).
  • Essential for maintaining posture and coordinating movement.
  • Muscle spindles: stretch mechanoreceptors monitoring muscle length.

Golgi Tendon Organ

  • Located in tendons and ligaments; monitors tension.
  • Inhibits excessive muscle contraction, reducing muscle tension to protect against tearing during heavy lifting.

Maintaining Balance

  • Signals from muscle spindles and Golgi tendon organs.
  • Visual input for orientation.
  • Pressure sensors in toes indicate lean direction.
  • Vestibular apparatus in inner ear for spatial orientation.
  • Conflicting messages can lead to motion sickness.

Sound

  • Sound is produced by pressure waves from vibrating air molecules.
  • Mechanoreceptors in the auditory system convert these pressure waves into electrical signals.

Process of Hearing

  • Outer ear (pinnae) collects sound waves and directs them to the auditory canal.
  • The tympanic membrane vibrates in response to sound waves, converting air pressure to physical forces in the middle ear.
  • Middle ear amplifies sound by a factor of 20 (larger eardrum to smaller oval window).

Middle Ear Mechanics

  • Converts sound to physical vibrations through three small bones:
    • Malleus (hammer) – attached to eardrum.
    • Incus (anvil).
    • Stapes (stirrup).
  • These ossicles magnify and transmit vibrations to the cochlea at the oval window, which creates fluid vibrations in the inner ear.
  • Cochlea contains fluid-filled chambers that respond to vibrations, stimulating hair cells that release neurotransmitters.

Equilibrium

  • The inner ear plays a crucial role in equilibrium, with the vestibular apparatus receiving motion and spatial orientation information.
  • Consists of three semicircular canals located at right angles, filled with fluid to detect rotational movement.
  • Inertia causes fluid lag when the head rotates, activating specific receptors.

Vestibular Responses

  • Signals from semicircular canals activate neck muscles for compensation during displacement, ensuring the head remains stable.
  • Head stabilization allows for better vision and awareness of surroundings, assisting in locating food and evading predators.

The Visual System

  • Approximately 70% of sensory receptors in the body are located in the eyes.
  • Retina contains millions of photoreceptors, converting light into action potentials.
  • Light travels in waves; visible light represents only a small part of the electromagnetic spectrum.

Eye Structure

  • Eyes are filled with fluids that maintain shape, protected by fat and muscles.
  • Eye walls include three layers:
    • Fibrous layer (sclera and cornea)
    • Vascular layer (choroid, ciliary muscles, iris)
    • Inner layer (retina)
Fibrous Layer
  • Sclera: white part of the eye, attaches muscles for movement.
  • Cornea: transparent anterior part that bends light.
Vascular Layer
  • Choroid membrane provides nutrients and blood supply.
  • Ciliary muscles surround the lens, controlling its shape.
  • Iris: colored muscle controlling light entry through the pupil, adjusting size in response to lighting.

Lens Function

  • The lens is a convex, transparent disc that focuses light onto the retina.
  • Ciliary muscles adjust lens shape:
    • Contracting thickens lens for near objects.
    • Relaxing thins lens for distant objects.
Retina and Photoreceptors
  • Retina contains photoreceptors:
    • Cone cells: function in high light, responsible for color vision and detail.
    • Rod cells: sensitive to low light, perceive shades of gray.

Optical Illusions

  • Optical illusions exploit differences between sensation (input from eyes) and perception (interpretation by the brain).
  • Equivocal illusions involve images with multiple interpretations that seem to change upon prolonged viewing.
  • Illusions can be further categorized into various types.
Examples of Optical Illusions
  • Various drawings and scenes that challenge perception, such as the Victorian couple kissing and ambiguous figures.
  • Different scenarios where lines appear non-parallel or distorted due to surrounding figures.
  • Illusions of depth where similar objects appear unequal.

Binocular Vision and Depth Perception

  • Each eye receives slightly different images, aiding distance perception.
  • Prey species, like rabbits, possess side-facing eyes for predator detection, sacrificing depth perception.