Role of PLACE and VOLLEY principles in determination of sound frequency

1. Place principle

• Different frequencies produce maximum vibration at different places on the basilar membrane.
• The site of maximal vibration stimulates the corresponding hair cells and cochlear nerve fibres.
• High-frequency sounds cause maximal vibration near the base of cochlea, close to the oval window.
• Low-frequency sounds cause maximal vibration near the apex of cochlea, close to the helicotrema.
• Thus, the brain determines frequency from the particular group of auditory nerve fibres activated. This orderly frequency map is called tonotopic organization.

2. Volley principle

• In low-frequency sounds, auditory nerve fibres fire action potentials in synchrony with a particular phase of the sound wave. This is called phase locking.
• A single nerve fibre cannot fire during every cycle of a high-frequency sound because of its refractory period.
• Therefore, many auditory nerve fibres fire alternately in groups or “volleys.”
• The total number and timing of impulses from the group corresponds to the sound-wave frequency.
• This enables coding of low and intermediate frequencies, where timing information is useful.

Conclusion

• Low frequencies: mainly coded by the volley/temporal principle, with contribution from place coding.
• High frequencies: mainly coded by the place principle.
• Both mechanisms together allow discrimination of the wide audible range of frequencies.

How to write this in a 4-mark answer

1. Frequency and pitch - one line
2. Place principle - 3 points
3. Volley principle - 3 points
4. One-line comparison/conclusion

Oval window / Base --------------------------- Apex / Helicotrema
        High frequency                         Low frequency
        PLACE principle                         PLACE principle

Low-frequency sound: multiple auditory nerve fibres fire in sequence
                    = VOLLEY principle

Other important physiology principles/laws for First MBBS short answers

A. General nerve and muscle physiology

1. All-or-none law
   • A nerve fibre or single muscle fibre responds maximally once threshold is reached.
   • Applied to action potential and cardiac muscle.
   • Exception: whole skeletal muscle shows graded contraction because of recruitment.
2. Refractory period
   • Absolute and relative refractory periods.
   • Importance in unidirectional conduction and tetany.
3. Size principle of motor-unit recruitment
   • Small motor neurons/motor units are recruited first, then larger ones as force demand rises.
   • Important in graded skeletal muscle contraction.
4. Reciprocal inhibition
   • Contraction of an agonist muscle is associated with inhibition of its antagonist.
   • Example: flexion reflex.
5. Renshaw cell inhibition
   • Recurrent inhibition of alpha motor neurons.
   • Helps stabilize motor-neuron activity.
6. Law of denervation
   • Denervated skeletal muscle becomes hypersensitive to acetylcholine and eventually atrophies.

B. Cardiovascular physiology

1. Frank-Starling law of the heart
   • Within physiological limits, force of cardiac contraction is proportional to initial fibre length/end-diastolic volume.
   • “Heart pumps what it receives.”
   • Role in matching right and left ventricular outputs.
2. Fick principle
   • Used to calculate cardiac output: [ \text{Cardiac output} = \frac{\text{Oxygen consumption per minute}} {\text{Arterial O}_2\text{ content} - \text{Venous O}_2\text{ content}} ]
3. Starling principle/equation for capillary fluid exchange
   • Explains filtration and reabsorption across capillaries.
   • Important for oedema.
4. Laplace’s law

• Wall tension is proportional to pressure and radius.
• Application: aneurysm, dilated heart, alveoli.

1. Poiseuille’s law

• Resistance is inversely proportional to the fourth power of radius.
• Explains why arterioles are the main resistance vessels.

1. Bayliss myogenic response

• Increased stretch of vascular smooth muscle causes constriction.
• Important in autoregulation of blood flow.

C. Respiratory physiology

1. Hering-Breuer inflation reflex

• Lung inflation stimulates pulmonary stretch receptors.
• Vagal afferents inhibit inspiration and prevent overinflation.

1. Bohr effect

• Increased CO₂, increased H⁺, increased temperature, and increased 2,3-BPG shift the oxygen-haemoglobin dissociation curve to the right.
• Promotes O₂ unloading in tissues.

1. Haldane effect

• Oxygenation of haemoglobin in lungs decreases its capacity to carry CO₂ and H⁺.
• Facilitates CO₂ removal in lungs.

1. Ventilation-perfusion matching

• Hypoxia produces pulmonary vasoconstriction, directing blood towards better-ventilated alveoli.

D. Renal and body-fluid physiology

1. Transport maximum (Tm)

• Maximum rate at which a renal tubule can reabsorb or secrete a substance due to carrier saturation.
• Classic example: glucose reabsorption and glucosuria in diabetes mellitus.

1. Counter-current principle

• Loop of Henle acts as a counter-current multiplier.
• Vasa recta acts as a counter-current exchanger.
• Produces concentrated urine.

1. Glomerulotubular balance

• Proximal tubule reabsorbs a relatively constant fraction of filtered sodium and water despite changes in GFR.

1. Pressure diuresis and natriuresis

• Rise in arterial pressure increases urine and sodium excretion.
• Important in long-term blood-pressure regulation.

E. Gastrointestinal physiology

1. Law of the gut

• Distension causes contraction above and relaxation below the stimulus.
• Produces movement of contents in the anal direction.
• Also called the peristaltic reflex.

1. Enterogastric reflex

• Duodenal distension, acidity, fat, or irritation inhibits gastric emptying.

F. Sensory physiology

1. Weber-Fechner law

• Sensation is proportional to the logarithm of stimulus intensity.
• Basis of sensory discrimination.

1. Labeled-line principle

• Each sensory pathway conveys one specific modality, regardless of how it is stimulated.
• Example: stimulation of optic pathway is perceived as light.

1. Place and volley principles

• Frequency/pitch coding in hearing, as explained above.

1. Purkinje shift

• In dim light, maximum visual sensitivity shifts toward blue-green wavelengths.
• Rod vision predominates in darkness.

Highest-yield priority list for your supplementary exam

1. Place and volley principle
2. All-or-none law
3. Frank-Starling law
4. Fick principle
5. Starling forces and oedema
6. Laplace’s law
7. Bohr and Haldane effects
8. Hering-Breuer reflex
9. Transport maximum
10. Counter-current mechanism
11. Law of gut/peristaltic reflex
12. Weber-Fechner law and labeled-line principle