Hoja de repaso: Spinal Cord Pathways and Reflexes

📋 Course Outline

1. Spinal cord funiculi locations and functions
2. Ascending spinal tracts and tract organization
3. Spinothalamic pathway for pain and temperature
4. Spinocervicothalamic conscious pathway
5. Motor systems and upper versus lower motor neurons
6. Descending extrapyramidal motor tracts
7. Corticospinal tract and pyramidal system
8. Spinal reflexes and reflex arc components
9. Patellar, withdrawal, panniculus and perineal reflexes

📖 1. Spinal cord funiculi locations and functions

🔑 Key Concepts & Definitions

• Funiculi : Funiculi are the spinal cord white-matter columns that divide into dorsal, lateral, and ventral regions.
• White matter tracts : White matter tracts are bundles of functionally related axons located within the spinal cord white matter.
• Ascending pathways : Ascending pathways carry sensory information from spinal cord receptors toward the brain.
• Descending pathways : Descending pathways convey motor commands from the brain to spinal cord neurons.

📝 Essential Points

• Tracts are bundles of functionally related axons within the CNS white matter.
• Spinal pathways use a sequence of neurons where excitability passes from one neuron to the next.
• Spinal pathways are classified as ascending (receptors → brain) or descending (brain → spinal cord).
• The spinal cord divides into dorsal, lateral, and ventral funiculus (columns).
• Tracts are named by their origin and destination/function, e.g., spinothalamic tract.
• Each funiculus region shares a common origin, destination, and function.

💡 Memory Hook

Dorsal = up (sensory), ventral = down (motor); lateral sits in between as the “side” column.

📖 2. Ascending spinal tracts and tract organization

🔑 Key Concepts & Definitions

• Dorsal funiculus : A spinal cord column that contains specific ascending sensory tracts in the dorsal part of the white matter.
• Lateral funiculus : A spinal cord column that contains specific ascending sensory tracts in the lateral part of the white matter.
• Ventral funiculus : A spinal cord column that contains specific ascending sensory tracts in the ventral part of the white matter.
• Spinothalamic tract : An ascending sensory pathway that carries pain and temperature information to the brain.
• Spinocerebellar tracts : Ascending pathways that convey sensory information to the cerebellum, organized into dorsal and ventral components.

📝 Essential Points

• Ascending tracts carry sensory information upward, while descending tracts carry motor commands downward.
• The white matter is divided into dorsal, lateral, and ventral funiculi (columns).
• Tracts are named by their origin and destination, such as spinothalamic tract.
• Ascending sensory pathways begin with a primary afferent neuron that terminates in the brain.
• Pain information is carried by the spinothalamic tract.

💡 Memory Hook

Ascending = sensory up; funiculi = dorsal/lateral/ventral columns; spinothalamic = pain/temperature to thalamus.

📖 3. Spinothalamic pathway for pain and temperature

🔑 Key Concepts & Definitions

• Spinothalamic pathway : A sensory pathway that carries pain and temperature signals from the spinal cord to the thalamus and then to the cerebral cortex.
• Third-order neuron : A neuron in the thalamus that receives spinothalamic input and relays it to specific cortical targets.
• Lateral thalamus : A thalamic region that receives spinothalamic projections linked to discriminative aspects of pain.
• Medial thalamus : A thalamic region that receives spinothalamic projections linked to motivational-affective aspects of pain.
• Somesthetic area of cerebral cortex : A cortical region that receives thalamic axons carrying somesthetic information including pain-related signals.

📝 Essential Points

• Thalamic projection neurons send axons to the somesthetic area of the cerebral cortex.
• Axons terminating in the lateral thalamus mediate discriminative aspects of pain.
• Axons terminating in the medial thalamus mediate motivational-affective aspects of pain.
• Motivational-affective pain includes emotional aspects and influences attention and memory of pain.
• Spinothalamic pathway organization uses thalamic termination targets to separate pain components.

