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Neurophysiology of the CNS — Physiology of the Spinal Cord — Reflex & Reflex Arc

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1. The Nervous System: Definition and Classification

Definition

Classification

NERVOUS SYSTEM
│
├── Central Nervous System (CNS)
│     ├── Brain (cerebrum, cerebellum, brainstem)
│     └── Spinal cord
│
└── Peripheral Nervous System (PNS)
      ├── Somatic Nervous System
      │     ├── Afferent (sensory) — carries signals TO CNS
      │     └── Efferent (motor) — carries signals FROM CNS to skeletal muscle
      │
      └── Autonomic Nervous System (ANS)
            ├── Sympathetic ("fight or flight")
            ├── Parasympathetic ("rest and digest")
            └── Enteric (intrinsic nervous system of the gut)

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2. Functions of the Nervous System

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3. Cells of the Nervous System: Neurons and Neuroglia

A. Neurons

• Excitable cells — the structural and functional units of the nervous system
• Transmit electrochemical impulses
• ~100 billion neurons in the human brain (Harrison's, p. 11976)
• Permanently post-mitotic in most regions; cannot regenerate once lost
• Vary enormously in size (5 µm granule cells → 100+ µm Betz cells)

B. Neuroglia (Glial Cells)

• Non-excitable supporting cells — 10× more numerous than neurons
• Do not generate action potentials in the classical sense
• Provide structural, metabolic, trophic, and immunological support
• Multiple subtypes (see Topic 5)

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4. The Neuron: Structure, Classification, Functions of Components, and Properties

Structure of a Neuron

        Dendrites (multiple, branching)
             ↓
        Cell Body (Soma / Perikaryon)
             ↓
        Axon Hillock
             ↓
        Axon (single, may be myelinated)
             ↓
        Axon Terminals (Synaptic Boutons / End Feet)

Classification of Neurons

• Golgi Type I — Long axons (projection neurons, e.g., pyramidal neurons)
• Golgi Type II — Short axons (local circuit neurons, interneurons)

• Cholinergic, glutamatergic, GABAergic, dopaminergic, serotonergic, noradrenergic, etc.

Properties of the Neuron

1. Excitability — Can respond to stimuli by generating an action potential
2. Conductivity — Can propagate action potentials along its membrane
3. All-or-none law — Once threshold is reached, a full AP is produced regardless of stimulus strength
4. Refractory period — Absolute (no AP possible) and relative (larger stimulus required); limits frequency of firing
5. Rhythmicity — Some neurons fire spontaneously in a rhythmic pattern (pacemaker neurons)
6. Synaptic transmission — Can release and receive neurotransmitters
7. Plasticity — Can modify synaptic strength in response to activity (basis of learning)
8. Metabolism — Highly active; dependent on continuous O₂ and glucose supply; cannot survive >5–10 min without O₂

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5. Neuroglia: Types and Functions

CNS Glia

PNS Glia

Key Differences: Oligodendrocytes vs. Schwann Cells

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6. Central Synapses: Types and Properties

Definition

Types of Synapses

Properties of Chemical Synapses

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7. Types of Interneuronal Connections (Chain and Circular)

A. Chain (Open) Circuits

• Multiple presynaptic neurons → single postsynaptic neuron
• Allows integration of information from many sources
• Increases sensitivity of the postsynaptic neuron

• Single presynaptic neuron → multiple postsynaptic neurons
• Amplifies a signal; one neuron can influence many
• Two types: within same tract (amplification) or to multiple tracts (different pathways)

• Neuron A → Neuron B → Neuron C → ...
• Simple relay; used in ascending/descending tracts

B. Circular (Reverberating) Circuits

• Axon collaterals feed back to re-excite the same chain of neurons
• Allows sustained, prolonged discharge after a single input
• Critical for:
  • Rhythmic activity (respiratory center, locomotion, sleep-wake cycles)
  • Short-term memory (reverberatory circuits in hippocampus)
  • After-discharge — prolonged response after stimulus ends

1. Single neuron with recurrent collateral → re-excites itself
2. Chain of neurons with collateral from last neuron back to first
3. Complex multi-loop reverberating circuits

• Multiple collateral pathways allow simultaneous processing

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8. Mediator Systems of the Brain

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9. Reflex: Definition and Significance — Classification of Reflexes

Definition

Significance

• Provides rapid, protective responses (withdrawal from pain)
• Maintains homeostasis (cardiovascular reflexes, postural reflexes)
• Coordinates complex motor activity (walking, swallowing)
• Allows clinical assessment of neurological integrity
• Forms the basis of all higher nervous activity (Pavlov's view)

Classification of Reflexes

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10. Reflex Arc: Structure and Significance of its Components

Definition

Components of the Reflex Arc

STIMULUS
    ↓
1. RECEPTOR
    ↓
2. AFFERENT (SENSORY) NERVE
    ↓
3. NERVE CENTER (REFLEX CENTER in CNS)
    ↓
4. EFFERENT (MOTOR) NERVE
    ↓
5. EFFECTOR ORGAN
    ↓
RESPONSE

Detailed Significance of Each Component

Clinical Significance

• Integrity of the reflex arc reflects the health of sensory/motor nerves and spinal cord segments
• Exaggerated reflexes (hyperreflexia) → upper motor neuron lesion
• Diminished/absent reflexes (hyporeflexia/areflexia) → lower motor neuron or sensory nerve lesion
• Used in clinical neurological examination (deep tendon reflexes, plantar response)

