Physiology
Medical physiology for MBBS: cell and body fluids, blood, nerve and muscle, and the cardiovascular, respiratory, gastrointestinal, renal, endocrine, reproductive and nervous systems, special senses and integrative physiology.
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Definition
Ascending (sensory) pathways carry sensory information from the body to the brain; the two major systems are the dorsal column and the spinothalamic tract.
Dorsal Column–Medial Lemniscus
- Carries: fine touch, vibration, proprioception
- Crosses in the medulla
- Receptor → dorsal column → medulla → thalamus → cortex
Spinothalamic Tract
- Carries: pain, temperature, crude touch
- Crosses in the spinal cord (at entry level)
- Receptor → dorsal horn → cross → thalamus → cortex
General Features
- Three neurons (1st, 2nd, 3rd order)
- Relay in the thalamus
- End in somatosensory cortex (post-central gyrus)
Three neurons relay sensation, crossing the midline before reaching the cortex. Feature Dorsal column Spinothalamic Carries Touch, vibration, proprioception Pain, temperature Crosses Medulla Spinal cord Applied
- Brown-Séquard (hemisection) → dissociated sensory loss
- Tabes dorsalis — dorsal column degeneration
🔑KEY POINTS TO REMEMBER- Dorsal column = touch/vibration/proprioception, crosses in medulla.
- Spinothalamic = pain/temperature, crosses in spinal cord.
- Both relay in thalamus → somatosensory cortex.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Descending (motor) pathways carry motor commands from brain to muscle; the main one is the corticospinal (pyramidal) tract.
Pyramidal Tract (Corticospinal)
- Voluntary, skilled movement
- Motor cortex → crosses in the medulla (pyramids) → spinal cord
- Lateral (crossed) + anterior (uncrossed)
Extrapyramidal Tracts
- Rubrospinal, vestibulospinal, reticulospinal, tectospinal
- Posture, tone, involuntary control
Upper vs Lower Motor Neuron
- UMN — cortex to anterior horn
- LMN — anterior horn to muscle
Voluntary commands run from cortex, cross in the medulla, and reach muscle via the LMN. Tract Function Corticospinal Voluntary movement Vestibulospinal Balance, posture Reticulospinal Muscle tone Applied
- UMN lesion → spastic paralysis
- Stroke affects the corticospinal tract
🔑KEY POINTS TO REMEMBER- Corticospinal (pyramidal) = voluntary movement; crosses in medulla.
- Extrapyramidal tracts control posture and tone.
- UMN vs LMN give opposite clinical signs.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
A reflex is an involuntary, stereotyped response to a stimulus via a reflex arc; the spinal cord mediates many reflexes.
Reflex Arc Components
- Receptor
- Afferent (sensory) neuron
- Centre (spinal cord / brain)
- Efferent (motor) neuron
- Effector
Types of Reflexes
- Monosynaptic — stretch reflex (knee jerk)
- Polysynaptic — withdrawal reflex
- Superficial, deep, visceral
Spinal Cord
- Grey matter (H-shaped) — neurons
- White matter — ascending/descending tracts
- Acts as reflex centre + conduction pathway
A reflex arc runs receptor → afferent → centre → efferent → effector. Reflex Synapses Knee jerk Monosynaptic Withdrawal Polysynaptic Applied
- Reflexes test nervous-system integrity
- Exaggerated in UMN lesion, lost in LMN lesion
🔑KEY POINTS TO REMEMBER- Reflex arc: receptor → afferent → centre → efferent → effector.
- Monosynaptic (knee jerk) vs polysynaptic (withdrawal).
- ↑ in UMN lesion, lost in LMN lesion.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The cerebellum coordinates movement, balance and tone; the basal ganglia regulate the initiation and control of voluntary movement.
