Sensory and Motor Mechanisms Chapter 50 Sensory and Motor Mechanisms

Overview: Sensing and Acting Bats use sonar to detect their prey Moths, a common prey for bats, can detect the bat’s sonar and attempt to flee Both organisms have complex sensory systems that facilitate survival These systems include diverse mechanisms that sense stimuli and generate appropriate movement

Fig. 50-1 Figure 50.1 Can a moth evade a bat in the dark?

Concept 50.1: Sensory receptors transduce stimulus energy and transmit signals to the central nervous system All stimuli represent forms of energy Sensation involves converting energy into a change in the membrane potential of sensory receptors Sensations are action potentials that reach the brain via sensory neurons The brain interprets sensations, giving the perception of stimuli

Sensory Pathways Functions of sensory pathways: sensory reception, transduction, transmission, and integration For example, stimulation of a stretch receptor in a crayfish is the first step in a sensory pathway

Fig. 50-2 Weak receptor potential Action potentials Membrane potential (mV) –50 Membrane potential (mV) –70 Slight bend: weak stimulus –70 1 2 3 4 5 6 7 Brain perceives slight bend. Dendrites Stretch receptor Time (sec) 1 2 4 Axon 3 Brain Muscle Brain perceives large bend. Action potentials Large bend: strong stimulus Strong receptor potential Membrane potential (mV) Figure 50.2 A simple sensory pathway: Response of a crayfish stretch receptor to bending Membrane potential (mV) –50 –70 1 Reception –70 1 2 3 4 5 6 7 Time (sec) 2 Transduction 3 Transmission 4 Perception

Sensory Reception and Transduction Sensations and perceptions begin with sensory reception, detection of stimuli by sensory receptors Sensory receptors can detect stimuli outside and inside the body

This change in membrane potential is called a receptor potential Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor This change in membrane potential is called a receptor potential Many sensory receptors are very sensitive: they are able to detect the smallest physical unit of stimulus For example, most light receptors can detect a photon of light

Transmission After energy has been transduced into a receptor potential, some sensory cells generate the transmission of action potentials to the CNS Sensory cells without axons release neurotransmitters at synapses with sensory neurons Larger receptor potentials generate more rapid action potentials

Integration of sensory information begins when information is received Some receptor potentials are integrated through summation

Perception Perceptions are the brain’s construction of stimuli Stimuli from different sensory receptors travel as action potentials along different neural pathways The brain distinguishes stimuli from different receptors by the area in the brain where the action potentials arrive

Amplification and Adaptation Amplification is the strengthening of stimulus energy by cells in sensory pathways Sensory adaptation is a decrease in responsiveness to continued stimulation

Types of Sensory Receptors Based on energy transduced, sensory receptors fall into five categories: Mechanoreceptors Chemoreceptors Electromagnetic receptors Thermoreceptors Pain receptors

Mechanoreceptors Mechanoreceptors sense physical deformation caused by stimuli such as pressure, stretch, motion, and sound The sense of touch in mammals relies on mechanoreceptors that are dendrites of sensory neurons

Heat Gentle touch Pain Cold Hair Epidermis Dermis Hypodermis Nerve Fig. 50-3 Heat Gentle touch Pain Cold Hair Epidermis Dermis Figure 50.3 Sensory receptors in human skin Hypodermis Nerve Connective tissue Hair movement Strong pressure

Chemoreceptors General chemoreceptors transmit information about the total solute concentration of a solution Specific chemoreceptors respond to individual kinds of molecules When a stimulus molecule binds to a chemoreceptor, the chemoreceptor becomes more or less permeable to ions The antennae of the male silkworm moth have very sensitive specific chemoreceptors

Fig. 50-4 Figure 50.4 Chemoreceptors in an insect 0.1 mm

Electromagnetic Receptors Electromagnetic receptors detect electromagnetic energy such as light, electricity, and magnetism Photoreceptors are electromagnetic receptors that detect light Some snakes have very sensitive infrared receptors that detect body heat of prey against a colder background

Eye Infrared receptor (a) Rattlesnake (b) Beluga whales Fig. 50-5 Figure 50.5 Specialized electromagnetic receptors (b) Beluga whales

Eye Infrared receptor (a) Rattlesnake Fig. 50-5a Figure 50.5a Specialized electromagnetic receptors (a) Rattlesnake

Many mammals appear to use Earth’s magnetic field lines to orient themselves as they migrate

(b) Beluga whales Fig. 50-5b Figure 50.5b Specialized electromagnetic receptors (b) Beluga whales