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11. Reflex Time: Definition and Factors Affecting It

Definition

Reflex Time = Receptor activation time + Afferent conduction time + Central (synaptic) delay time + Efferent conduction time + Effector latency

Central Synaptic Delay

• Each synapse contributes ~0.5 ms delay
• Monosynaptic reflex: ~1 ms total central delay
• Polysynaptic reflexes: longer; proportional to number of synapses

Factors Affecting Reflex Time

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12. Inhibition in the CNS: Types and Significance

Definition

Types of Inhibition

A. Postsynaptic Inhibition (Direct Inhibition)

• Inhibitory interneuron releases GABA (brain/spinal cord) or glycine (spinal cord) onto the postsynaptic membrane
• Opens Cl⁻ channels → Cl⁻ influx → IPSP (hyperpolarization)
• Membrane potential moves away from threshold

1. Renshaw cell inhibition (Recurrent inhibition)
   • Motor neuron sends axon collateral to Renshaw cell (inhibitory interneuron using glycine)
   • Renshaw cell feeds back and inhibits the same motor neuron
   • Prevents excessive, prolonged firing; provides negative feedback control
2. Reciprocal inhibition (Sherrington)
   • Excitation of flexor motor neurons → simultaneous inhibition of extensor motor neurons (via inhibitory interneuron)
   • Essential for coordinated limb movement (antagonist muscle relaxation during agonist contraction)

B. Presynaptic Inhibition

• An axoaxonic synapse releases GABA onto the presynaptic terminal (axon) of an excitatory neuron
• Opens Cl⁻ channels on the presynaptic axon → reduces amplitude of AP reaching the terminal → less Ca²⁺ entry → less neurotransmitter release
• Does not produce IPSP on postsynaptic membrane
• Very common in sensory pathways (pain gate control, dorsal horn)
• Acts as a "sensory gate" — can selectively suppress certain inputs

C. Feed-forward Inhibition

• Excitatory neuron A activates both target neuron B and an inhibitory interneuron that inhibits B
• Limits the duration and spread of excitation

D. Lateral Inhibition (Surround Inhibition)

• A neuron inhibits its neighbors while exciting itself
• Sharpens spatial discrimination and contrast
• Critical in sensory systems (skin, retina, auditory system)

E. Post-excitatory Inhibition

• After intense activity, neurons may enter a hyperpolarized refractory state

Significance of Inhibition

• Prevents excessive neuronal activity (seizures)
• Enables coordinated movement (reciprocal inhibition)
• Sharpens sensory discrimination (lateral inhibition)
• Allows selective attention (presynaptic inhibition)
• Creates rhythmic patterns of activity
• Prevents reflex irradiation (spread of reflexes)

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13. Spinal Cord: Structure and Functions

Structure

• Cylindrical structure, ~45 cm long, extending from foramen magnum (C1) to L1-L2 vertebra in adults (conus medullaris)
• Covered by three meninges: dura mater, arachnoid mater, pia mater
• 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
• Two enlargements: cervical enlargement (C5-T1) for upper limb; lumbar enlargement (L1-S2) for lower limb
• Below L2: cauda equina (horse's tail) of nerve roots

        Dorsal (Posterior)
              |
    Dorsal column (white)
    Posterior horn (grey)
              |
Lateral    --- ---   Lateral
column          |    column
(white)   Lateral   (white)
          horn (grey)
              |
    Anterior horn (grey)
    Ventral column (white)
              |
        Ventral (Anterior)

• Composed of myelinated axons organized into funiculi (columns):
  • Dorsal funiculus — ascending sensory tracts (gracile & cuneate fasciculi)
  • Lateral funiculus — ascending (spinothalamic, spinocerebellar) + descending (corticospinal, rubrospinal) tracts
  • Ventral funiculus — descending tracts (vestibulospinal, tectospinal, reticulospinal)

Functions of the Spinal Cord

1. Conduction function — serves as a bidirectional highway for ascending sensory and descending motor impulses between brain and body
2. Reflex function — contains centers for somatic and autonomic reflexes
3. Integration — integrates sensory input with motor output at the segmental level
4. Autonomic control — preganglionic neurons for sympathetic and sacral parasympathetic outflow

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14. Conduction Function: Ascending and Descending Pathways

Ascending (Sensory) Tracts

Descending (Motor) Tracts

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15. Reflex Function: Somatic and Autonomic Reflexes

A. Somatic Reflexes

1. Stretch Reflex (Myotatic Reflex) — e.g., Knee Jerk

• Receptor: Muscle spindle (intrafusal fibers; Ia afferents)
• Arc: Ia afferent → anterior horn → alpha motor neuron → same muscle
• Monosynaptic — only one synapse in CNS
• Function: Resists stretch; maintains muscle tone; anti-gravity posture
• Gamma loop: Gamma motor neurons maintain spindle sensitivity during muscle shortening
• Clinical use: Tests spinal cord segment integrity (patella → L3-L4)

2. Golgi Tendon Organ (Inverse Myotatic / Clasp-knife) Reflex

• Receptor: Golgi tendon organ (Ib afferents) — detects muscle tension
• Arc: Ib afferent → inhibitory interneuron (Ib interneuron) → inhibits homonymous motor neuron
• Disynaptic (two synapses)
• Function: Protective — prevents muscle damage from excessive tension; autogenic inhibition