Cerebellum Functions
- Coordination of movement
- Balance and posture
- Regulation of muscle tone
- Motor learning
Basal Ganglia Functions
- Initiation and regulation of movement
- Control of tone and posture
- Suppression of unwanted movements
Lesions
- Cerebellar — ataxia, intention tremor, nystagmus
- Basal ganglia — Parkinson’s disease, chorea
Cerebellum coordinates while basal ganglia initiate and regulate movement. Structure Function Lesion Cerebellum Coordination, balance Ataxia, tremor Basal ganglia Movement initiation Parkinson’s, chorea Applied
- Parkinson’s — dopamine loss (basal ganglia)
- Cerebellar signs — ataxia, dysmetria, nystagmus
🔑KEY POINTS TO REMEMBER- Cerebellum: coordination, balance, tone (lesion → ataxia, intention tremor).
- Basal ganglia: initiate/regulate movement (lesion → Parkinson’s, chorea).
- Cerebellar tremor = intention; Parkinson tremor = resting.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The cerebral cortex is the seat of higher functions; the thalamus relays sensory input; the hypothalamus controls homeostasis and the autonomic/endocrine systems.
Cerebral Cortex — Lobes
- Frontal — motor, personality
- Parietal — sensory
- Temporal — hearing, memory
- Occipital — vision
Thalamus
- Relay station for sensory input (except smell)
- Gateway to the cortex
Hypothalamus
- Temperature, hunger, thirst
- Autonomic and endocrine control
- Circadian rhythm, emotion
Thalamus relays sensation to the cortex; the hypothalamus runs homeostasis. Region Function Frontal lobe Motor, personality Thalamus Sensory relay Hypothalamus Homeostasis Applied
- Cortical lesions → localised deficits
- Hypothalamic lesion → temperature/appetite disorders
🔑KEY POINTS TO REMEMBER- Cortex lobes: frontal (motor), parietal (sensory), temporal (hearing), occipital (vision).
- Thalamus = sensory relay (except smell).
- Hypothalamus = homeostasis, autonomic & endocrine control.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
A reflex arc is the neural pathway that mediates a reflex, consisting of five components.
Components
- Receptor — detects the stimulus
- Afferent neuron — sensory
- Centre — spinal cord / brain
- Efferent neuron — motor
- Effector — muscle / gland
The reflex arc converts a stimulus into an automatic response through five parts. Type Example Monosynaptic Knee jerk Polysynaptic Withdrawal reflex Applied
- Basis of clinical reflex testing
- Loss or exaggeration localises lesions
🔑KEY POINTS TO REMEMBER- Reflex arc = receptor, afferent, centre, efferent, effector.
- Monosynaptic (knee jerk) vs polysynaptic (withdrawal).
- Used to test nervous-system integrity.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The muscle spindle is a stretch receptor in muscle; the stretch reflex is the monosynaptic contraction of a muscle when it is stretched.
Muscle Spindle
- Intrafusal fibres with a sensory (Ia) ending
- Detects muscle length and rate of stretch
- Innervated by gamma motor neurons
Stretch Reflex
- Muscle stretched → spindle activated
- Ia afferent → alpha motor neuron
- Muscle contracts (monosynaptic)
- Example: knee jerk
Stretching a muscle excites its spindle, which reflexly contracts the same muscle. Component Role Spindle Detects stretch Ia afferent Sensory limb Alpha motor neuron Contraction Applied
- Tests reflex-arc integrity
- Basis of muscle tone
🔑KEY POINTS TO REMEMBER- Muscle spindle detects length/stretch (Ia afferent, gamma efferent).
- Stretch reflex is monosynaptic (knee jerk).
- Basis of muscle tone.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Cerebrospinal fluid (CSF) is a clear fluid produced by the choroid plexus that surrounds and protects the brain and spinal cord.
Formation & Circulation
- Produced by choroid plexus (lateral, 3rd, 4th ventricles)
- ~500 mL/day produced; ~150 mL present at a time
- Ventricles → subarachnoid space
- Absorbed by arachnoid villi
Functions
- Protection (cushions the brain)
- Buoyancy (reduces effective brain weight)
- Removes wastes
- Maintains a stable environment
CSF is made in the ventricles, bathes the CNS, then is absorbed into venous blood. Feature Value Volume ~150 mL Production ~500 mL/day Source Choroid plexus Applied
- Hydrocephalus — blocked flow / absorption
- Lumbar puncture for diagnosis (meningitis)
🔑KEY POINTS TO REMEMBER- CSF from choroid plexus; ~150 mL, ~500 mL/day.
