THE BRAIN AND CRANIAL NERVES
Sensory Perception
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal
environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals
of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus
causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is
integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of
that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation
of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful
pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that
detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell
membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in
the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule
in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors,
are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane,
and can generate an action potential or a graded potential in the sensory neurons.
Sensory Receptors
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli
are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different
criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position
in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how
the mechanical stimulus, light, or chemical changed the cell membrane potential.
, Structural Receptor Types
The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with
dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the
sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell,
which has distinct structural components that interpret a specific type of stimulus (Figure 14.2). The pain and temperature
receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the
skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the
retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor.
Figure 14.2 Receptor Classification by Cell Type Receptor cell types can be classified on the basis of their
structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the
eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a
bipolar cell, which then synapses with the optic nerve neurons.
Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a
receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in
the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the
increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of
the body, such as a muscle, that interprets the positions of the tissues as they move.
Functional Receptor Types
A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are
of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these
chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor
cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only
electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack,
such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.
Receptor cells can be further categorized on the basis of the type of stimuli they transduce. Chemical stimuli can
be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors
respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence
of chemicals from tissue damage, or similar intense stimuli, through a nociceptor. Physical stimuli, such as pressure and
vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor.
Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that
is either sensitive to temperatures above (heat) or below (cold) normal body temperature.
Sensory Modalities
Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight.
However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to
simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the
type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by
thermoreceptors and pain perception by nociceptors.
Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is
distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin,
muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as
described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which
Sensory Perception
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal
environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals
of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus
causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is
integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of
that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation
of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful
pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that
detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell
membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in
the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule
in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors,
are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane,
and can generate an action potential or a graded potential in the sensory neurons.
Sensory Receptors
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli
are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different
criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position
in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how
the mechanical stimulus, light, or chemical changed the cell membrane potential.
, Structural Receptor Types
The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with
dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the
sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell,
which has distinct structural components that interpret a specific type of stimulus (Figure 14.2). The pain and temperature
receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the
skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the
retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor.
Figure 14.2 Receptor Classification by Cell Type Receptor cell types can be classified on the basis of their
structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the
eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a
bipolar cell, which then synapses with the optic nerve neurons.
Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a
receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in
the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the
increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of
the body, such as a muscle, that interprets the positions of the tissues as they move.
Functional Receptor Types
A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are
of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these
chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor
cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only
electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack,
such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.
Receptor cells can be further categorized on the basis of the type of stimuli they transduce. Chemical stimuli can
be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors
respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence
of chemicals from tissue damage, or similar intense stimuli, through a nociceptor. Physical stimuli, such as pressure and
vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor.
Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that
is either sensitive to temperatures above (heat) or below (cold) normal body temperature.
Sensory Modalities
Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight.
However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to
simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the
type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by
thermoreceptors and pain perception by nociceptors.
Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is
distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin,
muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as
described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which
