4 Sensory Systems

From stimuli to perception

Sensory systems detect changes in the body and environment, convert them into neural signals, and help the brain identify and respond to them. A typical sensory pathway includes a receptor, one or more neural relays, and brain regions that analyze the information. Sensation begins with transduction, while perception is the brain’s interpretation of the resulting neural activity.

How sensory signals are encoded

Receptors are specialized for particular kinds of stimuli. Mechanoreceptors respond to physical deformation; photoreceptors respond to light; chemoreceptors respond to chemicals; and thermoreceptors and nociceptors respond to temperature and potentially damaging stimuli.

Receptors convert stimulus energy into a change in membrane voltage called a receptor potential. If this change is large enough, it increases signaling by sensory neurons, often by increasing their action-potential firing rate. Some receptor cells release neurotransmitter onto a separate sensory neuron; in other cases, the sensory neuron itself detects the stimulus.

The brain can infer a stimulus’s modality, location, intensity, and duration from patterns of activity. The identity of a pathway helps specify modality: activity in a visual pathway is normally interpreted as vision. A receptive field is the region of sensory space that can change a neuron’s activity. Smaller, more densely sampled receptive fields support finer spatial discrimination.

Stimulus strength is often represented by firing rate and by how many receptors and neurons are recruited. Adaptation, a reduced response to a sustained, unchanging stimulus, makes changes in the stimulus more noticeable.

Touch, temperature, pain, and body position

Somatosensation includes skin sensations and proprioception, the sense of the body’s position and movement. Skin receptors detect features such as pressure, vibration, stretch, temperature, and tissue damage. Muscle spindles and other proprioceptive receptors signal muscle length, tension, and movement.

Two major pathways carry sensory information from the body toward the brain:

• The dorsal column–medial lemniscus pathway carries fine touch, vibration, and conscious proprioception. Signals ascend on the same side of the spinal cord, cross in the medulla, and relay through the thalamus to the primary somatosensory cortex.

• The anterolateral (spinothalamic) pathway carries pain, temperature, and aspects of crude touch. Its fibers generally cross within the spinal cord and ascend on the opposite side to the thalamus and other brain regions.

Sensory information from the face travels mainly through the trigeminal nerve and relays through the thalamus. The primary somatosensory cortex, in the parietal lobe, contains an organized map of the body. This map is not to scale: regions with high sensory acuity, such as the fingertips, receive relatively extensive representation.

Pain is not a simple readout of tissue damage. Nociceptors signal potentially harmful events, while spinal and brain circuits can amplify or inhibit those signals. The resulting experience depends on pathway activity and its modulation by attention, context, and other neural processes.

Vision

Light is focused onto the retina, neural tissue at the back of the eye. Rods are especially sensitive in dim light, while cones support color vision and fine detail, particularly in the central retina. Photoreceptors convert light into changes in electrical signaling and communicate with retinal circuits; the circuits’ output travels along the optic nerve.

At the optic chiasm, some retinal fibers cross. This arrangement sends information about the left side of visual space to the right cerebral hemisphere and information about the right side to the left. Most visual signals relay through the thalamus’s lateral geniculate nucleus before reaching primary visual cortex in the occipital lobe.

Processing continues through networks involved in features such as form, color, motion, and spatial relationships. Vision therefore depends on retinal encoding and the brain’s interpretation of activity across multiple regions.

Hearing and balance

Sound waves vibrate the eardrum and the small bones of the middle ear, transmitting motion to the fluid-filled cochlea. Movement in the cochlea bends hair-cell stereocilia and opens mechanically gated ion channels. Hair cells then alter their signaling to auditory-nerve fibers. Different places along the cochlea respond best to different frequencies, creating a tonotopic map that is maintained in parts of the auditory pathway and cortex.

Auditory signals pass through brainstem nuclei and the midbrain before relaying through the thalamus’s medial geniculate nucleus to auditory cortex in the temporal lobe. Information from each ear reaches both sides of the brain, allowing brainstem circuits to compare timing and intensity differences between the ears—cues that help locate sound.

The vestibular system in the inner ear detects head rotation and linear acceleration. Fluid movement and displaced sensory structures bend vestibular hair cells. Signals travel through the vestibular nerve to brainstem and cerebellar circuits that help stabilize gaze, maintain balance, and coordinate posture and movement.

Smell and taste

In olfaction, odor molecules bind to receptors on sensory neurons in the nasal epithelium. These neurons send axons to the olfactory bulb, where inputs are organized into glomeruli, and onward to olfactory cortical areas. Unlike the other major senses, olfaction’s initial route to primary cortex does not first relay through the thalamus; later olfactory processing can involve the thalamus. Connections with memory- and emotion-related regions help explain why odors can evoke vivid associations.

Taste begins when chemicals dissolved in saliva activate taste-receptor cells in taste buds. Taste signals travel in cranial nerves to the brainstem, then relay through the thalamus to cortical regions involved in taste, including the insula. The familiar experience of flavor also depends heavily on smell, along with texture, temperature, and other oral sensations.

Integration and interpretation

The thalamus relays and helps regulate much sensory information before it reaches the cortex. Olfaction is a notable exception to the usual first thalamic relay. In the cortex, specialized regions analyze different features, while communication among regions combines them into coherent perceptions.

Sensory processing is not merely passive. Attention, expectations, and context can influence which signals are emphasized and how they are interpreted. For example, touching a warm mug activates skin receptors. Activity in sensory pathways conveys the location and intensity of the contact and temperature; cortical processing and context help identify the object and determine whether it feels comfortable or too hot. The perception results from coordinated processing, not from a single receptor’s message.

Across the senses, receptors transduce specific forms of energy into neural signals. Pathways preserve and transform information about stimulus type, location, intensity, and change as it travels to the brain. Specialized circuits, including retinal and auditory maps and somatosensory pathways, contribute to processing, while brain regions integrate their activity to produce perception. Attention, context, and neural modulation shape how sensory input is experienced.