- Functions: protection, buoyancy, waste removal.
- Blocked flow → hydrocephalus.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Upper motor neuron (UMN) lesions affect neurons from cortex to anterior horn; lower motor neuron (LMN) lesions affect anterior horn to muscle — giving opposite signs.
UMN Lesion
- Spastic paralysis
- ↑ Tone, ↑ reflexes
- Babinski positive
- No wasting or fasciculation
LMN Lesion
- Flaccid paralysis
- ↓ Tone, ↓/absent reflexes
- Wasting and fasciculations
- Babinski negative
UMN lesions release reflexes (spastic); LMN lesions denervate muscle (flaccid). Feature UMN LMN Paralysis Spastic Flaccid Tone ↑ ↓ Reflexes ↑ ↓ Wasting No Yes Babinski Positive Negative Applied
- Stroke (UMN); poliomyelitis (LMN)
- Helps localise the lesion
🔑KEY POINTS TO REMEMBER- UMN = spastic, ↑ tone/reflexes, Babinski +, no wasting.
- LMN = flaccid, ↓ tone/reflexes, wasting, fasciculations.
- Stroke (UMN) vs polio (LMN).
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Pain is carried by the spinothalamic tract; referred pain is pain felt away from its source, due to shared spinal segments.
Pain Pathway
- Nociceptors → Aδ (fast) & C (slow) fibres
- Dorsal horn → cross → spinothalamic tract
- → thalamus → cortex
Referred Pain
- Visceral & somatic afferents converge on the same spinal segment
- Brain misinterprets the source
- Examples: cardiac → left arm; diaphragm → shoulder
Pain signals cross in the cord and ascend the spinothalamic tract to the cortex. Organ Pain referred to Heart Left arm, jaw Diaphragm Shoulder tip Appendix Umbilicus (early) Applied
- Gate-control theory of pain
- Referred pain aids clinical diagnosis
🔑KEY POINTS TO REMEMBER- Pain: nociceptor → spinothalamic tract → thalamus → cortex.
- Aδ = fast/sharp, C = slow/dull.
- Referred pain: shared segment (heart → left arm).
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The reticular activating system (RAS) is a network in the brainstem reticular formation that controls wakefulness, arousal and consciousness.
Functions
- Maintains wakefulness and alertness
- Controls the sleep–wake cycle
- Filters sensory input (attention)
- Helps regulate muscle tone
The RAS relays arousal signals that keep the cortex awake and alert. State RAS activity Awake / alert High Sleep Low Coma Damaged Applied
- RAS damage → coma
- General anaesthesia depresses the RAS
🔑KEY POINTS TO REMEMBER- RAS (brainstem) controls wakefulness and arousal.
- Filters sensory input (attention).
- Damage → coma.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Parkinson’s disease is a movement disorder due to degeneration of dopaminergic neurons in the substantia nigra (basal ganglia).
Cardinal Features
- Tremor — resting, pill-rolling
- Rigidity — cogwheel
- Bradykinesia — slow movement
- Postural instability, mask-like face
Mechanism
- Loss of dopamine (substantia nigra)
- Imbalance of dopamine vs acetylcholine
- ↓ initiation of movement
Loss of nigral dopamine unbalances the basal ganglia, causing the motor signs. Feature Description Tremor Resting, pill-rolling Rigidity Cogwheel Bradykinesia Slow movement Treatment
- Levodopa (dopamine precursor)
- Dopamine agonists
🔑KEY POINTS TO REMEMBER- Parkinson’s = loss of nigral dopamine neurons.
- Triad: resting tremor, rigidity, bradykinesia.
- Treated with levodopa/dopamine agonists.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).