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Chapter 5

PSY290H5 Chapter Notes - Chapter 5: Lamellar Corpuscle, Stimulus Modality, Spinal Nerve


Department
Psychology
Course Code
PSY290H5
Professor
Alison Fleming
Chapter
5

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Chapter 5: The Sensorimotor System
Part I: Sensory Processing and the Somatosensory System
- All animals have sensory organs containing receptor cells that sense some forms of energy (called stimuli) but not
others
- Receptor cell: a specialized cell that responds to a particular energy or substance in the internal or external
environment and converts this energy into a change in the electrical potential across its membrane
- Stimulus: a physical event that triggers a sensory response
- Receptor cells act as filters, ignoring the environmental background, and converting the key stimuli into the
language of the nervous system: electrical signals
- Different kinds of energy (light, sound, etc.) need different sensory organs to convert them into neural activity
- Although the end product of sensory receptors (action potentials) is the same for all the different sensory
modalities, the brain recognizes the modalities as separate and distinct because the action potentials for each
sense are carried in separate nerve tracts
- Labeled lines: the concept that each nerve input to the brain reports only a particular type of information
Receptor Cells Convert Sensory Signals into Electrical Activity
- The structure of a receptor cell determines the particular kind of energy or chemical to which it will respond
- Although a wide variety of cellular mechanisms are used to detect different stimuli, the outcome it always the
same: an electrical change in the receptor, called a generator potential, that resembles the excitatory
postsynaptic potentials
- Generator potential: a local change in the resting potential of a receptor cell in response to stimuli, which may
initiate an action potential
- Converting the signal in this way (from environmental stimuli into action potentials tat our brain can understand)
is called sensory transduction
- Sensory transduction: the process in which a receptor cell converts the energy in a stimulus into a change in the
electrical potential across its membrane
- Skin receptors experience a variety of different sensory experiences: pressure, vibration, tickle, “pins and needs”
and more complex dimensions (such as smoothness or wetness)
- Pacinian corpuscle: also called lamellated corpuscle. A skin receptor cell type that detects vibration and pressure
- Acting as a filter, the corpuscle allows only vibrations of more than about 200 cycles per second to stimulate the
sensory nerve ending inside it; this type of stimulation is what’s created when we feel a texture against our skin
- By stretching the membrane of the sensory nerve ending, stimuli cause mechanically gated sodium channels to
pop open, creating a graded generator potential
- The amplitude (size) of this generator potential is directly proportional to the strength of the stimulus that was
received
- If the generator potential exceeds the firing threshold, action potentials are generated that travel via sensory
nerves to the spinal cord

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- Threshold: here, the stimulus intensity that is just adequate to trigger an action potential in a sensory cell
- In contrast to the texture sensitivity of Pacinian corpuscles, Meissner’s corpuscles and Merkel’s discs mediate
most of our ability to perceive the forms of objects we touch
- While Merkel’s discs are especially responsive to edges and to isolated points on a surface, the more numerous
Meissner’s corpuscles seem to respond to changes in stimuli, allowing them to detect localized movement
between the skin and a surface
- Meissner’s corpuscle: Also called tactile corpuscle. A skin receptor type that detects light touch, responding
especially to changes in stimuli
- Merkel’s discs: a skin receptor cell type that detects light touch, responding especially to edges and isolated points
on a surface
- Ruffini corpuscles which are only sparsely distributed in the
skin, detect stretching of patches of the skin when we move
fingers of limbs
- Ruffini corpuscle: a skin receptor cell type that detects
stretching of the skin
- Pain, hear, and cold stimuli are detected by free nerve
endings in the skin
- Free nerve ending: an axon that terminates in the skin and
has no specialized cells associated with it. Free nerve
endings detect pain and/or changes in temperature
- All of these sensory receptors are found in their highest
concentrations in regions of the skin where our sense of
touch in finest (fingertips, tongue, and lips)
Sensory Information Processing is Selective and Analytical
- The sensory organs and pathways convey only limited even distorted information to the brain
- A good deal of selection and analysis takes place along sensory pathways, before the information ever reaches
the brain (so the brain receives a highly filtered representation of the external world)
- This processing and filtering is seen in several aspects of sensory transduction, including stimuli coding and
processing across receptive fields, as well as in adaptation and active suppression by the brain
SENSORY EVENTS ARE ENCODED AS STREAMS OF ACTION POTENTIALS
- The nervous system uses labeled lines to identify the type of stimulus that is encountered
- Because the action potentials produced by a sensory neuron always have the same size and duration, the other
characteristics of a sensory stimulus must be encoded in the number and frequency of the action potentials, the
rhythm in which clusters of action potentials occur, and so on
- Only a limited range of intensities can be represented by a single sensory receptor neuron because neurons can
only fire up to so fast
- Some sensory systems solve this problem by employing multiple sensory receptor cells, each specializing in just
one part of the overall range of intensities to cover the whole range
- As the strength of a stimulus increases, additional sensory neurons sensitive to the higher intensities are
“recruited”; thus, the intensity of a stimulus can be represented by the number and threshold of activated cells
- The position of a stimulus (either outside or inside the body) is also an important piece of information
- Some sensory systems (the somatosensory system, for example) reveal this information by the position of
receptors on the sensory surface
- Somatosensory system: a set of specialized receptors and neural mechanisms responsible for body sensations
such as touch and pain
- Neurons of all levels of the visual and touch pathways are arranged in an orderly, map-like manner
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