Thermoreceptors Thermoreceptors, which respond to heat or cold, help regulate body temperature by signaling both surface and body core temperature

Pain Receptors In humans, pain receptors, or nociceptors, are a class of naked dendrites in the epidermis They respond to excess heat, pressure, or chemicals released from damaged or inflamed tissues

Concept 50.2: The mechanoreceptors responsible for hearing and equilibrium detect moving fluid or settling particles Hearing and perception of body equilibrium are related in most animals Settling particles or moving fluid are detected by mechanoreceptors

Sensing Gravity and Sound in Invertebrates Most invertebrates maintain equilibrium using sensory organs called statocysts Statocysts contain mechanoreceptors that detect the movement of granules called statoliths

Ciliated receptor cells Fig. 50-6 Ciliated receptor cells Cilia Statolith Figure 50.6 The statocyst of an invertebrate Sensory axons

Many arthropods sense sounds with body hairs that vibrate or with localized “ears” consisting of a tympanic membrane and receptor cells

Tympanic membrane 1 mm Fig. 50-7 Figure 50.7 An insect “ear”—on its leg Tympanic membrane 1 mm

Hearing and Equilibrium in Mammals In most terrestrial vertebrates, sensory organs for hearing and equilibrium are closely associated in the ear

Figure 50.8 The structure of the human ear Middle ear Outer ear Inner ear Skull bone Stapes Semicircular canals Incus Malleus Auditory nerve to brain Cochlear duct Bone Auditory nerve Vestibular canal Tympanic canal Cochlea Oval window Eustachian tube Pinna Auditory canal Tympanic membrane Round window Organ of Corti Tectorial membrane Hair cells Figure 50.8 The structure of the human ear Hair cell bundle from a bullfrog; the longest cilia shown are about 8 µm (SEM). Basilar membrane Axons of sensory neurons To auditory nerve

Auditory nerve to brain Fig. 50-8a Middle ear Outer ear Inner ear Skull bone Stapes Semicircular canals Incus Malleus Auditory nerve to brain Figure 50.8 The structure of the human ear Cochlea Oval window Eustachian tube Pinna Auditory canal Round window Tympanic membrane

Vestibular canal Tympanic canal Fig. 50-8b Cochlear duct Bone Auditory nerve Vestibular canal Tympanic canal Figure 50.8 The structure of the human ear Organ of Corti

Axons of sensory neurons To auditory nerve Fig. 50-8c Tectorial membrane Hair cells Figure 50.8 The structure of the human ear Basilar membrane Axons of sensory neurons To auditory nerve

Fig. 50-8d Figure 50.8 The structure of the human ear Hair cell bundle from a bullfrog; the longest cilia shown are about 8 µm (SEM).

Hearing Vibrating objects create percussion waves in the air that cause the tympanic membrane to vibrate Hearing is the perception of sound in the brain from the vibration of air waves The three bones of the middle ear transmit the vibrations of moving air to the oval window on the cochlea

These vibrations create pressure waves in the fluid in the cochlea that travel through the vestibular canal Pressure waves in the canal cause the basilar membrane to vibrate, bending its hair cells This bending of hair cells depolarizes the membranes of mechanoreceptors and sends action potentials to the brain via the auditory nerve

Figure 50.9 Sensory reception by hair cells “Hairs” of hair cell Neuro- trans- mitter at synapse More neuro- trans- mitter Less neuro- trans- mitter Sensory neuron –50 –50 Receptor potential –50 –70 –70 –70 Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) Action potentials Signal Signal Signal –70 –70 –70 Figure 50.9 Sensory reception by hair cells 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Time (sec) Time (sec) Time (sec) (a) No bending of hairs (b) Bending of hairs in one direction (c) Bending of hairs in other direction

Membrane potential (mV) Fig. 50-9a “Hairs” of hair cell Neuro- trans- mitter at synapse Sensory neuron –50 –70 Membrane potential (mV) Action potentials Figure 50.9 Sensory reception by hair cells Signal –70 1 2 3 4 5 6 7 Time (sec) (a) No bending of hairs

Membrane potential (mV) Fig. 50-9b More neuro- trans- mitter –50 Receptor potential –70 Membrane potential (mV) Figure 50.9 Sensory reception by hair cells Signal –70 1 2 3 4 5 6 7 Time (sec) (b) Bending of hairs in one direction

Membrane potential (mV) Fig. 50-9c Less neuro- trans- mitter –50 –70 Membrane potential (mV) Figure 50.9 Sensory reception by hair cells Signal –70 1 2 3 4 5 6 7 Time (sec) (c) Bending of hairs in other direction

