Sensory input to the brain enters through pathways that travel through either the spinal cord (for somatosensory input from the body) or the brain stem (for everything else, except the visual and olfactory systems) to reach the diencephalon. In the diencephalon, sensory pathways reach the thalamus. This is necessary for all sensory systems to reach the cerebral cortex, except for the olfactory system that is directly connected to the frontal and temporal lobes.
The two major tracts in the spinal cord, originating from sensory neurons in the dorsal root ganglia, are the dorsal column system and the spinothalamic tract. The major differences between the two are in the type of information that is relayed to the brain and where the tracts decussate. The dorsal column system primarily carries information about touch and proprioception and crosses the midline in the medulla. The spinothalamic tract is primarily responsible for pain and temperature sensation and crosses the midline in the spinal cord at the level at which it enters. The trigeminal nerve adds similar sensation information from the head to these pathways.
The auditory pathway passes through multiple nuclei in the brain stem in which additional information is extracted from the basic frequency stimuli processed by the cochlea. Sound localization is made possible through the activity of these brain stem structures. The vestibular system enters the brain stem and influences activity in the cerebellum, spinal cord, and cerebral cortex.
The visual pathway segregates information from the two eyes so that one half of the visual field projects to the other side of the brain. Within visual cortical areas, the perception of the stimuli and their location is passed along two streams, one ventral and one dorsal. The ventral visual stream connects to structures in the temporal lobe that are important for long-term memory formation. The dorsal visual stream interacts with the somatosensory cortex in the parietal lobe, and together they can influence the activity in the frontal lobe to generate movements of the body in relation to visual information.
ascending pathway
fiber structure that relays sensory information from the periphery through the spinal cord and brain stem to other structures of the brain
association area
region of cortex connected to a primary sensory cortical area that further processes the information to generate more complex sensory perceptions
binocular depth cues
indications of the distance of visual stimuli on the basis of slight differences in the images projected onto either retina
chief sensory nucleus
component of the trigeminal nuclei that is found in the pons
circadian rhythm
internal perception of the daily cycle of light and dark based on retinal activity related to sunlight
decussate
to cross the midline, as in fibers that project from one side of the body to the other
dorsal column system
ascending tract of the spinal cord associated with fine touch and proprioceptive sensations
dorsal stream
connections between cortical areas from the occipital to parietal lobes that are responsible for the perception of visual motion and guiding movement of the body in relation to that motion
fasciculus cuneatus
lateral division of the dorsal column system composed of fibers from sensory neurons in the upper body
fasciculus gracilis
medial division of the dorsal column system composed of fibers from sensory neurons in the lower body
inferior colliculus
last structure in the auditory brainstem pathway that projects to the thalamus and superior colliculus
interaural intensity difference
cue used to aid sound localization in the horizontal plane that compares the relative loudness of sounds at the two ears, because the ear closer to the sound source will hear a slightly more intense sound
interaural time difference
cue used to help with sound localization in the horizontal plane that compares the relative time of arrival of sounds at the two ears, because the ear closer to the sound source will receive the stimulus microseconds before the other ear
lateral geniculate nucleus
thalamic target of the RGCs that projects to the visual cortex
medial geniculate nucleus
thalamic target of the auditory brain stem that projects to the auditory cortex
medial lemniscus
fiber tract of the dorsal column system that extends from the nuclei gracilis and cuneatus to the thalamus, and decussates
mesencephalic nucleus
component of the trigeminal nuclei that is found in the midbrain
multimodal integration area
region of the cerebral cortex in which information from more than one sensory modality is processed to arrive at higher level cortical functions such as memory, learning, or cognition
nucleus cuneatus
medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the upper body and arms
nucleus gracilis
medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the lower body and legs
optic chiasm
decussation point in the visual system at which medial retina fibers cross to the other side of the brain
optic tract
