Lectures 10-11: Sensory Systems I-IIILectures 10-11: Sensory Systems I-III

Reading: chapter 6, except section on special senses

Conduction speed on myelinated axons ~ 120 m/s

Longest axon ~ meter long (big toe to spinal cord)

Total travel time for a nerve impulse from head to toe: 10 ms + 10 ms + 1-2 ms per synapse = ~ 25 ms

Brain

sensory inputs

motor outputs

Body

Sensory-motor transformation

Sensory Systems - 6 major sensory systems in the mammalian brain, each is organized according to a common anatomical plan.

- Visual System vision, sleep-waking cycle

- Auditory System hearing

- Somatosensory System touch, pain, temperature, proprioception

- Olfactory System* smell

- Gustatory System taste

- Vestibular System posture and balance

General Organization:

receptors - stimuli are transduced by receptors grouped together to form a sensory surface

transduction - the conversion of stimulus energy to a neuronal signal

receptor - a cell whose axon or dendrite is capable of transduction in a particular sensory modality

Receptor Relay Nuclei Thalamus Cerebral CortexPrimary

Cerebral CortexSecondary

relay nuclei - Groups of neurons located in the central nervous system that process signals from receptor neurons and transmit signals to the thalamus.

thalamus - Groups of neurons, organized into nuclei within the thalamus, that process signals from relay nuclei and transmit signals to the cerebral cortex.

primary cerebral cortex - Anatomically defined areas of the cerebral cortex that process signals from the thalamus and transmit signals to secondary cerebral cortex.

secondary cerebral cortex - Anatomically defined areas of the cerebral cortex that process signals from primary sensory cortex and transmit signals to association cortex, motor cortex and subcortical structures.

Concepts in Receptor Physiology

Receptor potential

Modality specificity

Receptive field

Maps

Lateral inhibition

Acuity

receptor potential - change in membrane potential at the site of transduction

(example – photoreceptor) (example – pacinian corpuscl)

modality specificity - category of stimuli to which a receptor is sensitive

(example – photoreceptor)

(example – pacinian corpuscl)

receptive field - location on the sensory surface within which a stimulus (of the appropriate modality)

can influence the activity of a sensory neuron

(example – auditory)

(example – somatosensory)

map: organized array of surface of receptor cells on the sensory surface

somatotopic map

tonotopic map

retinotopic or visuotopic map

lateral inhibition - inhibition of adjacent neurons in a map - facilitates localization of stimuli

2 point discrimination threshold

simultaneous presentation of stimuli

minimum audible angle

sequential presentation

1st 2nd

MAA

acuity

Visual Receptors :

Photoreceptors (rods and cones) located in the retina

The Visual System

Structure of the Eye

- Cornea- Pupil- Lens

- Aqueous humor- Vitreous humor

- Retina- Choroid- Sclera

- Optic disc- Optic nerve

Surface of the Retina

= Optic disc = Location of Fovea

Optics of the Eye

- the lens inverts and focuses the visual stimulus onto the surface of the retina

Anatomy of the Retina

- light passes through the neural circuitry of the retina before making contact with the photoreceptors in the back of the eye

- Cell Types: photoreceptors, bipolar cells, ganglion cells, horizontal cells, amacrine cells

- Two classes of photoreceptors - rods and cones (serve scotopic and photopic vision)

- 3 regions of the photoreceptor - outer segment, inner segment, synaptic terminal

- Outer segment contains discs that house photopigment molecules (rhodopsin or coneopsin)

- Outer segments are embedded in the pigment epithelium to reduce the absorption of scattered light

3 Classes of Cones: Red, Green, and Blue

Phototransductioncell membrane of outer segment

disc membrane

rhodopsinPDE

Na+

cGMP

In Dark - high levels of intracellular cGMP keep the cGMP-gated Na+ channel opens- cell depolarized- cell constituitively releases neurotransmitter

transducin

(Adapted from Figure 6-25)

Phototransductioncell membrane of outer segment

disc membrane

rhodopsinPDE

In Light: 1) light stimulation of rhodopsin leads to activation of a G protein, transducin

Na+

light

transducin

2) activated G protein activates cGMP phosphodiesterase

3) PDE hydrolyzes cGMP, reducing its concentration

4) cGMP-gated Na+ channels close, the cell hyperpolarizes, and the cell stops releasing neurotransmitter

cGMP GMP

(Adapted from Figure 6-25)

Summary - Photoreceptors

- Structure of the eye.

- Anatomy of the retina (rods, cones, bipolar, horizontal, amacrine, ganglion).

- Anatomy of photoreceptors (outer segment, inner segment, synaptic terminals).

- Phototransduction cascade: light absorption leads to hyperpolarization and a decrease in the

amount of neurotransmitter release.

Visual System

Visual System – (cont.)

Visual System – (cont.)

Primary visual cortex, V1, area17: binocularity, orientation selectivityDorsal stream: where pathway e.g. area MT involved in motion detectionVentral stream: what pathway e.g. area IT involved in face recognition

Auditory Receptors: Hair Cells located in Cochlea

The Auditory System

Structure of the Ear

- External ear: pinna, external auditory meatus, tympanic membrane- Middle ear: tympanic membrane, ossicles, oval window- Inner ear: oval window, cochlea, vestibular apparatus, round window

Cochlea – Structure and Sound Transduction

Cochlea – Structure and Sound Transduction

Cochlea – Tonotopic Organization

Sound Waves

Vibration of Tympanic Membrane

Vibration of Oval Window

Vibration of Middle Ear Bones

Fluid Movement within the Cochlea

Vibration of Basilar Membrane

Bending of Hair Cells

Action Potentials Generated in Auditory Nerve

Graded Receptor Potential

Propagation to Auditory Cortex

Vibration of Round Window

Dissipation of Energy (no sound perception)

Summary – sound transduction

Auditory SystemJust as the auditory nerve branches to innervateseveral targets in the cochlear nuclei, the neuronsin these nuclei give rise to several different pathways.

Beyond the cochlear nucleus, there is a high degreeof bilateral connectivity.

The best-understood function mediated by the auditory brainstem nuclei is sound localization.

Humans use two binaural cues to localize the horizontal position of sound sources, depending on the frequencies in the stimulus.- interaural timing differences (if < 3 kHz)- interaural intensity differences (if > 3 kHz)

This figure is not in any version of the textbook

Somatosensory Receptors

Specific receptors for different modalities/sensations

- touch (mechanoreceptors)

- pain (nociceptors)

- temperature (thermoreceptors)

- proprioception (mechanoreceptors)

The Somatosensory System

Pacinian corpuscle

Somatosensory System – (cont.)

Spinal Cord

Relay Nuclei(dorsal columns)

Thalamus

Dorsal Root Ganglion

1o Somatosensory Cortex

Skin

Somatosensory System

Sensory HomunculusSomatosensory System – (cont.)

Summary – Sensory Systems, Organization and Receptor Physiology

6 Major Sensory Systems - visual, auditory, somatosensory, olfactory, gustatory, vestibular.

General Organization - receptor, relay nuclei, thalamus, primary cerebral cortex, secondary cerebral cortex.

Receptor Physiology – signal flow, receptor potential, modality specificity, receptive field, maps, lateral inhibition.

Visual System

Somatosensory System

Auditory System