CLOCKS

Clocks:

What is a clock?How does a biological clock compare with a mechanical clock?Why have a circadian clock?What are the properties of circadian clocks?How do we set the circadian clock?

CLOCKS

Measure Time

All clocks must have two basic components:

1. a regular, constant or repetitive process or action or OSCILLATION to mark off equal increments of time

2. a means of keeping track of the increments of time and displaying the result.

Oscillations are turned into time by matching them to solar

(light/dark) cycle

Text

0 90 180 270 360

External Clock time (Time of Day)

Internal clock time

Earths rotation

6 a.m.

Mechanical Clock

Oscillator: a regular, constant or repetitive process or action to mark off equal increments of time.• Pendulum• Balance wheels• Vibrating crystal - quartz• electromagnetic waves – atomic clocks

Output: a means of keeping track of the increments of time and displaying the

result.• position of clock hands• digital time displays.

MECHANICAL CLOCKS

Pendulum clock 1656 Christiaan Huygens

1721 George Graham

1889 Siegmund Riefler

Balance Wheels 1876

Quartz clocks 1920’s

1969 Seiko

1972 Pulsar

1980 Swatch

Atomic clocks 1955 cesium

Mechanical Clock

Oscillator: a regular, constant or repetitive process or action to mark off equal increments of time.• Pendulum• Balance wheels• Vibrating crystal - quartz• electromagnetic waves – atomic clocks

Output: a means of keeping track of the increments of time and displaying

the result.• position of clock hands• digital time displays.

Resetting Mechanism•manual

Biological Clock

Oscillator: pacemaker neurons

Output: overt biological rhythms

Resetting Mechanism - zeitgeber

Anticipation

Properties of Biological Clocks

They continue to run under constant conditions.

tau = not exactly 24h

They are Temperature Compensated

They are affected by Light Intensity.

They are under Genetic Control.

They are Adjustable

Zeitgeber

Range of Entrainment

Why have a circadian clock?

Coordinates body activities with each other

Synchronizes individual’s activity with the environment

Anticipates predictable rhythmic changes in our environment

Gives animal best chance for survival

Where is the circadian clock?

hypothalamus

scn

Optic Nerve

Optic Chiasm

Suprachiasmatic Nucleus (SCN)

Neuropeptides

VIP (vasoactive intestinal polypeptide

GRP (Gastrin releasing peptide)

PHI (peptide histidine isoleucine)

Dorsomedial divisionVentrolateral Division

Curtis Richter

Lesion

Stephan and Zucker

Ablation

Arrhythmic

Ablation eliminates circadian rhythms

Criteria for identification of the clock:

Ablation eliminates circadian rhythms Electrolytic Lesion Chemical Lesion

Circadian rhythm of activity exists within the clock Electrophysiological Metabolic -2Deoxyglucose Neurochemical synthesis

Isolation of clock from rest of brain in vivo eliminates overt rhythms but does not eliminate rhythm within the clock.

Remove clock from the animal and clock still exhibits a sustained circadian rhythm

Transplant clock cells back into lesioned, arrhythmic animal restores rhythms

Stereotaxic instrument

Rhythmic Activity in situ

Inouye and Kawamura – 1970s – Electrical Activity

Schwartz et al – 1980 – Metabolic Activity

Remove clock from the animal and clock still exhibits a sustained circadian rhythm

Rhythms within SCN

Electrical activity – high during day, low at night

Metabolic activity – active during day; inactive at night

Neurotransmitter synthesis

Oscillator

Lesion

Arrhythmia

Ablation

Humoral

SCNx

temporal

A structure exhibiting a regular variation

Injury

Without a rhythm

Total removal

Transported in the blood

SCN lesioned

Related to time

Tau mutant A circadian period different from 24h

Wild type Non-mutant, control organism

Homozygous 2 identical alleles for a trait

Heterozygous 2 different alleles for a trait

In vitro in an artificial environment outside the

living organism

In vivo In the living organism

Electrode An instrument which conducts an

electric current

Explant tissue removed from its normal place of

growth and placed into tissue culture

Organotypic

Electrolyic made with an electricl current

Torpor dormant or lethargic state

Peripheral Oscillators

Retina

Fibroblasts

Liver