3. Flexor (Withdrawal) Reflex

• Stimulus: Nociceptive (pain)
• Response: Flexion of the stimulated limb (withdrawal)
• Arc: Pain receptor → multiple interneurons → flexor motor neurons ipsilaterally
• Polysynaptic
• Irradiation: With increasing stimulus intensity, spreads to more segments

4. Crossed Extension Reflex

• Occurs simultaneously with withdrawal reflex
• Ipsilateral limb flexes → contralateral limb extends
• Pathway: Pain afferents → interneurons that cross midline → excite contralateral extensors + inhibit contralateral flexors
• Function: Maintains balance and posture when one leg withdraws from pain

5. Superficial Reflexes (Clinical examples)

B. Autonomic Reflexes (Visceral Reflexes)

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16. Spinal Shock: Definition, Causes, and Features

Definition

Spinal shock = immediate and temporary depression of ALL spinal cord functions below the level of the lesion, due to sudden withdrawal of supraspinal (higher center) influences.

Causes

1. Traumatic transection — road traffic accidents, falls, gunshot wounds causing complete or near-complete spinal cord injury
2. Surgical transection — in animal experiments (historically used to study spinal physiology)
3. Severe cord contusion — blunt trauma
4. Vascular injury — anterior spinal artery occlusion causing cord infarction
5. Tumors — rapid cord compression (rare cause of true spinal shock)

Pathophysiology

• The spinal cord below the injury is intact but is suddenly deprived of all descending excitatory inputs from the brain (corticospinal, reticulospinal, vestibulospinal tracts)
• The normal background excitatory tone from higher centers that maintains motor neuron excitability is lost
• This results in hyperpolarization and functional depression of spinal neurons below the lesion
• Intrinsic spinal reflexes are intact anatomically but functionally depressed

Features During Spinal Shock

Duration

• In humans: lasts days to weeks (typically 3–6 weeks; may be months)
• In higher primates: 1–2 weeks
• In lower animals (e.g., frogs): minutes to hours (less dependent on higher centers)
• The more complex the nervous system, the longer the duration of spinal shock

Recovery from Spinal Shock (Sequence)

Mechanisms of Reflex Recovery After Spinal Shock

1. Denervation hypersensitivity — postsynaptic receptors up-regulate after loss of supraspinal input
2. Unmasking of latent synapses — previously inactive synapses become functional
3. Axon terminal sprouting — remaining intact axons sprout new terminals
4. Increased excitability of spinal motor neurons

Spinal Shock vs. Neurogenic Shock

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Summary Table: Key Spinal Cord Reflexes and Their Levels

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Brainstem & Cerebellum — Physiology

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1. Physiology of the Medulla Oblongata

Anatomy Overview

• The pyramids anteriorly (corticospinal fibers; pyramidal decussation at the caudal medulla — 85–90% of fibers cross here)
• The inferior olivary nucleus (relay for cerebellar input)
• The nucleus gracilis and nucleus cuneatus (relay for dorsal column–medial lemniscal pathway)
• The reticular formation (diffuse network throughout)
• Nuclei of cranial nerves VIII through XII

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Functions of the Medulla Oblongata

A. Vital Centers (Life-Sustaining)

• Located in the reticular formation of the medulla (rostral ventrolateral medulla — RVLM, and nucleus tractus solitarius — NTS)
• Two components:
  • Cardioacceleratory center — increases heart rate and contractility via sympathetic outflow (C1 neurons of RVLM → intermediolateral cell column → sympathetic chain → heart)
  • Cardioinhibitory center — decreases heart rate via the dorsal motor nucleus of vagus (CN X); parasympathetic slowing
• Receives input from baroreceptors (carotid sinus, aortic arch via CN IX and X → NTS) and from higher centers (hypothalamus, cortex)
• Vasomotor center — controls vasomotor tone of arterioles and veins via sympathetic outflow; tonic firing maintains resting vascular resistance; NTS mediates baroreflex

• The medullary component of the respiratory center is the primary rhythm generator for breathing
• Two groups of neurons:
  • Dorsal Respiratory Group (DRG) — located in the NTS; primarily responsible for inspiration; fires rhythmically; sets baseline respiratory rhythm; receives afferents from peripheral chemoreceptors (CN IX, X) and lung stretch receptors
  • Ventral Respiratory Group (VRG) — located in the nucleus ambiguus and nucleus retroambiguus; active during forced expiration and deep breathing; includes the pre-Bötzinger complex (rostral VRG), the primary respiratory rhythm generator (pacemaker cells with intrinsic bursting activity)
• Mechanism: Pre-Bötzinger complex generates inspiratory bursts → DRG drives phrenic nerve (C3-C5) → diaphragm contracts → inspiration; VRG activates intercostal and abdominal muscles during forced breathing
• Apneustic and pneumotaxic centers in the pons modulate medullary rhythm (see Topic 2)

• Located in the nucleus tractus solitarius (NTS) and nucleus ambiguus
• Coordinates the pharyngeal and esophageal phases of swallowing — a highly complex, patterned reflex
• Afferents: CN V, IX, X; Efferents: CN IX, X, XII (Bailey & Love, p. 796)

• Located in the lateral reticular formation (nucleus tractus solitarius and adjacent reticular formation)
• Coordinates the entire sequence of emesis: retroperistalsis, abdominal contraction, glottis closure, relaxation of esophageal sphincter
• Receives inputs from the chemoreceptor trigger zone (CTZ) in the area postrema (floor of 4th ventricle; outside BBB), vestibular system (CN VIII), GI tract afferents (CN X), higher cortical centers (sight/smell)
• Efferents: phrenic nerve, spinal motor neurons (abdominal muscles), vagus (esophagus, stomach)

B. Non-Vital Centers

C. Conduction Function

• All ascending sensory tracts (spinothalamic, dorsal column) and descending motor tracts (corticospinal) pass through the medulla
• Pyramidal decussation at the caudal medulla: lateral corticospinal tract crosses here → explains contralateral hemiplegia with cortical lesions

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Cranial Nerves Originating from the Medulla Oblongata

Note: CN VIII exits at the pontomedullary junction; CN IX, X, XI exit from the lateral medulla (postolivary sulcus); CN XII exits from the anterior medulla (preolivary sulcus).

