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Chapter 49 – Sensory/Motor Systems Part 1 By: Becky Christian Elise Devaney
Sensory and Perceptions • Sensations – action potentials that reach the brain via sensory neurons • Perceptions – the interpretation of stimuli formed in the brain and do not exist outside of it • Ex. Colors, smells, sounds, tastes
Begins with sensory reception, which is the detection of a stimulus by sensory receptors (sensory cells) • Most are specialized neurons or epithelial cells that exist singly or in groups with other cell types in sensory organs (such as eyes and ears) • Two kinds of sensory receptors: • Exteroreceptors – detect stimuli coming from outside the body • Ex. Heat, light, pressure, chemicals • Interoreceptors – detect stimuli coming from within the body • Ex. Blood pressure, body position
Functions Performed by Sensory Receptors • Sensory Transduction – conversion of stimulus energy into a change in the membrane potential of a sensory receptor • Receptor Potential – change in the membrane potential itself • Result from opening or closing of ion channels in sensory receptor’s plasma membrane • Structure/Function: The stretching or bending of the membrane causes ion channels to open or close • Sensory receptors have extreme sensitivity and can change with conditions
Amplification – strengthening of stimulus energy by cells in sensory pathways • Ex. Action potential conducted from eye to brain has about 100,000 times as much energy as the few photons of light that triggered it
Transmission • Energy in a stimulus is transduced into a receptor potential • Action potentials are then transmitted to the Central Nervous System (CNS) • Some sensory receptors are sensory neurons that produce action potentials and have an axon that extends into the CNS (49.2a) • Other receptors release neurotransmitters and synapses with sensory neurons (49.2b) • In almost all pathways where synapse with sensory neurons receptor releases an excitatory neurotransmitter, causing sensory neuron to transmit action potential to the CNS • Such neurons alert the CNS not only to presence or absence of a stimulus but also to changes in intensity or direction
Integration – processing of sensory information that begins as soon as information is received • Sensory Adaptation – decrease in responsiveness during continued stimulation. Reduces likelihood that a maintained stimulus will result in continuous sensation • Ex. Without it, you would feel every heartbeat or every piece of clothing on your body
Types of Receptors • Mechanoreceptors – sense physical deformation caused by stimuli such as pressure, touch, stretch, motion, and sounds, all forms of mechanical energy • Ex. Sense of touch – relies on mechanoreceptors that are dendrites of sensory neurons • Structure/Function: structure of connective tissue and location of receptors affect type of mechanical energy that best stimulates them
Chemoreceptors – includes both general receptors that transmit information about the total solute concentration of a solution and specific receptors that respond to individual kinds of molecules • Stimulus binds to a specific site on the membrane of the receptor cell and initiates changes in membrane permeability • Ex. Osmoreceptors in mammalian brain detect changes in solute concentration of the blood and stimulates thirst when osmolarity increases
Electromagnetic Receptors – detect various forms of electromagnetic energy, such as visible light, electricity, and magnetism • Photoreceptors – electromagnetic receptors that detect visible light and often organized into eyes • Ex. Some fish use electric currents to locate objects; many animals use Earth’s magnetic field to orient themselves
Thermoreceptors – respond to heat or cold • Helps regulate body temperature by signaling surface and body core temperature
Pain Receptors – aka nociceptors – a class of naked dendrites in the epidermis • Important – leads to a defense reaction, such as withdrawing from danger • Those born with CIPA (Congenital Insensitivity to Pain with Anhidrosis) cannot feel pain and are thus much more likely to die young. Most with the condition die before the age of 3 due to overheating
Gravity in Invertebrates • Statocysts – contain mechanoreceptors and function in their sense of equilibrium • Common type consists of a layer of ciliated receptor cells surrounding a chamber that contains one or more statoliths (grains of sand or dense granules) • Gravity causes statoliths to settle in low point of chamber • Ex. Lobsters and crayfish have statocysts near base of their antennules, indicating body position
Sound in Invertebrates • Have hair that are tuned to frequencies of sounds produced by other organisms. Vibrate in response to sound waves • Ex. Caterpillars have vibrating body hairs that can detect the buzzing wings of predatory wasps, warning them of danger • Also have ‘ears’ – eardrum (tympanic membrane) is stretched over an internal air chamber • Sound waves vibrate the membrane that stimulates attached receptor cells which result in nerve impulses sent to the brain
Hearing in Mammals • Vibrating objects create percussion waves in surrounding air that cause the tympanic membrane to vibrate at same frequency • Pressure waves travel through the vestibular canal, around the tip of the cochlea, and through the tympanic canal, dissipating as they strike the round window • Waves then push down on cochlear duct and basitar membrane which vibrates causing hair cells to bend in one direction then the other • Depolarizes cells, increasing neurotransmitter release and frequency of synapsing of sensory neurons. These neurons carry sensations to brain via auditory nerve
Volume and Pitch • Volume – determined by amplitude of sound wave • Higher the amplitude, greater bending of hair cells, more action potentials in sensory neurons • Pitch – a function of a sound waves frequency (# of vibrations/second) expressed in hertz • High frequency waves produce high-pitched sounds and vice-versa • Structure/Function: Cochlea can distinguish pitch because it is narrow at stiff and base and wider and flexible at apex (tip).
Equilibrium in Mammals • Hair cells in utricle and saccule respond in changes to head position with respect to gravity and movement in one direction • Hair cells are arranged in clusters in a gelatinous material containing small otoliliths (calcium carbonate particles) that pull hair cells down because they are heavier • Is in effect a specialized type of statolith • When force on hair cell changes, it changes its rate of release of neurotransmitters • Brain interprets changes in impulse production as change in head position
Hearing In Fish • No cochlea, but saccule, utricle, and semicircular canals, homologous to equilibrium senses of human ears • No eardrum and does not open to outside of body • Vibrations of water form sound waves are conducted to inner ear
Lateral Line System • Most fish and aquatic animals have them along both sides of body • Detects low frequency waves by a mechanism similar to function of inner ear • The information helps animal perceive it’s movement through water or direction of water currents over body, and vibrations by moving objects (such as prey or predators)
Vision in Invertebrates • Ocellus – simple, light detecting organ of planarians that provides information about light intensity and direction • Surrounds photoreceptors and formed by a layer of cells containing a screening pigment that blocks light • Ocelli are both pointed forward, allowing planarian to find shade and protect itself from predators • Compound eye – consists of up to several thousand light detectors called ommatidia, each with its own light focusing lens • Differences in intensity of light results in a mosaic image, which may be sharpened by brain • Single-lens eyes – iris changes the diameter of the pupil, where a single lens focuses light on a layer of photoreceptors behind it
Structure of Eye • Sclera – tough, white outer layer of connective tissue • Choroid – thing pigmented inner layer • Conjunctiva – delicate layer of epithelial cells forming a mucous membrane that keeps eye moist • Cornea – transparent part of sclera that allows light into eye • Pupil – hole in center of eye • Retina – innermost layer of eyeball
