Spinal cord and reflex arc detail information about spinal cord and reflex arc

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Spinal Cord and Reflex Arc - Detailed Information

Sources: Gray's Anatomy for Students, Costanzo Physiology 7th Ed., Ganong's Review of Medical Physiology 26th Ed., Neuroanatomy through Clinical Cases 3rd Ed., Medical Physiology

PART 1: THE SPINAL CORD

1. Overview and Location

The spinal cord is the most caudal portion of the CNS. It extends from the foramen magnum down to approximately the disc between vertebrae L1 and L2 in adults (it can end as high as T12 or as low as L2-L3). In neonates, it extends to approximately L3 (sometimes reaching L4). The distal cone-shaped end is called the conus medullaris, from which a fine connective tissue filament called the filum terminale (pial part) continues inferiorly.

Spinal Cord - gross anatomy showing cervical and lumbosacral enlargements, conus medullaris, and filum terminale

The spinal cord has 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) containing both sensory (afferent) and motor (efferent) nerves.

2. Enlargements

The cord is not uniform in diameter:

Enlargement	Segments	Innervates
Cervical enlargement	C5 to T1	Upper limbs
Lumbosacral enlargement	L1 to S3	Lower limbs

The white matter is thickest in the cervical region (where most ascending fibers have entered and descending fibers have not yet terminated). The sacral cord is mostly gray matter.

3. External Surface Features

The external surface carries several longitudinal grooves:

Anterior median fissure - extends the full length of the anterior surface
Posterior median sulcus - along the posterior surface
Posterolateral sulcus (each side) - where posterior rootlets of spinal nerves enter

4. Internal Structure - Gray and White Matter

Spinal cord cross-section showing gray matter (dorsal horn, ventral horn, intermediate zone) and white matter columns with dorsal root ganglion, sensory neuron, motor neuron, dorsal and ventral roots, and spinal nerve

Gray Matter (H-shaped, butterfly appearance)

Rich in nerve cell bodies, forming longitudinal columns. In cross-section it shows:

Dorsal (posterior) horn - mainly involved in processing sensory input
Ventral (anterior) horn - contains lower motor neurons (alpha and gamma motoneurons) that innervate skeletal muscle
Intermediate zone - contains Clarke's nucleus (spinocerebellar relay) and intermediolateral nucleus (autonomic preganglionic neurons)

Rexed's Laminae - The gray matter is organized into 10 functional layers (Rexed's laminae):

Region	Nucleus	Laminae
Dorsal horn	Marginal zone	I
Dorsal horn	Substantia gelatinosa	II
Dorsal horn	Nucleus proprius	III, IV
Dorsal horn	Neck of dorsal horn	V
Dorsal horn	Base of dorsal horn	VI
Intermediate zone	Clarke's nucleus, intermediolateral nucleus	VII
Ventral horn	Commissural nucleus	VIII
Ventral horn	Motor nuclei	IX
Around central canal	Commissural cells	X

White Matter

Surrounds the gray matter and is rich in myelinated nerve fiber tracts. Organized into three columns on each side:

Dorsal (posterior) columns
Lateral columns
Ventral (anterior) columns

5. Spinal Nerve Roots

Every spinal nerve is formed by the joining of:

Dorsal root (sensory/afferent) - carries information FROM the periphery (skin, joints, muscles, viscera) INTO the cord; cell bodies located in the dorsal root ganglion (DRG)
Ventral root (motor/efferent) - carries motor information FROM the cord TO the periphery; includes somatic motor nerves (to skeletal muscle) and autonomic motor nerves

6. Ascending and Descending Tracts

Spinal cord tracts - ascending tracts on the left (fasciculus gracilis, fasciculus cuneatus, spinothalamic, spinocerebellar), descending tracts on the right (lateral corticospinal, rubrospinal, vestibulospinal, reticulospinal, tectospinal)

Major Ascending Tracts (carry sensory information to the brain)