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2. Physiology of the Pons

Anatomy Overview

• Basis pontis (ventral pons) — contains corticospinal (pyramidal) fibers descending through, pontine nuclei (relay corticopontocerebellar fibers), and transverse pontocerebellar fibers crossing to the contralateral middle cerebellar peduncle
• Tegmentum (dorsal pons) — contains nuclei of cranial nerves V, VI, VII, VIII; reticular formation; ascending sensory tracts; medial longitudinal fasciculus (MLF)

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Functions of the Pons

A. As a Bridge (Relay / Conduction Function)

• Motor cortex and association cortex → corticofugal fibers → pontine nuclei (basis pontis) → synapse → transverse pontine fibers cross midline → exit via middle cerebellar peduncle → cerebellar cortex (contralateral)
• This is the principal pathway by which the cerebral cortex informs the cerebellum about intended motor commands
• The enormous size of the middle cerebellar peduncle reflects the massive corticopontocerebellar traffic

• All ascending sensory tracts (spinothalamic, medial lemniscus, trigeminal lemniscus) pass through the pons to reach the thalamus
• Trigeminal lemniscus originates partly in the pons

• Corticospinal and corticobulbar fibers pass through the basis pontis (scattered among pontine nuclei and transverse fibers, unlike the compact pyramid of the medulla)

• Superior cerebellar peduncle — primarily efferent from cerebellum (dentate nucleus → contralateral red nucleus and VL thalamus); passes through pons/midbrain junction
• Middle cerebellar peduncle — entirely afferent to cerebellum (pontocerebellar fibers)
• Inferior cerebellar peduncle — mixed; enters cerebellum at pontomedullary junction

• Runs through pons; connects CN III, IV, VI nuclei and vestibular nuclei
• Coordinates conjugate horizontal and vertical eye movements
• Internuclear ophthalmoplegia from MLF lesion in MS is a classic clinical sign

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B. Role in Regulation of Respiration

• Located in the rostral pons — nucleus parabrachialis and Kölliker-Fuse nucleus
• Switches off inspiration — limits the duration of inspiratory bursts from the medullary DRG
• Increases respiratory rate — more frequent switching means shorter, more frequent breaths
• Sends inhibitory signals to the apneustic center and the DRG
• If pneumotaxic center is destroyed (but apneustic center intact): prolonged inspiratory gasping called apneusis

• Located in the lower pons (reticular formation)
• Sends tonic excitatory signals to the DRG to sustain/prolong inspiration
• Normally checked/suppressed by the pneumotaxic center and vagal afferents (Hering-Breuer reflex)
• If vagus is cut AND pneumotaxic center removed: apneustic breathing (sustained inspiration)
• If only vagus is cut (pneumotaxic intact): breathing becomes slower and deeper

Pre-Bötzinger Complex (medulla) — PRIMARY RHYTHM GENERATOR
         ↑ modulated by ↓
Apneustic Center (lower pons) — PROMOTES AND PROLONGS INSPIRATION
         ↑ inhibited by ↓
Pneumotaxic Center (upper pons) — TERMINATES INSPIRATION, increases rate
         ↑ modulated by ↓
Vagal afferents (lung stretch receptors — Hering-Breuer reflex)

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C. Cranial Nerves from the Pons

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D. Other Functions of the Pons

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3. Physiology of the Midbrain (Mesencephalon)

Anatomy Overview

MIDBRAIN (cross-section, rostral to caudal)
│
├── TECTUM (dorsal / posterior)
│     ├── Superior colliculi (x2) — rostral tectum
│     └── Inferior colliculi (x2) — caudal tectum
│
├── TEGMENTUM (central, surrounds aqueduct)
│     ├── Red nucleus
│     ├── Substantia nigra
│     ├── Periaqueductal grey (PAG)
│     ├── CN III and IV nuclei
│     └── Reticular formation
│
└── CEREBRAL PEDUNCLES / CRURA CEREBRI (ventral)
      ├── Corticospinal, corticobulbar, and corticopontine fibers

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Functions of the Midbrain

A. Superior Colliculi — Visual Reflexes

• Located on the dorsal surface of the rostral midbrain (tectum)
• Primary visual reflex center — receives direct input from the optic tract (retinotectal fibers), visual cortex, auditory system (inferior colliculus), somatosensory system
• Each superior colliculus contains a topographic map of the visual field (retinotopic map) and is also mapped for auditory and somatosensory space — a multimodal sensory integration center

B. Inferior Colliculi — Auditory Reflexes

• Located on the dorsal surface of the caudal midbrain
• Mandatory relay station in the ascending auditory pathway

