Biology Chapter on Sensory Systems

Animal Body Systems - Sensory Systems

• Animals detect a wide range of environmental variables.
• Sensory systems are crucial for homeostasis.
• Basic neuronal physiology principles apply, but specialized adaptations exist.
• Sensory systems start with sensory receptors (transducers).
• Receptors detect sensory input, convert it to neural activity, and send it to the CNS.
• Sensory receptors are formed by dendrites of afferent neurons or specialized receptor cells.

Sensory Transduction

• Sensory transduction is when stimuli cause changes in membrane potentials in sensory receptors.
• Stimuli can be light, heat, sound, mechanical stress, or chemicals.
• If receptors are present, the cell reacts.

Sensory Receptors

• Receptors respond to stimuli in their receptive fields with changes in receptor potential.
• Receptor potential varies with the stimulus magnitude (graded potential).
• Changes in receptor potential are due to changes in the rate of positive ion (Na+, K+, Ca2+) conduction across the plasma membrane.

Strength of Stimulus

• Stimulus intensity and extent are encoded in multiple ways:
• The frequency of action potentials generated in the afferent neuron (number per unit time).
• The number of afferent neurons sending action potentials in response to the stimulus.

Structural Forms of Sensory Receptors

• Sensory receptors have three structural forms:
  • Specialized cell synapsing with an afferent neuron.
  • Peripheral endings of an afferent neuron (two forms).

Sensory Receptor: Free Nerve Endings

• A stimulus causes a change in membrane potential, generating an action potential (AP) in the axon.
• Examples include pain receptors and some mechanoreceptors.

Sense Organ: Enclosed Nerve Endings

• A stimulus affecting a specialized structure triggers an action potential in the afferent neuron.
• Examples include some mechanoreceptors.

Sensory Cell Consisting of Two Separate Cells

• Stimulus changes the membrane potential, causing neurotransmitter release.
• Neurotransmitter triggers an AP in the afferent neuron axon.
• Examples include photoreceptors, chemoreceptors, and some mechanoreceptors.

Major Categories of Sensory Receptors

• Mechanoreceptors: Respond to mechanical deformation (e.g., auditory receptors in ears, temperature receptors in skin).
• Thermoreceptors: Respond to cold and heat (e.g., temperature receptors).
• Nocioreceptors: Respond to pain (e.g., skin and internal organs).
• Electromagnetic Receptors: Respond to electrical and magnetic fields, including infrared, and ultraviolet light (detecting earth's magnetic fields, visual receptors).
• Photoreceptors: Respond to visible light (e.g., visual receptors).
• Chemoreceptors: Respond to various chemicals (e.g., taste buds).

Sensory Cell Membrane Proteins

• Mechano-, thermo-, electro-, chemo-, and photosensors show how sensory cell membrane proteins respond to stimuli.

Mechanoreceptors: Touch and Pressure

• Detect mechanical stimuli like touch and pressure.
• Vertebrates have touch and pressure receptors embedded in skin, skeletal muscles, blood vessels, and internal organs.
• Four mechanoreceptor types detect tactile stimulation in human skin (free nerve endings, Pacinian corpuscles, Ruffini endings, Meissner's corpuscles).

Mechanoreceptors: Proprioceptors

• Detect stimuli used to monitor and maintain body and limb positions.
• Examples include statocysts in aquatic invertebrates (jellyfish, gastropods, arthropods).
• Sensory hair cells generate action potentials when hairs move.
• Detect pressure/tension changes in muscles, tendons, and joints (more detail in musculoskeletal system).

Two More Examples of Proprioceptors

• Halteres: Vestigial hind wings of flies that transduce information about pitch, roll, and yaw during flight.
• Lateral Line System of Fishes: Neuroblasts contain a gelatinous cupula that vibrates and pushes hair cells, generating action potentials.

Vertebrate Vestibular Apparatus: Balance and Orientation

• Used for balance, orientation, perception of head position/movement, and maintaining equilibrium.
• Consists of three semicircular canals and two chambers (utricle and saccule).

Human Vestibular Apparatus

• Diagram shows ampulla of a semicircular canal, direction of head rotation, and receptor of an utricle or saccule with otolithic membrane.

Detection of Sound

• Sound is vibrations as waves from alternating compression and decompression in air or water.
• Water transmits sound roughly three times faster than air.
• Invertebrates: Mechanoreceptors detect sound and vibrations (skin of other surface structures).
• Vertebrates: Auditory structures transduce vibrations in air, triggering action potentials in specialized hair cells.

Terrestrial Vertebrate Ear

• Pinna (outer ear) concentrates and focuses sound waves.
• Middle ear consists of malleus, incus, stapes, & oval window.
• Inner ear (cochlea, Organ of Corti, round window) transmits vibrations from the tympanic membrane to the fluid of the inner ear, triggering action potentials in hair cells.

Structures of the Human Ear

• Vibrations from eardrum through the inner ear's fluids make the basilar membrane vibrate, causing hair cells to bend, and triggering actions potentials.

Some Animals have Pinnae: Some Don't

• Images show whales, reptiles, birds, and bats, illustrating variations in outer ear structures.

Photoreceptors

• Detect light at specific wavelengths.
• Convert stimuli to action potentials which transmit information to the CNS visual centers/central ganglion.
• In all animals, one key light-absorbing pigment, retinal (made from vitamin A), is used.
• Simplest eyes can distinguish light from dark.
• Most complex eyes can distinguish shapes, colors and have focused images.

Invertebrate Eyes Take Many Forms

• Earthworms have photoreceptors in skin, allowing them to sense light.
• Ocellus, the simplest eye, has no lens and doesn't create images. It contains photoreceptor cells in a cup or a pit (planarians, insects, arthropods, molluscs).

Compound Eyes

• Faceted visual units/ommatidia in insects & crustaceans.
• Light enters through a cornea, focused by a crystalline cone onto photoreceptor cells with microvilli rich in rhodopsin.
• The brain receives a motion-sensitive mosaic image (like a picture with many tiny pieces).

Single-Lens Eyes

• Operate like a camera, with light entering through cornea, and a lens focusing light on photoreceptor cells (retina).
• Iris muscles adjust the pupil's size to control light entering.
• Lens adjusts for focusing on objects at different distances (accommodation).

Accommodation: Focusing on a Distant Object

• Diagram showing how ciliary muscles relax to focus on a distant object.
• When focusing on a near object, ciliary muscles contract, loosening ligaments and allowing the lens to become rounded.
• The rounded lens focuses the image on the retina.

Photoreceptors of the Retina

• Specialized for low-intensity light detection (Rods) or light of different wavelengths (cones).
• Rods and cones are linked to neurons in the retina.

Converting Signals to Electrical Impulses

• Photoreceptor has outer and inner segments (including discs with light-absorbing pigment), a synaptic terminal (releasing neurotransmitters).
• Different light-absorbing pigments (retinal) exist in different animals.

Photoreceptors: Rhodopsin

• Found in rods; consists of opsin protein and retinal.
• Retinal changes shape (bent to straight) in response to light, activating rhodopsin.

Retina: Initial Integration

• Diagram illustrates the retina's initial layers of cells involved in processing visual input (optic nerve, bipolar cells, ganglion cells).

Opsins and Colour Vision

• Colour vision relies on cones containing photopsins that combines with retinal.
• Most mammals have 2, humans and other primates have 3 types of cones.

Neural Pathways for Vision

• Diagram illustrates visual field overlap
• Optic nerves, optic chiasm, lateral geniculate nucleus (of the thalamus), and visual cortex are part of the pathway.