Sensory Systems (INCLUDE AUDITORY PATHWAYS)

NSC4354 Lecture 1 - Introduction and course topics

Afferents

signals traveling from sensory receptors to the CNS

Trigeminal Ganglia

Sensory receptors for the face

Dorsal Root Ganglia

Sensory receptors for the body

Receptive fields

• Skin surface over which stimulation results in a significant change in rate of action potentials

• Has center and surround area → center increases cell firing

Somatotopic maps

The foot, leg, trunk, forelimbs and face are represented in a medial to lateral arrangement

Homunculus

• Illustrates the proportion of representation in cortical processing

• Facial expression, speaking and hand use require lots of cortical circuitry

Somatosensory cortex plasticity in adults

Cortical circuits are capable of reorganization in adults

Immediately following digit 3 lesion, the corresponding cortical region is unresponsive

→ After a few weeks, the unresponsive area becomes responsive to stimulation of neighboring regions of skin

peripheral lesions

• Immediately following, the corresponding cortical region is unresponsive

• After a few weeks, the unresponsive area becomes responsive to stimulation of neighboring regions of skin

On-center cells

Increase firing when luminance increases in their receptive field center

Off-center cells

Increase firing when luminance decreases in their receptive field center

3-neuron chain

(Visual Pathway)

Photoreceptors (rods and cones) → Horizontal cells (maybe) → Bipolar Cells → Amacrine Cell* (maybe) → Ganglion Cells → to optic nerve

lateral geniculate nucleus

Neurons here show similar arrangement as in retina

→ center-surround receptive fields and selectivity for increases/decreases in luminance

Visual area of the thalamus

primary visual cortex

Cells here respond selectively to oriented bars/edges

→ The “preferred” orientation is the orientation to which a cell is most responsive

Primary auditory cortex (A1)

• combination-sensitive neurons

• species-specific sounds

• Speech comprehension (Wernicke’s area)

- occupies superior temporal gyrus

- projections from the ventral division of the medial geniculate (thalamus) maintains tonotopic map

Belt areas

Adjacent areas to A1 receive projections from the medial & dorsal medial geniculate

characteristic frequency map

Displays what frequencies in different areas of auditory cortex that they respond to

Motor Cortex

Planning, initiating, and directing voluntary movements

Descending system (upper motor neurons)

Brainstem centers

Rhythmic, stereotyped movements, and postural control

Descending system (upper motor neurons)

Basal Ganglia

Initiation of intended movement and suppression of unwanted movement

Works with the motor cortex

Cerebellum

Coordination of ongoing movement

Works with the brainstem centers

Local circuit neurons

Sensorimotor integration and central pattern generation

Part of the spinal cord and brainstem circuits

Receives sensory inputs

Motor neuron pools (lower motor neurons)

Sends information to the skeletal muscles

Receives inputs from local circuit neurons and brainstem centers

intracortical microstimulation (ICMS)

• found that small currents initiated the excitation of several muscles

• motor map in the precentral gyrus is much less precise than the somatotopic map in the postcentral gyrus

• regions responsible for initiating different movements overlap substantially

Effects of visual deprivation

• Normal distribution of ocular dominance can be altered by visual experience

→Sometime between when the kitten’s eyes open and 1 year of age, visual experience determines how the visual cortex is wired with respect to eye dominance

• Very few cortical cells could be driven from the deprived (previously sutured) eye

• Recordings from the retina and LGN were normal

• Deprived eye gets functionally disconnected from the visual cortex

• The same manipulation – closing one eye – performed in adulthood has no effect on the responses of cells in the mature visual cortex

Patient S.M.

Suffers from a rare condition called Urbach-Wiethe disease → Bilateral calcification and atrophy of the anterior-medial temporal lobes, including the amygdala

Cannot recognize the emotion of fear + has no fear