Tract	Location	Carries	Destination
Fasciculus gracilis + cuneatus (posterior columns)	Dorsal column	Fine touch, proprioception, vibration	Thalamus via medial lemniscus
Spinothalamic tract	Lateral column	Pain, temperature, crude touch	Thalamus
Dorsal spinocerebellar	Lateral column	Proprioception (ipsilateral)	Cerebellum
Ventral spinocerebellar	Lateral column	Proprioception (bilateral)	Cerebellum

Major Descending Tracts (carry motor commands from the brain)

Tract	Location	Function
Lateral corticospinal	Lateral column	Voluntary movement of limbs (contralateral)
Anterior corticospinal	Ventral column	Axial muscle control (ipsilateral)
Rubrospinal	Lateral column	Limb motor control
Lateral vestibulospinal	Ventral column	Balance, extensor tone
Reticulospinal (pontine + medullary)	Ventral column	Posture, tone, reflexes
Tectospinal	Ventral column	Head-eye reflex movements

The lateral corticospinal tract is clinically the most important - upper motor neuron axons decussate at the caudal medulla (pyramidal decussation), descend in the lateral column, and synapse on lower motor neurons in the anterior horn.

7. Blood Supply

The spinal cord receives arterial supply from two sources:

Anterior spinal artery (single) - supplies the anterior 2/3 of the cord
Posterior spinal arteries (paired) - supply the posterior 1/3

These are supplemented by radicular arteries at multiple levels, with the artery of Adamkiewicz (great anterior radicular artery, usually arising from T9-T12) being the dominant supply to the thoracolumbar cord.

PART 2: THE REFLEX ARC

1. Definition

A reflex is a stereotyped, specific motor response to a particular stimulus. The neuronal circuit that directs this response is the reflex arc.

The reflex arc - showing sense organ (generator potential) -> afferent neuron (action potentials) -> synapse (EPSPs/IPSPs) -> efferent neuron (action potentials) -> neuromuscular junction (endplate potentials) -> muscle (action potentials)

2. Components of the Reflex Arc

The basic unit consists of 5 components:

Sensory receptor - detects the stimulus; generates a graded receptor potential proportional to stimulus strength
Afferent (sensory) neuron - carries the signal to the CNS via dorsal root; cell bodies in the dorsal root ganglion; generates all-or-none action potentials
Integrating center (CNS interneurons) - in the spinal gray matter; generates graded EPSPs and IPSPs
Efferent (motor) neuron - lower motor neuron; carries the command from the ventral horn out via the ventral root
Effector (muscle/gland) - produces the response; generates graded endplate potentials

The alpha motoneurons supplying extrafusal skeletal muscle fibers are the "final common pathway" - all neural influences on muscular contraction converge here.

Spinal reflex arc diagram showing somatic sensory receptor -> primary sensory neuron (via dorsal root) -> interneuron in dorsal horn -> motor neuron in ventral horn (via ventral root) -> skeletal muscle

3. Types of Spinal Cord Reflexes

Type of Reflex	Example	Synapses	Stimulus	Afferent Fiber	Response
Stretch (myotatic) reflex	Knee jerk	1 (monosynaptic)	Stretch (lengthening) of muscle	Group Ia	Contraction of same muscle
Golgi tendon reflex	Clasp-knife	2 (dysynaptic)	Contraction (shortening) of muscle	Group Ib	Relaxation of same muscle
Flexor-withdrawal reflex	Touching a hot stove	Many (polysynaptic)	Pain / noxious stimulus	Groups II, III, IV	Ipsilateral flexion + contralateral extension

4. Stretch Reflex (Monosynaptic)

Stretch reflex - muscle spindle stretched -> Group Ia afferent -> alpha motoneurons activated -> homonymous muscle contracts; also activates synergistic muscles and inhibits antagonistic muscles

Steps:

