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Sensory and Motor Mechanisms (Sensory receptors transduce stimulus energy…
Sensory and Motor Mechanisms
Sensory receptors transduce stimulus energy and transmit signals to the central nervous system
A sensory pathway begins with sensory reception, the detection of stimulus by sensory cells
The term sensory receptor is used to describe a sensory cell or organ, as well as the subcellular structure that detects stimuli
The conversion of a physical or chemical stimulus to a change in the membrane potential of a sensory receptor is called sensory transduction, and the change in membrane potential itself is known as a receptor potential
When action potentials reach the brain via sensory neurons, circuits of neurons process this input, generating the perception of the stimuli
Amplification refers to the strengthening of a sensory signal during transduction
Upon continued stimulation, many receptors undergo a decrease in responsiveness termed sensory adaptation
Mechanoreceptors sense physical deformation caused by forms of mechanical energy such as pressure, touch, stretch, motion, and sound
Chemoreceptors include both general receptors - those that transmit information about total solute concentration - and specific receptors - those that respond to individual kinds of molecules
Electromagnetic receptors detect forms of electromagnetic energy, such as light, electricity, and magnetism
Thermoreceptors detect heat and cold
To detect stimuli that reflect such noxious conditions, animals rely on nociceptors, also called pain receptors
The mechanreceptors responsible for hearing and equilibrium detect moving fluid or settling particles
To sense gravity and maintain equilibrium, most invertebrates rely on mechanoreceptors located in organs called statocysts
In a typical statocyst, statoliths, granules formed by grains of sand or other dense materials, sit freely in a chamber lined with ciliated cells
To hear music, speech, or other sounds in our environment, we rely on hair cells, sensory cells with hairlike projections that detect motion
The chambers called the uticle and saccule allow us to perceive position with respect to gravity or linear movement
Most fishes and aquatic amphibians are able to detect low-frequency waves by means of a lateral line system along both sides of their body
The diverse visual receptors of animals depend on light-absorbing pigments
These diverse light detectors all contain photoreceptors, sensory cells that contain light-absorbing pigment molecules
A compound eye consists of up to several thousand light detectors called ommatidia, each with its own light-focusing lens
Among invertebrates, single-lens eyes are found in some jellies and polychaste worms, as well as in spiders and many molluscs
The eye of an octopus or squid, for example, has a small opening, the pupil, through which light enters. Like a camera's adjustable aperture, the iris contracts or expands, changing the diameter of the pupil to let in more or less light
The physical interaction of protein filaments is required for muscle function
The major component of thin filaments is the globular protein actin
The thick filaments are staggered arrays of myosin molecules
Vertebrate skeletal muscle, which moves bones and body, has a hierarchy of smaller and smaller units
Inside a muscle cells lies a longitudinal bundle of myofibrils, which contain the thin and thick filaments
The myofibrils in muscle fibers are made up of repeating sections called sarcomeres, which are the basic contractile units of skeletal muscle
According to the well-accepted sliding-filament model, the thin and thick filaments ratchet past each other, powered by myosin molecules
Fast-twitch fibers develop tension two to three times faster than slow-twitch fibers
Vertebrate cardiac muscle is found only in the heart
Smooth muscle in vertebrates is found mainly in the walls of hollow organs, such as blood vessels and organs of the digestive tract
The senses of taste and smell rely on similar sets of sensory receptors
The perceptions of gustation and olfaction both depend on chemoreceptors
In the case of terrestrial animals, taste is the detection of chemicals called tastants that are present in a solution, and smell is the detection of odorants
The receptor cells for taste in mammals are modified epithelial cells organized into taste buds, which are scattered in several areas of the tongue and mouth
Skeletal systems transform muscle contraction into locomotion
A hydrostatic skeleton consists of fluid held under pressure in a closed body compartment
These shape changes bring about peristalsis, a movement produced by rhythmic waves of muscle contractions passing from front to back
The clan shell you find on a beach once served as an exoskeleton, a hard covering deposited on an animal's surface. Animals ranging from sponges to mammals have a hardened internal skeleton, or endoskeleton, buried within their soft tissues
These activities involve locomotion - active travel from place to place