C. Red Nucleus — Motor Functions

• Located in the tegmentum of the rostral midbrain (at the level of CN III)
• A large, ovoid nucleus with a slightly reddish color (due to high vascularity and iron content)
• Two parts: parvocellular (rostral; larger, phylogenetically newer) and magnocellular (caudal; origin of rubrospinal tract)

• Cerebellum (contralateral dentate and interposed nuclei → via superior cerebellar peduncle → decussation in midbrain → contralateral red nucleus) — the most important input
• Motor cortex (ipsilateral; corticorubral fibers)
• Globus pallidus

• Rubrospinal tract — exits red nucleus → immediately decussates (anterior tegmental decussation / Forel's decussation) → descends in lateral funiculus of spinal cord → synapse on contralateral alpha motor neurons (especially cervical cord; facilitates flexor motor neurons)
• Rubroolivary fibers → inferior olivary nucleus → cerebellum (feedback loop)
• Rubrobulbar fibers → brainstem motor nuclei

• Flexor motor control — facilitates flexor muscles, particularly of the upper extremity; complements corticospinal tract
• Alternative motor pathway — in animals, rubrospinal tract is critical for fine limb movement; in humans, its role is supplementary (corticospinal tract dominates)
• Cerebellar output relay — serves as a key relay for cerebellar influence on motor activity
• Tremor generation — damage to red nucleus or superior cerebellar peduncle → rubral (Holmes) tremor (intention tremor at rest; "wing-beating" tremor)
• Lesion = Benedikt's syndrome: ipsilateral CN III palsy + contralateral tremor, chorea, athetosis (Harrison's, p. 985)

D. Substantia Nigra — Motor and Dopaminergic Functions

• Located in the ventral tegmentum of the midbrain, extending throughout its length
• Largest nucleus of the midbrain; appears dark due to neuromelanin (oxidation product of dopamine synthesis)
• Two parts:
  • Pars compacta (SNc) — dorsal part; densely packed dopaminergic neurons (A9 group); source of nigrostriatal dopaminergic pathway
  • Pars reticulata (SNr) — ventral part; GABAergic neurons; functionally similar to globus pallidus interna (GPi)

• Striatum (caudate + putamen) → GABAergic striatonigral fibers
• Subthalamic nucleus → glutamatergic input
• Cerebral cortex, thalamus, raphe nuclei

• Nigrostriatal pathway (SNc → striatum) — dopaminergic; the most important output
• SNr → thalamus (VA/VL) — GABAergic; inhibits thalamic output (part of direct/indirect pathway of basal ganglia)
• SNr → superior colliculus — controls saccadic eye movements

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4. Functional Anatomy and Histology of the Cerebellum

Gross Anatomy

• Located in the posterior cranial fossa, behind the brainstem, beneath the tentorium cerebelli
• Connected to brainstem by three cerebellar peduncles on each side:

• Two hemispheres (lateral) + vermis (midline)
• Multiple transverse folds called folia
• Three lobes:
  • Anterior lobe — separated from posterior by primary fissure; spinocerebellum
  • Posterior lobe — largest; cerebrocerebellum (neocerebellum) in lateral hemispheres; spinocerebellum in paravermal zone
  • Flocculonodular lobe — most ancient; vestibulocerebellum; separated by posterolateral fissure

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Histology of the Cerebellar Cortex

SURFACE (outer)
│
├── 1. MOLECULAR LAYER (outermost)
│
├── 2. PURKINJE CELL LAYER (middle; single cell thick)
│
└── 3. GRANULE CELL LAYER (innermost / deepest)
│
WHITE MATTER + DEEP CEREBELLAR NUCLEI

Layer 1: Molecular Layer (Outermost)

• Relatively cell-sparse layer
• Contains:
  • Basket cells — inhibitory interneurons (GABAergic); axons wrap around Purkinje cell somata ("basket"); receive parallel fiber input → inhibit Purkinje cells
  • Stellate cells — inhibitory interneurons (GABAergic); contact Purkinje cell dendrites; receive parallel fiber input → inhibit Purkinje cells
  • Parallel fibers — axons of granule cells that ascend to molecular layer and bifurcate into T-shape, running parallel to folia for up to 6 mm; synapse on Purkinje cell dendrites (excitatory; glutamate)
  • Purkinje cell dendrites — extensive, flat, fan-shaped dendritic tree perpendicular to the folium; receives ~200,000 parallel fiber synapses + climbing fiber synapses

Layer 2: Purkinje Cell Layer (Middle)

• Single row of large Purkinje cell bodies
• Purkinje cells are the sole output neurons of the cerebellar cortex
• They are large (50-80 µm), pear-shaped neurons with an extensively branched flat dendritic tree (fan-shaped, spanning the molecular layer)
• Neurotransmitter: GABA — Purkinje cell output is ALWAYS INHIBITORY to deep cerebellar nuclei and vestibular nuclei
• Each Purkinje cell receives:
  • ~200,000 parallel fiber synapses (from granule cells) — weak individual synapses; summation needed
  • 1 climbing fiber synapse (from contralateral inferior olivary nucleus) — extremely powerful; a single climbing fiber winds around the Purkinje cell like a vine and makes 300–500 synaptic contacts; produces complex spikes; this 1:1 relationship is unique
  • Inhibitory input from basket cells and stellate cells

Layer 3: Granule Cell Layer (Innermost / Deepest)