name for the fiber structure containing axons from the retina posterior to the optic chiasm representing their CNS location
primary sensory cortex
region of the cerebral cortex that initially receives sensory input from an ascending pathway from the thalamus and begins the processing that will result in conscious perception of that modality
sensory homunculus
topographic representation of the body within the somatosensory cortex demonstrating the correspondence between neurons processing stimuli and sensitivity
solitary nucleus
medullar nucleus that receives taste information from the facial and glossopharyngeal nerves
spinal trigeminal nucleus
component of the trigeminal nuclei that is found in the medulla
spinothalamic tract
ascending tract of the spinal cord associated with pain and temperature sensations
superior colliculus
structure in the midbrain that combines visual, auditory, and somatosensory input to coordinate spatial and topographic representations of the three sensory systems
suprachiasmatic nucleus
hypothalamic target of the retina that helps to establish the circadian rhythm of the body on the basis of the presence or absence of daylight
ventral posterior nucleus
nucleus in the thalamus that is the target of gustatory sensations and projects to the cerebral cortex
ventral stream
connections between cortical areas from the occipital lobe to the temporal lobe that are responsible for identification of visual stimuli
vestibular nuclei
targets of the vestibular component of the eighth cranial nerve
vestibulo-ocular reflex (VOR)
reflex based on connections between the vestibular system and the cranial nerves of eye movements that ensures images are stabilized on the retina as the head and body move
Watch this video to learn more about how the brain perceives 3-D motion. Similar to how retinal disparity offers 3-D moviegoers a way to extract 3-D information from the two-dimensional visual field projected onto the retina, the brain can extract information about movement in space by comparing what the two eyes see. If movement of a visual stimulus is leftward in one eye and rightward in the opposite eye, the brain interprets this as movement toward (or away) from the face along the midline. If both eyes see an object moving in the same direction, but at different rates, what would that mean for spatial movement?
Whereas the video shows opposite movement information in each eye for an object moving toward the face on the midline, movement past one side of the head will result in movement in the same direction on both retinae, but it will be slower in the eye on the side nearer to the object.
The inability to recognize people by their faces is a troublesome problem. It can be caused by trauma, or it may be inborn. Watch this video to learn more about a person who lost the ability to recognize faces as the result of an injury. She cannot recognize the faces of close family members or herself. What other information can a person suffering from prosopagnosia use to figure out whom they are seeing?
Even if a person cannot recognize a person’s face, other cues such as clothing, hairstyle, or a particular feature such as a prominent nose or facial hair, can help make an identification.
1. Which of these sensory modalities does not pass through the ventral posterior thalamus?
A) gustatory
B) proprioception
C) audition
D) nociception
C
2. Which nucleus in the medulla is connected to the inferior colliculus?
A) solitary nucleus
B) vestibular nucleus
C) chief sensory nucleus
D) cochlear nucleus
D
3. Visual stimuli in the upper-left visual field will be processed in what region of the primary visual cortex?
A) inferior right
B) inferior left
C) superior right
D) superior left
A
4. Which location on the body has the largest region of somatosensory cortex representing it, according to the sensory homunculus?
A) lips
B) thigh
C) elbow
D) neck
A
5. Which of the following is a direct target of the vestibular ganglion?
A) superior colliculus
B) cerebellum
C) thalamus
D) optic chiasm
B
1. Following a motorcycle accident, the victim loses the ability to move the right leg but has normal control over the left one, suggesting a hemisection somewhere in the thoracic region of the spinal cord. What sensory deficits would be expected in terms of touch versus pain? Explain your answer.
The right leg would feel painful stimuli, but not touch, because the spinothalamic tract decussates at the level of entry, which would be below the injury, whereas the dorsal column system does not decussate until reaching the brain stem, which would be above the injury and thus those fibers would be damaged.
2. A pituitary tumor can cause perceptual losses in the lateral visual field. The pituitary gland is located directly inferior to the hypothalamus. Why would this happen?
As the tumor enlarges, it would press against the optic chiasm, and fibers from the medial retina would be disrupted. These fibers carry information about the lateral visual field because the visual scene is reversed as the light passes through the pupil and lens.