💡 Memory Hook

Lateral thalamus = “discriminate”; medial thalamus = “mood/attention/memory” for pain.

📖 4. Spinocervicothalamic conscious pathway

🔑 Key Concepts & Definitions

• Spinocervicothalamic pathway : A conscious ascending pathway that carries sensory information from the spinal cord to higher brain centers via the cervical region and thalamus.
• Conscious sensory pathway : A sensory pathway that supports conscious perception rather than purely reflex or subconscious processing.
• Thalamus : A brain relay structure that receives sensory inputs and helps route them to cortical areas for conscious experience.
• Cervical relay : A cervical-level processing step where ascending sensory signals are relayed toward the thalamus.

📝 Essential Points

• The spinocervicothalamic pathway is an ascending route associated with conscious perception of sensory information.
• It conveys information from the spinal cord upward through the cervical region before reaching the thalamus.
• The thalamus functions as a relay for sensory signals on their way to cortical processing.
• This pathway is distinct from subconscious or reflex pathways that do not support conscious awareness.
• The pathway’s name reflects its route: spinal cord to cervical region to thalamus.

💡 Memory Hook

Think “S-C-T” = Spinal → Cervical → Thalamus for conscious sensation.

📖 5. Motor systems and upper versus lower motor neurons

🔑 Key Concepts & Definitions

• Rubrospinal tract : A descending motor pathway from the red nucleus that supports voluntary and skilled movement control.
• Corticospinal tract : A descending motor pathway originating in the cerebral cortex that drives voluntary skilled movements.
• Pyramidal system : The corticospinal pathway group named for axons passing through the medulla’s pyramid region.
• Extrapyramidal system : Descending motor pathways other than the pyramidal tract that contribute to movement control via multiple synapses.
• Upper motor neurons : Motor neurons in the brain that send descending commands to spinal or cranial motor circuits.

📝 Essential Points

• The red nucleus in the mesencephalon is the most significant source for the rubrospinal pathway.
• The corticospinal tract is mainly responsible for voluntary skilled movements requiring conscious thought and concentration.
• Corticospinal axons pass through the ventral medulla oblongata in the pyramid region, giving the pyramidal system its name.
• The extrapyramidal system is multisynaptic, shown by interruptions in its descending tract column.
• Spinal cord tracts link spinal nerves to the brain via ascending and descending pathways in spinal white matter.
• Spinal cord tracts also generate spinal reflexes through pre-determined interneuronal circuits that integrate reflex activity.

💡 Memory Hook

Pyramids = corticospinal (cortex → pyramid medulla); Extrapyramidal = extra synapses (interruptions).

📖 6. Descending extrapyramidal motor tracts

🔑 Key Concepts & Definitions

• Extrapyramidal motor tracts : Extrapyramidal motor tracts are descending pathways that influence lower motor neuron activity from the central nervous system.
• Upper motor neurons : Upper motor neurons are neurons whose cell bodies lie in the brain stem motor nuclei or motor cortex and whose axons stay within the CNS.
• Lower motor neurons : Lower motor neurons are neurons in the spinal ventral horn and somatic cranial motor nuclei that send axons to skeletal muscle.
• Interneuronal circuits : Interneuronal circuits are networks within the spinal cord that integrate reflex inputs and help shape reflex output.

📝 Essential Points

• Descending extrapyramidal tracts modulate lower motor neuron activity rather than directly projecting to muscle fibers.
• Upper motor neuron axons connect to lower motor neurons by direct synapses or via interneurons.
• Upper motor neurons can inhibit or facilitate lower motor neuron-driven activity.
• Lower motor neuron axons travel through cranial or spinal nerves to reach skeletal muscle and terminate on muscle fiber groups.
• Lower motor neurons provide the efferent limb for simple reflexes, while upper motor neurons mostly regulate reflex activity.
• Spinal reflexes are integrated by pre-determined interneuronal circuits that link stimulus to reflex output.