• Densest packing of neurons in the entire brain — ~100 billion granule cells (equal to or exceeding all other CNS neurons combined)
• Contains:
  • Granule cells — very small, excitatory neurons (glutamatergic); receive mossy fiber input (from spinal cord, pons, vestibular system); their axons ascend to molecular layer and bifurcate into parallel fibers
  • Golgi cells — large inhibitory interneurons (GABAergic); receive parallel fiber input + mossy fibers → inhibit granule cells (feedback inhibition); slow, inhibitory control of granule cell output
  • Mossy fiber synaptic glomeruli — synaptic complexes where mossy fiber terminals, granule cell dendrites, and Golgi cell axon terminals all meet in a cerebellar glomerulus

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Deep Cerebellar Nuclei

1. Inhibitory (GABAergic) input from Purkinje cells
2. Excitatory (glutamatergic) collateral input from mossy fibers and climbing fibers

Memory aid (medial to lateral): "Don't Eat Greasy Food" — Dentate, Emboliform, Globose, Fastigial (reversed: F, G, E, D medial → lateral)

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5. Functional Divisions and Connections of the Cerebellum

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A. Vestibulocerebellum (Archicerebellum)

• Equilibrium and balance control (primary function)
• Vestibulo-ocular reflex (VOR) — stabilizes gaze during head movement
• Controls eye movements: smooth pursuit, nystagmus suppression
• Axial and proximal limb posture

• Truncal ataxia — inability to maintain balance while sitting/standing; tendency to fall
• Nystagmus (especially gaze-evoked)
• Gait ataxia — wide-based, staggering gait
• Lesions of the vermis/flocculonodular lobe → midline cerebellar syndrome

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B. Spinocerebellum (Paleocerebellum)

• Ongoing motor correction — compares intended movement (efference copy from cortex) vs. actual movement (afferent proprioception); detects and corrects errors in real-time
• Provides smooth, coordinated limb movements
• Controls muscle tone and posture
• Serves as a "comparator" (see Topic 6)

• Limb ataxia — clumsy, dysmetric movements of the limbs
• Dysmetria — past-pointing, overshoot or undershoot
• Hypotonia in limbs

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C. Cerebrocerebellum (Neocerebellum / Pontocerebellum)

• Motor planning and programming — plans the sequence, timing, and trajectory of complex voluntary movements before they are executed
• Cognitive functions — language processing timing, working memory, attention, mental imagery of movement
• Motor learning — adaptation of motor programs through climbing fiber–mediated LTD at Purkinje cells
• Acts as a "servomechanism" — predictive feedforward control (see Topic 6)
• Coordinates movements between different joints and muscle groups

• Decomposition of movement — complex movements broken into sequential simple steps
• Dysmetria, dysdiadochokinesia
• Intention tremor
• Scanning speech (cerebellar dysarthria) — slow, irregular, explosive speech
• Cognitive-affective cerebellar syndrome (Schmahmann syndrome)

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Summary Table: Three Functional Divisions

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6. Cerebellar Functions and Applied Physiology

A. Mechanisms of Cerebellar Function

1. The "Damping" Mechanism

• The cerebellum acts as a damping system to prevent oscillation and overshoot of movements
• When the motor cortex initiates a movement, it also sends a signal to the cerebellum (efference copy)
• The cerebellum predicts when the movement should stop and sends an inhibitory "braking" signal back to the motor cortex just before the limb reaches its target
• Without this braking: the limb would overshoot, then the nervous system would try to correct, overshoot again → intention tremor (oscillatory tremor during movement, worsening as target is approached)
• This is why cerebellar lesions cause dysmetria and intention tremor — the damping mechanism is lost
• Analogous to a servo-controlled mechanical system with a "brake"

2. The "Comparator" (Error Detection) Mechanism

• Primarily a function of the spinocerebellum
• The cerebellum acts as a comparator — it continuously compares:
  • Intended movement (efference copy from motor cortex → via corticopontocerebellar pathway)
  • Actual movement (proprioceptive feedback → via spinocerebellar tracts)
• If a discrepancy (error) is detected → cerebellum sends corrective signals via interposed nuclei → red nucleus → rubrospinal tract and via VL thalamus → motor cortex → modifies ongoing motor command
• This is feedback (reactive) control — operates during execution of movement
• Allows smooth, accurate movements despite changing loads and unpredictable perturbations

3. The "Servomechanism" (Predictive / Feedforward Control)

• Primarily a function of the cerebrocerebellum (lateral hemisphere / dentate nucleus)
• The cerebellum is capable of predictive, feedforward control — anticipating sensory consequences of movements BEFORE feedback arrives (sensory feedback is too slow for fast movements)
• The cerebellum develops internal models of the body and its dynamics through motor learning:
  • Forward model — predicts the sensory consequences of a motor command
  • Inverse model — computes the motor command needed to achieve a desired outcome
• When a new motor skill is being learned, the cerebellum uses climbing fiber error signals (from inferior olivary nucleus) to induce long-term depression (LTD) at the parallel fiber–Purkinje cell synapse → reduces Purkinje cell inhibition of deep nuclei → more effective motor output next time
• Once learned, movements can proceed rapidly and accurately without relying on slow feedback
• This is why the cerebellum is essential for motor learning and skill acquisition (writing, playing an instrument, sports)
• The cerebellum also predicts the timing of sequential movements

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B. Clinical Signs of Cerebellar Lesions (DANISH Mnemonic)

Cerebellar signs are ipsilateral to the lesion (cerebellum controls the same side of the body — double-crossing: spinocerebellar tracts enter ipsilaterally; dentatorubrothalamic tract crosses in the midbrain and the corticospinal tract crosses again; net result = ipsilateral cerebellar control of ipsilateral body)

