10: The Physiology of Sensory
- Page ID
- 175108
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Our ability to sense and interpret the world depends on a complex network of sensory pathways that convert physical or chemical signals into patterns of nerve activity. This process, called sensory transduction, begins when specialized receptors detect a stimulus such as light, sound, pressure, or a chemical odor. Each receptor type is tuned to respond best to a specific form of energy, called its adequate stimulus. When stimulation reaches a certain threshold, it produces a graded potential that can trigger action potentials in sensory neurons. These signals travel through the nervous system and are ultimately interpreted by the brain as sight, sound, taste, touch, or other sensations.
The brain identifies several features of every incoming stimulus: modality (the type of stimulus), location, intensity, and duration. This information is encoded by the pattern of activated neurons and the frequency of their action potentials. For example, the brain can locate a touch on the skin by comparing which receptive fields are activated or determine the direction of a sound by comparing the timing of receptor activation in each ear. Sensory neurons may adapt to continuous stimulation. Tonic receptors continue to fire as long as the stimulus is present, while phasic receptors respond briefly and then stop firing if the stimulus remains constant.
The somatic senses include touch, temperature, pain, itch, and proprioception, which is the body’s awareness of position and movement. These sensations begin with receptors in the skin, muscles, and joints and ascend through specific neural pathways to the somatosensory cortex. Pain and itch are detected by nociceptors, which respond to damaging or irritating stimuli. Fast pain travels through myelinated fibers and is sharp and localized, while slow pain moves through unmyelinated fibers and produces a dull ache. Sometimes, sensory signals from internal organs merge with skin pathways, creating referred pain, such as the left arm pain that can occur during a heart attack.
Chemical senses, including smell (olfaction) and taste (gustation), rely on receptors that bind to specific molecules. Olfactory neurons detect airborne chemicals in the nasal cavity and send signals directly to the olfactory cortex, which is unique among the senses because it bypasses the thalamus. Taste buds on the tongue contain receptor cells that respond to five primary taste sensations: sweet, sour, salty, bitter, and umami. The activation of these receptors leads to the release of neurotransmitters that stimulate sensory neurons.
Hearing and balance both depend on the inner ear. Sound waves are transformed from air vibrations into mechanical movements of the middle ear bones, then into fluid waves in the cochlea. Here, sensory hair cells convert mechanical bending of their cilia into electrical signals that travel to the auditory cortex. Different regions of the cochlea respond to different sound frequencies, allowing the brain to perceive pitch. The sense of equilibrium relies on hair cells located in the vestibular apparatus, which detect head position, movement, and acceleration.
Vision, our most complex sense, begins when light enters the eye and is focused onto the retina, where photoreceptors convert light energy into electrical activity. Rods detect low-light and gray-scale information, while cones provide sharp color vision. The visual pigment rhodopsin in rods changes shape when exposed to light, setting off a cascade that alters neurotransmitter release. Signals travel through the optic nerve, cross partially at the optic chiasm, and reach the visual cortex, where the brain constructs a coherent image of the environment.
Together, the sensory systems allow the brain to create a continuous representation of the body and its surroundings. They transform energy from the external and internal world into the language of electrical impulses, allowing perception, awareness, and coordinated response.