Lecture Outline: Sensory Systems

OO. Brain Organization

Hind Brain; Mid Brain; Fore Brain / Cerebral Cortex

O. Touch Experiment

1. Temporal Aspects: Adaptation
a. One finger to the back of your hand, steady pressure
2. Spatial Aspects
a. two sharpish points (e.g. pencil) / how close and still detect?
3. Somatosensory System - Sensory Map in the brain

I. Sensory Systems: Evaluating the World, Evaluating the Body

A. The organism is irrelevant in the absence of environmental information.
B. Environmental interaction provides selective pressure "driving" evolution.
C. Senses - Signals - Detectors
D. Sensory Transduction: translating environment into neuronal activity

II. Sensory Neurons / Sensory Organs

A. Sensory neurons are neurons:
B. Sensory neurons are effectively organized as sampling devices

III. Sensory Transduction: Environment to Electrical / Spatial

A. The Environmental Signal MUST be Translated into Neural Activity
B. Electrical Activity : Signal Transduction

IV. Specific Examples

A. Example 1: Smell & Taste
B. Example 2: Vision
C. In General - understanding sensory systems

1. Consider the mechanism of detecting the signal
2. Consider how the sensory system recreates the pattern of the signal
3. Consider what the animal does with the information (behaviorally).

Lecture Notes: Sensory Systems

OO. Brain Organization

A. Hind Brain - motor (movement) coordination
B. Mid Brain - relays
C. Fore Brain -

Cerebral Cortex - multiple tasks, including sorting of sensory information

O. Touch Experiment

A. Temporal Aspects: Adaptation

1. One finger to the back of your hand, steady pressure

- notice that you stop sensing it - adaptation
- adaptation serves to adjust to the noise level

B. Spatial Aspects

1. two sharpish points (pencil?)

- How close can they be and still be recognized as two?
- Is this distance the same all over your body?
- how close can they

C. Somatosensory System - Sensory Map in the brain

1. Touch (pressure) sensitive sensory neurons all over body
2. more sensory neurons in some areas than other areas
3. sensory neurons project to the cortex

spatially distributed - cortical distribution resembles body distribution
A Sensory Map is recreated in the brain

I. Sensory Systems: Evaluating the World, Evaluating the Body

A. The organism is irrelevant in the absence of environmental information.

B. Environmental interaction provides selective pressure "driving" evolution.

C. Senses - Signals - Detectors

Smell / Taste : Chemoreceptors
Vision / Light : Photoreceptors
Touch / Hearing / balance / gravity : Mechanoreceptors
Echo / Sonar
Electric Fields / Magnetic Fields


D. Sensory Transduction: translating environment into neuronal activity

1. detector mechanism
2. information extraction: a more complex feature of the system

- a photoreceptor cell detects light but an eye detects an image and

presents a 2-dimensional neural projection (map) to the brain

- an auditory neuron detects vibration but ear discriminates frequencies

and presents a frequency based spatial pattern (map) to the brain

II. Sensory Neurons / Sensory Organs

A. Sensory neurons are neurons:

1. cell body
2. receptive region ("dendrite"): place where signal stimulates neuron
3. axon - (may be absent: vertebrate taste and hearing - NT released from cell body)


B. Sensory neurons are effectively organized as sampling devices

1. eye may be image forming AND a collection of light sensitive neurons
2. nose is a gas sampler AND a collection of odor sensitive neurons
3. ear separates sound frequencies AND a group of sound sensitive neurons
4. other systems may seem less organized, but both components:

neuron and organization appropriate to signal and animals requirements.

III. Sensory Transduction: Environment to Electrical / Spatial

A. The Environmental Signal MUST be Translated into Neural Activity

- Electrical Activity
- "Spatial" Representation of the Signal in the Environment


B. Electrical Activity : Signal Transduction

1. Receptors / Receptor Mechanisms

a. some systems activate second messenger pathways
b. some systems directly open ion channels - ion currents

