Updated Physio Complete Unit 2 exam review

Dorsal Column System

Conveys discriminative touch, pressure, vibration, and proprioception

Anterolateral System

Conveys pain, temperature, and light touch

Dorsal Column decussation point

Medulla (second-order neurons cross midline)

Anterolateral System decussation point

Spinal cord (second-order neurons cross midline)

First-order neuron cell body location

Dorsal root or cranial ganglion

Second-order neuron cell body location

Medulla (Dorsal Column) or Spinal Cord (Anterolateral)

Third-order neuron cell body location

Thalamus

Fourth-order neuron cell body location

Somatosensory cortex

Receptor potential

Graded change in membrane potential in response to a stimulus; not an action potential

Sensory transduction

Conversion of a physical stimulus into electrochemical energy (change in membrane potential)

Modality (encoding)

Type of stimulus detected; encoded by dedicated pathways

Location (encoding)

Where the stimulus occurs; encoded by receptive field

Duration (encoding)

How long the neuron fires; encoded by duration of action potentials

Intensity (encoding)

Strength of stimulus; encoded by frequency or population coding

Receptive field

Area of the body that alters a sensory neuron’s firing rate when stimulated

Smaller receptive field

Higher spatial resolution (better localization)

Lateral inhibition

Inhibition of neighboring neurons to sharpen sensory discrimination

Frequency coding

Intense stimulus → higher frequency of action potentials

Population coding

Intense stimulus → more receptors firing simultaneously

Adaptation

Decline in receptor firing rate with sustained stimulus

Phasic receptor

Responds quickly and adapts (e.g. Pacinian corpuscle)

Tonic receptor

Maintains response to prolonged stimulus (e.g. Merkel receptor)

A fibers

Myelinated fibers with faster conduction

C fibers

Unmyelinated fibers with slower conduction

Aβ fibers

Large, fast fibers for touch and pressure

Aδ fibers

Small, myelinated fibers for pain and temperature

Dermatome

Skin region innervated by sensory nerves from one spinal segment

Myotome

Muscle group innervated by motor nerves from one spinal segment

Sclerotome

Bony element associated with one spinal segment

Pacinian corpuscle

Detects vibration and tapping (phasic)

Meissner corpuscle

Detects fine touch, point discrimination, flutter

Merkel disks

Detect sustained pressure and texture (tonic)

Ruffini endings

Detect stretch and joint rotation

Hair follicle receptor

Detects velocity of skin movement

Stevens’ Power Law

Ψ = kΦⁿ; sensation increases as a power function of stimulus intensity

Auditory system function

Transduces sound waves into electrical signals interpreted as hearing

Human hearing range

10 Hz – 20,000 Hz

Most sensitive frequency range

500 – 5,000 Hz

Pitch or tone

Determined by frequency of sound waves

Loudness

Determined by intensity (amplitude) of sound waves

Loudness in decibels formula

Loudness (dB) = 10 × log10(Isound / Iref)

Reference intensity for hearing threshold

~0.5 × 10⁻¹² W/cm² at 2000 Hz

Sound intensity proportional to

Pressure amplitude squared

Pressure sound level formula

Loudness (dB) = 20 × log10(ΔPsound / ΔPref)

Middle ear boundaries

Tympanic membrane to oval window of cochlea

Auditory ossicles

Malleus, Incus, Stapes

Function of ossicles

Amplify and transmit vibrations from tympanic membrane to oval window

Oval window

Membrane where stapes transmits vibrations into cochlea

Round window

Releases pressure waves from cochlea

Amplification factor of ossicles

~15-fold increase in pressure

Cochlea

Coiled structure in inner ear responsible for sound transduction

Scala vestibuli

Upper chamber of cochlea filled with perilymph

Scala media

Middle chamber filled with endolymph (high K⁺, low Na⁺)

Scala tympani

Lower chamber of cochlea filled with perilymph

Reissner’s membrane

Separates scala vestibuli and scala media

Basilar membrane

Separates scala tympani and scala media; supports organ of Corti

Helicotrema

Opening connecting scala vestibuli and scala tympani at apex

Organ of Corti

Contains hair cells that serve as auditory receptors

Hair cells

Mechanoreceptors with stereocilia that transduce sound vibrations

Tectorial membrane

Gelatinous structure contacting hair cell cilia

Endolymph composition

High K⁺, low Na⁺; creates +80 mV endocochlear potential

Perilymph composition

Low K⁺, high Na⁺; surrounds basolateral surface of hair cells

Hair cell depolarization

Bending stereocilia toward tallest cilium → K⁺ influx → depolarization

Hair cell hyperpolarization

Bending stereocilia away → K⁺ channels close → hyperpolarization

Depolarization effect

Opens Ca²⁺ channels → glutamate release → stimulates cochlear nerve

Sound frequency encoding

Determined by location of maximum basilar membrane displacement

High frequency location

Base of cochlea (narrow and stiff)

Low frequency location

Apex of cochlea (wide and flexible)

Tonotopic organization

Frequency mapping preserved from cochlea to auditory cortex

First-order auditory neurons

Spiral ganglion cells in modiolus (Cochlear Nerve CN VIII)

Second-order auditory neurons

Cochlear nuclei in medulla

Decussation of auditory pathway

Fibers cross midline via lateral lemniscus

Third-order auditory neurons

Inferior colliculus (midbrain)

Fourth-order auditory neurons

Thalamus (medial geniculate nucleus)

Final destination of auditory pathway

Auditory cortex in temporal lobe

Broca’s area

Controls motor function of speech (frontal lobe)

Wernicke’s area

Responsible for understanding spoken language (temporal lobe)

Expressive aphasia

Damage to Broca’s area → difficulty producing speech

Receptive aphasia

Damage to Wernicke’s area → difficulty understanding speech

Olfactory system function

Detects volatile chemical odorants in the air and converts them into neural signals

Cribriform plate

Thin, perforated bone separating nasal cavity from brain; olfactory nerve fibers pass through it

Olfactory epithelium

Region in nasal cavity containing olfactory receptor neurons

Olfactory receptor neurons

Type of neuron

Odorants

Volatile chemical molecules that bind to receptors on olfactory cilia

Olfactory ensheathing cells

Glial-like cells that support and guide olfactory axons through the cribriform plate

Bowman’s glands

Produce mucus covering olfactory epithelium

Olfactory mucus

Contains mucopolysaccharides, antibodies, electrolytes, and odorant-binding proteins

Mucus renewal time

Approximately every 10 minutes

Odorant-binding proteins

Concentrate odorants and facilitate binding to olfactory receptors

Olfactory receptor mechanism

Odorant binds → activates Golf (G-protein) → α-subunit activates adenylyl cyclase

Adenylyl cyclase function

Converts ATP to cyclic AMP (cAMP)

Effect of cAMP

Binds to cyclic nucleotide–gated (CNG) cation channels → opens Ca²⁺ channels

Ca²⁺ influx

Depolarizes olfactory receptor cell and opens Cl⁻ channels

High intracellular Cl⁻ in olfactory neurons

Causes Cl⁻ efflux, further depolarizing the cell

Result of depolarization

Triggers action potentials transmitted to olfactory bulb

Signal termination

Golf α-subunit hydrolyzes GTP → GDP → inactivates adenylyl cyclase

cAMP degradation

Phosphodiesterase converts cAMP → AMP, closing CNG channels

Ca²⁺ removal

Actively transported out of cell or into endoplasmic reticulum

First-order olfactory neurons

Receptor neurons in olfactory epithelium

Second-order neurons

Mitral and tufted (M & T) cells in olfactory bulb