Spinal Cord Physiology

Spinal Cord Physiology

I. Structure of the Spinal Cord

• Location: Located in the vertebral canal.

• Division: Consists of 32 segments:

• 8 cervical segments (C1-C8)

• 12 thoracic segments (Th1-Th12)

• 5 lumbar segments (L1-L5)

• 4 sacral segments (S1-S4)

• 1 coccygeal segment (Co)

• Each spinal segment:

• Has a pair of spinal nerve roots (or dorsal root ganglia)

• Consists of anterior and posterior roots, which combine to form the spinal nerve.

• Anterior root:

• Also known as the motor root.

• Posterior root:

• Also known as the sensory root.

• Contains the posterior root ganglion, which houses the cell bodies of sensory neurons.

II. Anatomy of a Cross-Section of the Spinal Cord:

• Gray matter:

• Located in the center.

• Divided into:

• Anterior horn: Contains alpha and gamma motor neurons, which control the contraction of skeletal muscles.

• Posterior horn: Receives axons from sensory neurons.

• Lateral horn (intermediate zone):

• Only present in certain areas:

• Cervical segments C8 to L2.

• Sacral segments S2 to S4.

• Lateral horn in cervical segments C8-L2: Contains neurons of the sympathetic ganglion.

• Lateral horn in sacral segments S2-S4: Contains neurons of the parasympathetic ganglion.

• White matter:

• Surrounds the gray matter.

• Divided into:

• Anterior column: Contains the anterior corticospinal tract.

• Lateral column: Contains the spinothalamic tract, lateral corticospinal tract, and spinocerebellar tract.

• Posterior column: Contains the gracile fasciculus and cuneate fasciculus.

III. Functions of the Spinal Cord:

• Conduction:

• Sensory conduction.

• Motor conduction.

• Reflexes:

• Spinal reflexes.

IV. Conduction Function:

1. Sensory Conduction:

• Tactile, pain, and temperature sensation:

• Transmitted through the spinothalamic tract.

• Conscious proprioception:

• Transmitted through the gracile fasciculus and cuneate fasciculus.

• Unconscious proprioception:

• Transmitted through the spinocerebellar tract.

a. Pathway for conscious proprioception:

• Pathway:

• Receptors in tendons, skeletal muscles, and joints.

• Posterior root ganglion neuron.

• Axon enters the posterior horn of the spinal cord.

• Ascends to the brainstem, synapsing with the second neuron in the brainstem.

• Crosses to the contralateral thalamus at the posterior ventral nucleus.

• From the thalamus, axons travel to the sensory cortex of the brain, specifically the parietal lobe.

• Posterior root ganglion neuron: Also known as the first-order neuron.

• Gracile fasciculus and cuneate fasciculus:

• Convey conscious proprioception: touch, two-point discrimination, vibration.

• Gracile fasciculus: Receives sensation from the lower part of the body (legs).

• Cuneate fasciculus: Receives sensation from the upper part of the body (arms).

b. Pathway for tactile and crude sensation:

• Pathway:

• Receptors in the skin.

• Posterior root ganglion neuron.

• Synapses with the second neuron in the spinal cord.

• Crosses to the contralateral gray matter one or two segments above the level of entry.

• Travels to the posterior ventral nucleus of the thalamus.

• Sensory cortex of the body.

c. Pathway for pain and temperature sensation:

• Pathway:

• Similar to the anterior spinothalamic tract.

• Receives:

• Sensory signals from the skin.

d. Decussation:

• Gracile fasciculus and cuneate fasciculus: Decussate in the brainstem.

• Anterior spinothalamic tract and lateral spinothalamic tract: Decussate in the gray matter of the spinal cord one or two segments above the level of entry.

e. Termination:

• Cuneate fasciculus, gracile fasciculus, anterior spinothalamic tract, lateral spinothalamic tract:

• All reach the posterior ventral nucleus of the thalamus.

f. Lesions:

• Lesion of the anterior spinothalamic tract: Loss of skin sensation one or two segments below the level of spinal cord injury.

• Lesion of the gracile fasciculus and cuneate fasciculus: Causes Tabes dorsalis, leading to loss of sensation of pressure, weight, and impaired coordination.

2. Motor Conduction:

• Pyramidal tract: Conducts voluntary movement.

• Extrapyramidal tract: Conducts involuntary movement.

a. Pyramidal tract:

• Includes:

• Corticospinal tract.

• Corticobulbar tract.

• Corticospinal tract:

• From the cerebral cortex to the spinal cord, controlling muscles in the trunk and limbs.

• Corticobulbar tract:

• From the cerebral cortex to the cranial nerve nuclei in the brainstem, controlling muscles in the head, face, and neck.

b. Extrapyramidal tract:

• Includes:

• Rubrospinal tract.