The fluid waves dissipate when they strike the round window at the end of the tympanic canal

{ { Fig. 50-10 Figure 50.10 Transduction in the cochlea 500 Hz (low pitch) Axons of sensory neurons 1 kHz Apex Flexible end of basilar membrane Oval window Vestibular canal Apex 2 kHz Basilar membrane Stapes { Vibration 4 kHz { Basilar membrane Figure 50.10 Transduction in the cochlea Tympanic canal 8 kHz Base Fluid (perilymph) Base (stiff) Round window 16 kHz (high pitch)

Axons of sensory neurons Apex Fig. 50-10a Axons of sensory neurons Apex Oval window Vestibular canal Stapes Vibration Figure 50.10a Transduction in the cochlea Basilar membrane Tympanic canal Base Fluid (perilymph) Round window

The ear conveys information about: Volume, the amplitude of the sound wave Pitch, the frequency of the sound wave The cochlea can distinguish pitch because the basilar membrane is not uniform along its length Each region vibrates most vigorously at a particular frequency and leads to excitation of a specific auditory area of the cerebral cortex

Flexible end of basilar membrane Fig. 50-10b 500 Hz (low pitch) 1 kHz Flexible end of basilar membrane Apex 2 kHz Basilar membrane 4 kHz Figure 50.10b Transduction in the cochlea 8 kHz Base (stiff) 16 kHz (high pitch)

Equilibrium Several organs of the inner ear detect body position and balance: The utricle and saccule contain granules called otoliths that allow us to detect gravity and linear movement Three semicircular canals contain fluid and allow us to detect angular acceleration such as the turning of the head

Semicircular canals Flow of fluid Vestibular nerve Cupula Hairs Fig. 50-11 Semicircular canals Flow of fluid Vestibular nerve Cupula Hairs Hair cells Vestibule Figure 50.11 Organs of equilibrium in the inner ear Axons Utricle Body movement Saccule

Hearing and Equilibrium in Other Vertebrates Unlike mammals, fishes have only a pair of inner ears near the brain Most fishes and aquatic amphibians also have a lateral line system along both sides of their body The lateral line system contains mechanoreceptors with hair cells that detect and respond to water movement

Opening of lateral line canal Cupula Epidermis Fig. 50-12 Lateral line Surrounding water Scale Lateral line canal Opening of lateral line canal Cupula Epidermis Sensory hairs Figure 50.12 The lateral line system in a fish Hair cell Supporting cell Segmental muscles Lateral nerve Axon Fish body wall

Concept 50.3: The senses of taste and smell rely on similar sets of sensory receptors In terrestrial animals: Gustation (taste) is dependent on the detection of chemicals called tastants Olfaction (smell) is dependent on the detection of odorant molecules In aquatic animals there is no distinction between taste and smell Taste receptors of insects are in sensory hairs called sensilla, located on feet and in mouth parts

Taste in Mammals In humans, receptor cells for taste are modified epithelial cells organized into taste buds There are five taste perceptions: sweet, sour, salty, bitter, and umami (elicited by glutamate) Each type of taste can be detected in any region of the tongue

Sensory receptor cells Fig. 50-13 Sugar molecule G protein Sweet receptor Tongue Phospholipase C SENSORY RECEPTOR CELL Sugar molecule Taste pore PIP2 Taste bud Sensory receptor cells IP3 (second messenger) Sodium channel Sensory neuron IP3-gated calcium channel Nucleus Figure 50.13 Sensory transduction by a sweet receptor ER Ca2+ (second messenger) Na+

When a taste receptor is stimulated, the signal is transduced to a sensory neuron Each taste cell has only one type of receptor

PBDG receptor expression in cells for sweet taste Fig. 50-14 RESULTS 80 60 40 20 PBDG receptor expression in cells for sweet taste No PBDG receptor gene Relative consumption (%) PBDG receptor expression in cells for bitter taste Figure 50.14 How do mammals detect different tastes? 0.1 1 10 Concentration of PBDG (mM); log scale

Smell in Humans Olfactory receptor cells are neurons that line the upper portion of the nasal cavity Binding of odorant molecules to receptors triggers a signal transduction pathway, sending action potentials to the brain

Brain Olfactory bulb Odorants Nasal cavity Bone Epithelial cell Fig. 50-15 Brain Action potentials Olfactory bulb Odorants Nasal cavity Bone Epithelial cell Odorant receptors Chemo- receptor Figure 50.15 Smell in humans Plasma membrane Cilia Odorants Mucus