Sensory Systems in the Brain

Sensory Perception

sensory_systems.jpg The human brain is capable of processing information in five senses, or modalities. The five types of modal information are visual, auditory, olfactory, somatosensory, and gustatory. Each type of modality-specific information is first encoded by a sensory organ, specifically designed for each type of information. This information then travels through different neural regions until it reaches a region of the cerebral cortex designated for that modality.

1. Visual System

visual_system.jpg Visual information is first processed in the retina, a thin sheet of neural tissue embedded with photoreceptors sensitive to light. Information then travels through the optic tract to the superior colliculus and lateral geniculate nucleus (LGN) of the thalamus. The thalamus is considered to be a major relay station for most of the sensory systems (the olfactory system is the only sensory system that does not relay in the thalamus). The thalamus then relays the visual information to the primary visual area (V1) in the occipital lobes. From there the information can travel to different visual areas processing aspects of the stimulus such as color and motion. auditorypath.jpg

2. Auditory System

An auditory stimulus is first detected by means of hair cells on the cochlea within the inner ear. The vibratory quality of sound moves these hair cells ever so slightly, and their motion is detected by cells underneath that then transmit this information to the cochlear nucleus. The information then travels through the inferior colliculi and medial geniculate nucleus (MGN) of the thalamus, whereupon it is relayed to the primary auditory cortex in the temporal lobes.

3. Olfactory System

Volatile, odorous substances travel up through the nose where they are detected by sensory cells in the olfactory epithelium. From there the information travels to the olfactory bulb in which a topographical representation of the odor stimulus is created. The output cells from the olfactory bulb then synapse onto regions of the olfactory cortex (paleocortex). Though olfactory information does not relay through the thalamus, information is sent to the thalamus after arrival in the olfactory cortex.

4. Somatosensory System

somatosensory.jpg Cells detecting touch, as well as pain, have their cell bodies in the dorsal root ganglion of the spinal cord. Information then passes through the brain stem and relays in the ventral posterior nucleus (VPN) of the thalamus before projecting to the primary somatosensory cortex, located on the postcentral gyrus of the parietal lobe.

5. Gustatory System

Tastant molecules are detected by taste receptors, located on cells within taste buds on the tongue. From here, information travels through cranial nerves until it reaches the first taste relay in the nucleus of the solitary tract. From here, gustatory information travels to the gustatory cortex (located in the insular cortex), as well as other brain regions such as the amygdala.

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