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C. Localization of Cerebellar Lesions

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D. Cerebellar Motor Learning — Climbing Fiber Hypothesis

• Proposed by Marr and Albus (theoretical) and demonstrated by Ito (experimental)
• Error signal generated by inferior olivary nucleus → climbing fiber → Purkinje cell → produces complex spikes
• If a parallel fiber is active simultaneously with a climbing fiber → long-term depression (LTD) at that parallel fiber–Purkinje cell synapse → less Purkinje cell inhibition of deep nuclei next time that movement is attempted → movement improves
• This represents supervised learning — the inferior olive acts as a "teacher" providing error signals
• Explains how cerebellar circuits progressively optimize motor performance with practice

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Physiology of the Autonomic Nervous System (ANS)

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Quick Overview: Classification of the Nervous System

NERVOUS SYSTEM
│
├── Central Nervous System (CNS) — Brain + Spinal cord
│
└── Peripheral Nervous System (PNS)
      │
      ├── SOMATIC Nervous System
      │     (voluntary; controls skeletal muscles)
      │
      └── AUTONOMIC Nervous System (ANS)
            (involuntary; controls internal organs)
            │
            ├── Sympathetic NS ("fight or flight")
            ├── Parasympathetic NS ("rest and digest")
            └── Metasympathetic NS (Enteric NS — gut's own nervous system)

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1. Somatic Nervous System: Characteristics

Simple Definition

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Key Characteristics

1. Voluntary Control

• Movements are conscious and deliberate
• You can decide to raise your hand, and it happens
• Controlled by the motor cortex of the brain

2. Effector Organ

• Acts ONLY on skeletal (striated) muscles
• Does NOT act on smooth muscle, cardiac muscle, or glands

3. Nerve Pathway — One Neuron from CNS to Muscle

Motor Cortex (Brain)
        ↓
Upper Motor Neuron (in CNS)
        ↓
Lower Motor Neuron (exits spinal cord via anterior horn)
        ↓
Skeletal Muscle

• Only ONE efferent (motor) neuron from spinal cord to muscle — no ganglion in between
• This is called a monosynaptic or direct pathway at the neuromuscular junction (NMJ)

4. Neurotransmitter at the Effector

• The lower motor neuron releases Acetylcholine (ACh) at the neuromuscular junction (NMJ)
• Receptor on skeletal muscle: Nicotinic receptor (NM type)
• Effect is ALWAYS excitatory — ACh always causes muscle contraction
• The somatic nervous system cannot inhibit skeletal muscle directly (inhibition happens in the CNS, not at the muscle)

5. Myelination

• Somatic efferent fibers are heavily myelinated (A-alpha fibers for motor; A-beta for touch; A-delta for pain/temp)
• Fast conduction velocity (up to 70–120 m/s for motor fibers)

6. Reflex Arc

• Somatic reflexes (e.g., knee jerk) are mediated through somatic NS
• Both afferent (sensory) and efferent (motor) limbs involve somatic nerves

7. Fatigue

• Skeletal muscle does fatigue with prolonged use (unlike cardiac muscle)

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Summary Table

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2. Autonomic Nervous System (ANS): Characteristics

Simple Definition

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Key Characteristics

1. Involuntary Control

• Functions are unconscious and automatic
• You do NOT consciously control your heart rate, blood pressure, or digestion
• Controlled by the hypothalamus (the main ANS control center), brainstem, and spinal cord

2. Effector Organs (Wide Variety)

• Smooth muscle (blood vessels, bronchi, GI tract, bladder, iris, etc.)
• Cardiac muscle (heart rate and contractility)
• Glands (salivary, sweat, lacrimal, adrenal medulla, digestive glands)
• Adipose tissue (fat metabolism)

3. Nerve Pathway — TWO Neurons from CNS to Organ

CNS (Brain / Spinal cord)
        ↓
PREGANGLIONIC Neuron (myelinated, thin — B fiber)
        ↓  (synapse in a GANGLION)
POSTGANGLIONIC Neuron (unmyelinated — C fiber)
        ↓
Effector Organ (smooth muscle / gland / heart)

• There is always a ganglion (a relay station) between the CNS and the effector
• Preganglionic neuron = from CNS to ganglion (myelinated, B fiber)
• Postganglionic neuron = from ganglion to organ (unmyelinated, C fiber)
• Conduction is slow compared to somatic NS

4. Neurotransmitters

5. Can Be Both Excitatory and Inhibitory

• Unlike the somatic NS, the ANS can either stimulate or inhibit an effector depending on the receptor type
• Example: Sympathetic stimulation accelerates the heart (via β1 receptors) but inhibits GI motility (via α2, β receptors)

6. Diffuse and Widespread Effects

• ANS produces widespread, diffuse effects on many organs simultaneously
• Especially the sympathetic division — during stress, nearly every organ in the body is affected at once (mass discharge)

7. Dual Innervation

• Most organs receive both sympathetic AND parasympathetic supply
• The two divisions oppose each other in most cases (antagonistic)
• But they can also be synergistic (e.g., both contribute to erection + ejaculation)

8. Higher Control

• Hypothalamus — master control center of ANS; integrates ANS with endocrine system and emotional states
• Brainstem (medulla) — cardiovascular and respiratory ANS centers
• Limbic system — emotional responses → ANS activation (stress, fear → sympathetic surge)
• Cortex — can modulate ANS (e.g., blushing, vasovagal syncope from emotion)

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Summary Table: Somatic vs. Autonomic

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3. Sympathetic Nervous System: Characteristics

Simple Definition

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Origin (Where Does It Come From?)