💡 Memory Hook

Extrapyramidal = CNS “managers” that tune LMN reflex output; LMN = “workers” that directly drive muscle contraction.

📖 7. Corticospinal tract and pyramidal system

🔑 Key Concepts & Definitions

• Upper motor neurons : Upper motor neurons are neurons located within the central nervous system that regulate lower motor neuron activity.
• Lower motor neurons : Lower motor neurons are neurons that directly drive lower motor outputs to muscles and other target effectors.
• Upper motor disease signs : Upper motor disease signs are clinical findings that reflect impaired upper motor neuron control over lower motor neurons.
• Lower motor disease signs : Lower motor disease signs are clinical findings that reflect impaired lower motor neuron function at the level of the effector.

📝 Essential Points

• Upper motor neuron lesions are confined to the central nervous system and therefore produce different clinical signs than lower motor neuron lesions.
• Upper motor disease signs differ from lower motor neuron disease signs because the level of the lesion changes the pattern of downstream motor effects.
• A reflex pathway includes a sensory input, a connection within the CNS, and then motor output to the neuromuscular junction and muscle.
• Reflexes can be somatic or autonomic and may involve cranial or spinal nerves and either striated or smooth muscle.
• Reflexes use exteroceptors, interoceptors, or proprioceptors as sensory sources.

💡 Memory Hook

UMN = CNS control; LMN = direct output—different lesion level → different sign pattern.

📖 8. Spinal reflexes and reflex arc components

🔑 Key Concepts & Definitions

• Myotatic reflex : A spinal reflex where muscle stretch triggers a reflex contraction that resists the stretch.
• Patellar reflex : A myotatic (muscle stretch) reflex elicited by tapping the patellar tendon, used to assess pelvic limb reflex function.
• Reflex arc : The pathway that carries a reflex stimulus from sensory input through the spinal cord to motor output.
• Withdrawal (flexor) reflex : A spinal reflex in which a limb flexes to withdraw from a noxious stimulus.
• Panniculus reflex : A cutaneous reflex where pricking the skin causes contraction of the cutaneous trunci (panniculus) muscle.

📝 Essential Points

• Muscle stretch is resisted by reflex contraction of the stretched muscle.
• The patellar reflex is the most reliable pelvic limb reflex.
• In the patellar reflex, the stimulus on the tendon travels via an afferent neuron to the spinal cord.
• In the patellar reflex, the impulse is transmitted to an efferent neuron that stimulates the quadriceps muscle.
• The motor component of the myotatic reflex arises from L4–L6 spinal cord segments, the origin of the femoral nerve.
• The femoral nerve innervates the quadriceps as the effector muscle for the patellar reflex and also causes reciprocal inhibition simultaneously.

💡 Memory Hook

Stretch → L4–L6 → Femoral → Quadriceps contracts (and reciprocal inhibition).

📖 9. Patellar, withdrawal, panniculus and perineal reflexes

🔑 Key Concepts & Definitions

• Panniculus reflex : A cutaneous reflex in which pricking the skin triggers bilateral contraction of the cutaneous trunci (panniculus) muscles.
• Perineal reflex : A reflex in which mild compression of the perineal or anal skin causes anal sphincter contraction and tail flexion.
• Cutaneous trunci muscles : The panniculus muscles whose contraction produces the visible skin “twitch” during the panniculus reflex.
• Pudendal nerve : The peripheral nerve involved in the perineal reflex pathway to the anal sphincter and related responses.

📝 Essential Points

• In the panniculus reflex, skin sensory input ascends bilaterally to the C8–T1 spinal cord segments.
• In the panniculus reflex, the sensory pathway synapses with the lateral thoracic nerve at the C8–T1 level.
• Pricking the skin over the panniculus region triggers bilateral contraction of the cutaneous trunci muscles.
• In the perineal reflex, mild forceps compression of perineal or anal skin causes anal sphincter contraction and tail flexion.
• The perineal reflex evaluates S1–S3 spinal nerves and, peripherally, the pudendal nerve.