Muscle is stretched → muscle spindles (intrafusal fibers) are stretched
Group Ia afferent fibers (detecting velocity of stretch) and Group II fibers (detecting length) increase firing rate
Group Ia afferents enter spinal cord via dorsal root and synapse directly on alpha motoneurons (the single synapse)
Alpha motoneurons activate extrafusal fibers of the homonymous (same) muscle → muscle contracts, opposing the stretch
Simultaneously, synergistic muscles contract and antagonistic muscles relax (reciprocal inhibition via Ia inhibitory interneurons)
Gamma motoneurons are co-activated to maintain spindle sensitivity during contraction

Neurotransmitter at central synapse: Glutamate

Clinical example - Knee jerk (patellar reflex):

Tap patellar tendon → stretches quadriceps → Group Ia afferents fire → alpha motoneurons fire → quadriceps contracts → leg extends
Spinal level: L3-L4
Grading: 0 (absent) to 5+ (sustained clonus)

DTR spinal levels for clinical testing:

Reflex	Spinal Level
Biceps	C5, C6
Triceps	C7
Patellar (knee jerk)	L4
Achilles (ankle jerk)	S1

5. Golgi Tendon Reflex (Dysynaptic - Inverse Myotatic)

The Golgi tendon organ is located in tendons, arranged in series with extrafusal fibers (unlike muscle spindles, which are in parallel).

Steps:

Muscle contracts → extrafusal fibers shorten → Golgi tendon organs activated
Group Ib afferent fibers activated → synapse on inhibitory interneurons in spinal cord
Inhibitory interneurons inhibit alpha motoneurons → muscle relaxes
Synergistic muscles also relax; antagonistic muscles contract

This reflex acts as a protective mechanism against excessive muscle tension.

The clasp-knife reflex is an abnormal exaggeration seen in spasticity: initial resistance to passive stretch suddenly gives way as the Golgi tendon reflex is triggered.

6. Flexor-Withdrawal Reflex (Polysynaptic)

Flexor-withdrawal reflex showing pain stimulus activating Groups II, III, IV fibers -> ipsilateral flexion (flexor muscles contract, extensor muscles relax) + contralateral extension (extensor muscles contract, flexor muscles relax)

Steps:

Painful/noxious stimulus activates flexor reflex afferent fibers (Groups II, III, IV) entering via dorsal root
These synapse on multiple interneurons in the spinal cord (hence polysynaptic)
Ipsilateral side: Flexors contract, extensors relax → withdrawal of the limb
Contralateral side: Extensors contract, flexors relax → crossed-extension reflex to maintain balance
Afterdischarge - persistent neural firing in polysynaptic circuits keeps the contracted muscles contracted even after the stimulus is removed

7. Modulation of Reflexes

Reflex activity is not fixed. It is continuously modulated by:

Descending pathways from the cerebral cortex and brainstem
Convergence of multiple inputs onto alpha motoneurons - on average ~10,000 synaptic knobs converge on a single motor neuron
Higher center damage (e.g., upper motor neuron lesion) hyperactivates stretch reflexes (hyperreflexia, clonus) because inhibitory descending input is lost
Lower motor neuron or peripheral nerve damage abolishes the reflex (areflexia)

Summary Table: UMN vs LMN Lesion Effects on Reflexes

Feature	Upper Motor Neuron (UMN) Lesion	Lower Motor Neuron (LMN) Lesion
Deep tendon reflexes	Hyperreflexia	Hyporeflexia / absent
Tone	Spasticity	Flaccidity
Babinski sign	Present	Absent
Muscle atrophy	Disuse (mild)	Denervation (severe)
Fasciculations	Absent	Present

Sources: Gray's Anatomy for Students, p. 119-120 | Costanzo Physiology 7th Ed., pp. 76, 111-113 | Ganong's Review of Medical Physiology 26th Ed., pp. 235-236 | Neuroanatomy through Clinical Cases 3rd Ed., pp. 62-63, 250 | Medical Physiology (Boron & Boulpaep), p. 419

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