• Also called the Thoracolumbar outflow
• Preganglionic neurons originate from the lateral horn (intermediolateral cell column) of the spinal cord at levels:
  • T1 to L2 (or L3) — "Thoracolumbar"
  • Therefore, sympathetic activity affects the whole body from top to bottom

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Pathway and Ganglia

Lateral horn of spinal cord (T1–L2)
        ↓
Preganglionic fiber (myelinated, SHORT)
        ↓  exits via anterior root → white ramus communicans
SYMPATHETIC GANGLION (close to spinal cord)
        ↓
Postganglionic fiber (unmyelinated, LONG)
        ↓  exits via gray ramus communicans
Effector organ (heart, lungs, blood vessels, etc.)

• The adrenal medulla is a modified sympathetic ganglion
• The preganglionic fiber goes directly to the adrenal medulla (no postganglionic neuron)
• Adrenal medulla releases Epinephrine (80%) and Norepinephrine (20%) directly into the bloodstream
• These hormones amplify and prolong the sympathetic response throughout the body

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Neurotransmitters and Receptors

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Effects of Sympathetic Stimulation on Organs

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"Fight or Flight" Response — Physiological Purpose

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General Features of the Sympathetic System

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4. Parasympathetic Nervous System: Characteristics

Simple Definition

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Origin (Where Does It Come From?)

• Also called the Craniosacral outflow
• Preganglionic neurons originate from TWO regions:
  • Cranial region — brainstem nuclei associated with CN III, VII, IX, X
  • Sacral region — lateral horn of S2, S3, S4 spinal segments

• S2, S3, S4 → pelvic splanchnic nerves (nervi erigentes)
• Terminal ganglia near or within organs
• Supplies: descending colon, sigmoid colon, rectum, bladder (detrusor), external genitalia (erection)

Important: CN X (Vagus) accounts for 75% of all parasympathetic activity in the body — it supplies almost all thoracic and abdominal viscera.

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Pathway and Ganglia

Brainstem / S2–S4 lateral horn
        ↓
Preganglionic fiber (myelinated, LONG)
        ↓  (synapse in ganglion NEAR or WITHIN the organ)
TERMINAL GANGLION (close to or inside organ wall)
        ↓
Postganglionic fiber (unmyelinated, VERY SHORT)
        ↓
Effector organ

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Neurotransmitters and Receptors

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Effects of Parasympathetic Stimulation on Organs

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"Rest and Digest" Response — Physiological Purpose

• Heart slows down → conserves energy
• Pupils constrict → reduced light needed at rest
• Airways narrow slightly
• GI tract works actively → digests food
• Saliva flows → begins digestion in mouth
• Bladder prepares to empty
• Sexual arousal (erection) is facilitated

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General Features of the Parasympathetic System

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Comparison: Sympathetic vs. Parasympathetic

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5. Metasympathetic Nervous System: Characteristics

Simple Definition

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Structure and Location

Wall of the GI Tract
│
├── MYENTERIC PLEXUS (Auerbach's plexus)
│     Location: Between the outer longitudinal and inner circular muscle layers
│     Function: Controls GI MOTILITY (peristalsis, segmentation, muscle contraction)
│
└── SUBMUCOSAL PLEXUS (Meissner's plexus)
      Location: In the submucosa (between circular muscle and mucosa)
      Function: Controls GI SECRETION and blood flow; senses luminal environment

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Key Characteristics

1. Autonomy (Self-Sufficient)

• The ENS can function completely independently of the CNS
• Even if all nerves connecting the gut to the brain are cut (vagotomy + sympathectomy), peristalsis and basic gut functions continue
• It has its own sensory neurons, interneurons, and motor neurons forming complete local reflex arcs
• This is what makes it unique — no other organ has its own complete nervous system

2. Neurotransmitters Used by the ENS

3. Functions of the Metasympathetic (Enteric) Nervous System

• Peristaltic reflex (Law of the intestine / Bayliss and Starling):
  • Distension of the gut wall → ENS detects it via sensory neurons (IPANs — Intrinsic Primary Afferent Neurons)
  • Above the distension → ACh + Substance P released → ascending excitation → contraction (circular muscle contracts, pushing content forward)
  • Below the distension → NO + VIP released → descending inhibition → relaxation (circular muscle relaxes, allowing content to move forward)
  • This wave of contraction above + relaxation below = peristalsis

• Submucosal plexus (Meissner's) controls:
  • Water and electrolyte secretion into the gut lumen
  • Enzyme secretion from intestinal glands
  • Mucus secretion

• Controls local blood flow to the gut wall (vasodilation during digestion)

• ENS communicates with gut-associated lymphoid tissue (GALT)
• Participates in gut immunity and mucosal defense

• ENS communicates bidirectionally with the CNS via the vagus nerve
• 80–90% of vagal fibers are afferent (from gut to brain) — the gut talks more to the brain than the brain talks to the gut!
• This is the basis of the gut-brain axis — explaining why emotional states affect digestion and why gut disorders affect mood

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4. Relationship with Sympathetic and Parasympathetic Systems

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5. Clinical Importance

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Summary Table: Three Divisions of ANS

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Easy-to-Remember Summary

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