💡 Memory Hook

Panniculus = C8–T1 skin prick → bilateral cutaneous trunci; Perineal = S1–S3 perineum pinch → anal squeeze + tail flex.

📊 Synthesis Tables

Ascending vs descending spinal pathways

Lateral vs medial thalamus in spinothalamic pain

⚠️ Common Pitfalls & Confusions

1. Mixing up pathway direction: ascending pathways carry information from receptors to the brain, while descending pathways convey information from the brain to spinal cord neurons.
2. Confusing funiculi with tracts: funiculi are dorsal/lateral/ventral white-matter columns, whereas tracts are bundles of functionally related axons within white matter.
3. Assuming spinothalamic “first-order” neurons terminate in the thalamus; the source states first-order neurons terminate in the brain, while the contralateral thalamus is reached via the spinothalamic tract with a third-/
4. Thinking the spinocervicothalamic pathway is the same as spinothalamic; the source distinguishes them by content (superficial pain/tactile vs pain) and by being the primary conscious pathway in carnivores.
5. Forgetting that UMNs are entirely within the CNS and do not project directly to muscle fibers, whereas LMNs project via cranial/spinal nerves to skeletal muscle.
6. Misidentifying the patellar reflex effector: the source specifies quadriceps as the effector muscle, and that the motor component arises from L4–L6 (femoral nerve).
7. Confusing reflex evaluation segments: patellar reflex tests L4–L6 and femoral nerve, while withdrawal reflex tests L6–L7–S1 (sciatic nerve) and panniculus/perineal reflexes test C8–T1 and S1–S3 respectively.

✅ Exam Checklist

1. State what funiculi are (dorsal, lateral, ventral) and what they share (common origin, destination, and function).
2. Define a tract as a bundle of functionally related axons in CNS white matter and explain how spinal pathways use a sequence of neurons with transmitted excitability.
3. Classify spinal pathways as ascending or descending and state the direction and general content of each (sensory vs motor).
4. Name how tracts are named (by origin and destination/function) and give the example naming style such as spinothalamic tract.
5. Describe the organization of ascending sensory pathways: primary afferent neuron begins and terminates in the brain.
6. Identify which tract carries pain and temperature information (spinothalamic tract) and state that it begins with a primary afferent (first-order) neuron.
7. Explain the spinothalamic pathway pain relay logic: primary afferent to spinal dorsal horn, second-order axons cross and form the spinothalamic tract, and thalamic projection neurons send axons to the somesthetic area of
8. Differentiate lateral vs medial thalamus roles in spinothalamic pain (discriminative vs motivational-affective/emotional attention and memory).
9. Identify the spinocervicothalamic pathway as a conscious ascending pathway, and state its key features from the source (superficial pain and tactile sensations; primary conscious pathway in carnivores).
10. Describe the spinocervicothalamic pathway neuron sequence and key nuclei/levels: first-order spinal ganglion, second-order marginal nucleus/nucleus proprius, third-order lateral cervical nucleus (C1–2), crossing to contr
11. Define upper motor neurons and lower motor neurons using the source criteria (UMN in CNS; LMN in ventral horn/somatic cranial motor nuclei) and state how UMNs connect to LMNs (directly or via interneurons).
12. For reflexes, list the five basic components (sensory receptor, sensory neuron, CNS synapse, motor neuron, target organ/effector) and then describe the patellar reflex pathway steps and the spinal segments/nerve involved
13. For spinal reflex examples, state what each reflex does and what it evaluates: withdrawal (flexion; sciatic nerve and L6–L7–S1), panniculus (bilateral C8–T1 to lateral thoracic nerve; cutaneous trunci), and perineal (S1–
14. Explain how higher brain centers can override spinal reflexes by sending impulses down descending pathways to efferent neurons to prevent muscle contraction despite harmful stimuli.