• Reticulospinal tract.

• Tectospinal tract.

• Vestibulospinal tract.

• Olivospinal tract.

• Function:

• Regulates muscle tone.

• Balance reflexes.

• Posture.

• Coordination.

c. Corticospinal tract:

• Originates: Motor cortex.

• Pathway:

• Group 1: Travels directly from the brainstem to the spinal cord, branching to motor neurons in the anterior horn bilaterally, forming the anterior corticospinal tract (direct pyramidal tract).

• Group 2: Decussates in the brainstem, traveling down to synapse with motor neurons in the anterior horn bilaterally, forming the lateral corticospinal tract (crossed pyramidal tract).

• Lateral corticospinal tract/ crossed pyramidal tract: Represents the majority.

d. Decussation:

• Corticospinal tract: Decussates in the brainstem.

• Rubrospinal tract, tectospinal tract: Decussate immediately after their origin, synapsing with neurons in the anterior horn of the contralateral spinal cord.

• Reticulospinal tract: Travels directly to the anterior column, reaching neurons in the same side of the anterior horn.

e. Termination:

• Corticospinal tract, rubrospinal tract, tectospinal tract, reticulospinal tract: Terminate in the anterior horn of the spinal cord.

f. Lesions:

• Lesions in the motor cortex: Reduced movement on the contralateral side of the body.

V. Spinal Reflexes:

• Characteristics:

• Follow general principles of reflexes.

• Specific laws:

• Ipsilateral law.

• Bilateral law.

• Irradiation law.

• Summation law.

• Reflex arc:

• Consists of 5 components:

• Sensory receptor.

• Afferent nerve (posterior root).

• Reflex center (spinal cord gray matter).

• Efferent nerve (anterior root).

• Effector organ.

VI. Classification of Spinal Reflexes:

• Two-neuron reflex (monosynaptic):

• Also known as a monosynaptic reflex.

• No presence of an interneuron.

• Three-neuron reflex (polysynaptic):

• Also known as a polysynaptic reflex.

• Presence of an interneuron.

VII. Basic Spinal Reflexes:

• Stretch reflex (Golgi tendon reflex):

• Receptors: Muscle spindle receptors, Golgi tendon receptors.

• Mechanism:

• Sudden stretching of a muscle.

• Receptor stimulation.

• Afferent fiber enters the posterior horn, reaches the anterior horn, and synapses with gamma motor neurons, causing muscle contraction.

• Purpose: Protect the body from injury.

• Flexor/withdrawal reflex:

• Mechanism:

• Stimulation of skin receptors.

• Afferent fiber enters the posterior horn, synapses with an interneuron, then synapses with motor neurons in the anterior horn, causing flexion of the limb, withdrawing it from the stimulus.

• Purpose: Withdraw from danger, avoid injury to the body.

• Crossed extensor reflex:

• Mechanism:

• Painful stimulation of a limb causes flexion of that limb.

• Afferent fiber enters the posterior horn, synapses with an interneuron, then synapses with motor neurons in the anterior horn, causing extension of the contralateral limb, helping to maintain balance.

• Purpose: Maintain balance, prevent falling.

• Cutaneous reflex:

• Mechanism:

• Stimulation of skin receptors.

• Afferent fiber enters the posterior horn, synapses with an interneuron, then synapses with motor neurons in the anterior horn, causing contraction of muscles in the stimulated area of skin.

• Purpose:

• Body protection.

• Neurological function assessment.

• Tonic reflex:

• Mechanism:

• Stimulation of skin, muscle, and joint receptors.

• Afferent fiber enters the posterior horn, synapses with an interneuron, then synapses with motor neurons in the anterior horn, maintaining muscle tone, assisting in balance.

• Purpose: Maintain posture, balance, and allow for flexible movement.

VIII. Autonomic Reflexes of the Spinal Cord:

• Autonomic reflexes without clear localization:

• Sweating.

• Goosebumps.

• Vasomotor reflexes.

• Autonomic reflexes with localization:

• Bladder reflex.

• Genital reflex.

• Blink reflex.

IX. Spinal Cord Injuries:

• Spinal shock:

• The spinal cord is transected at a specific segment.

• The area below the lesion becomes paralyzed, loses sensation, muscle tone, reflexes, and blood pressure drops rapidly.

• The duration of spinal shock is approximately 2-3 weeks.

X. Conclusion:

• The spinal cord plays a crucial role in conducting sensory and motor signals, reflexes, and controlling autonomic functions of the body.

• Examining spinal reflexes helps pinpoint the location of spinal cord injuries.

Note: This text has been rewritten based on the provided data, however, there may still be gaps or missing information.