2. Object is usually the regulation of neurotransmitter release to stimulate

a next neuron

IV. Specific Examples

A. Example 1: Smell & Taste

1. olfactory neurons have dendrite, cell body, axon

a. receptor proteins in dendritic membrane: "G-protein coupled receptors"
b. ODOR > receptor activation > G-protein activation > second messenger >

ion channels open > ionic currents > APs > neurotransmitter release

2. Taste transduction for large molecules similar to olfactory transduction

a. true for amino acids, glucose, bile acids, etc.
b. salt reception may use different transduction mechanisms consistent with

ionic currents

3. Smell and Taste are anatomically distinguished

a. olfactory neurons and taste neurons go to different regions of brain
b. aquatic animals smell and taste the same molecules
c. terrestrial animals smell volatile molecules (often from plants)

but taste water soluble molecules (glucose, Na₊, Cl_-, etc.)
4. Humans distinguish 10,000 odor molecules with 1 million olfactory

neurons each expressing 1 of 1,000 different receptor proteins (genes).
a. system allows great flexibility in recognition of complex odor
(i.e. chemical mixtures, as in perfumes)

5. Vertebrate olfactory system enters brain through limbic system:

emotional center


B. Example 2: Vision

1. Light sensitive neurons have specialized membrane for light detection;

also cell body and axon
a. Light Receptor Proteins: Rhodopsin-retinal complex

- Rhodopsin: G-protein coupled receptor (like odor receptors)
- retinal is like an odor molecule that is covalently bound to receptor
- light changes retinal from cis (bent) to trans (straight) conformation

-- trans-conformation / receptor is active
-- enzyme returns retinal to cis-conformation

- active receptor > G-protein activation >>> WHAT? (see below)

b. color specificity - specific rhodopsins

- retinal conversion sensitive to specific ranges of wave length of light
- sensitive wave lengths shifted by rhodopsin amino acid sequence
- several rhodopsin genes - rhodopsins with different color specificities

c. invertebrates: light stimulates APs / ionic currents in light

- cGMP low in dark, cGMP opens Na+ channels
- LIGHT > receptor activation > G-protein activation >

cGMP production > Na+ channels OPEN > ionic currents > AP >
neurotransmitter release

d. vertebrates: light inhibits APs / ionic currents in dark

- cGMP high in dark, cGMP opens Na+ channels
- LIGHT > receptor activation > G-protein activation >

phosphodiesterase activation > degrade cGMP >
Na+ channels CLOSE > no ionic currents > no AP >
no neurotransmitter release

2. Eye Structure: enormous variety

a. light sensitive cells highly conserved

- light receptors (rhodopsin) highly conserved (bacteria to humans)
- transduction mechanisms show small variation

b. eye, as an image forming structure, very diverse

- some believe has evolved independently MANY times;

compare insect, octopus and mammalian eyes; all form images

- simple light detectors to several kinds of image formers

lenses or not, focusing vs. fixed focus, compound vs. simple eyes

3. Vertebrate retina recreated in the brain: "topographic" map

a. retina is a 2 dimensional "ordered array" of neurons
b. image is focused on the retina, stimulation of retinal neurons is

image/position dependent

c. retinal neurons connected to interneurons which enter visual cortex

the neurons create an "ordered array" in the cortex that matches the
"ordered array in the retin, preserving spatial organization; recreating the
external world in neuronal pattern

4. The brain interprets color, space, distance, etc., based on:

a. color sensitivity of neurons
b. position of neurons in retina
c. binocular vision: comparisons of right and left eyes
d. circuitry that compares relative activity of neighboring neurons


C. In General - understanding sensory systems

1. Consider the mechanism of detecting the signal

a. what is actually stimulating the neuron
b. how the neuron translates the env. signal into an electrical signal

2. Consider how sensory systems recreate the pattern of the signal

a. vertebrate hearing

- frequencies are separated in the ear
- neurons stimulated by specific frequencies connect to frequency

specific regions in the cortex

b. vertebrate vision

- retinal neurons connect to the cortex in the same X-Y coordinate

positions (actually crossed and inverted)

c. touch

- neurons from the body connect to the somatosensory cortex in positions

that look like the body (if somewhat distorted)

d. smell

- specific odors are random in position in the air
- neurons expressing specific odor receptors are randomly distributed in the

olfactory epithelium

- neurons expressing specific odor receptors converge on the same "next"

neuron in the brain (olfactory bulb).

3. Consider what the animal does with the information (behaviorally).

a. stimulus may "release" or trigger a behavioral response
b. response may be to both component and spatial aspects of the signal

Vocabulary