318 Lect. #3 - Spinal Reflexes, Muscle Spindle Physiology & Spinal Tracts

Objectives

• Describe the basic components of a reflex arc
• Differentiate types of reflex arcs (mono- vs. polysynaptic, ipsilateral vs. contralateral, etc.)
• Explain the functional anatomy of a muscle spindle
• Trace how/where signals are sent to and from the CNS
  • Ascending (sensory) and descending (motor) spinal tracts

Spinal Cord: Core Functions

• Conduction
  • “Up” pathway = sensory information → brain
  • “Down” pathway = motor commands → effectors
• Neural integration
  • Networks in gray matter integrate multiple inputs; e.g., visceral control of the bladder
• Locomotion
  • Central pattern generators in the cord automate repetitive sequences (walking rhythm)
• Reflexes
  • Provide postural, co-ordination, and protective responses without cortical delay

General Reflex Physiology

• Spinal reflex arc = rapid, involuntary, stereotyped response
• Advantages
  • Fast (few synapses, local circuitry)
  • Involuntary but can be modulated by supraspinal centers
  • Require stimulation (no spontaneous firing)
  • Highly predictable—important for neurological exams
• Basic components (in order of information flow)
  1. Sensory receptor (e.g., nociceptor in skin)
  2. Sensory neuron (cell body in dorsal root ganglion)
  3. Integration center (synapse(s) in gray matter)
  4. Motor neuron (cell body in ventral horn; axon out ventral root)
  5. Effector organ (skeletal muscle, gland)
• Arc variants
  • Monosynaptic: one synapse, e.g., patellar stretch reflex
  • Polysynaptic: ≥1 interneuron; enables reciprocal inhibition, crossed extension, etc.
  • Ipsilateral vs. contralateral vs. intersegmental distribution

Muscle Spindle: In-Depth

• Definition: Fusiform stretch receptor embedded parallel to regular (extrafusal) muscle fibers
• Density is highest in muscles performing fine motor control (e.g., extraocular, intrinsic hand muscles)
• Proprioceptive role: conveys continual info on muscle length & rate of change to CNS

Structural Elements

• Intrafusal fibers (modified muscle fibers)
  • Contractile only at polar ends; middle region non-contractile & wrapped by sensory endings
• Gamma ((\gamma)) motor neurons
  • Adjust spindle tension so receptor doesn’t fall slack during overall muscle shortening; maintains sensitivity
• Primary (Group Ia) sensory endings
  • Annulospiral; encode length AND velocity (especially sudden stretch)
• Secondary (Group II) sensory endings
  • Flower-spray; encode static length
• Alpha ((\alpha)) motor neurons
  • Innervate extrafusal fibers → force generation

Functional Output

• Stretch reflexes supply constant, subconscious postural tone
• Aid in movement coordination by opposing undesired stretch, smoothing actions
• Support equilibrium in tandem with vestibular and visual input
• Work via reciprocal inhibition: activation of agonist simultaneous with inhibition of antagonist
• Classify surrounding muscles:
  • Synergists aid prime mover; antagonists oppose
  • Flexors ↓ joint angle, extensors ↑ joint angle

Key Stretch Reflex Characteristics

• Mediated by brain (cerebellum & motor cortex set (\gamma) activity) yet spinal component executes the rapid contraction
• Stronger when stretch is sudden
• Absolutely dependent on functional reciprocal inhibition circuitry

Flexor (Withdrawal) Reflex & Crossed Extension

• Triggered by injurious stimulus (e.g., stepping on glass)
• Polysynaptic: interneurons route divergent signals
• Same-side (ipsilateral) limb → flexor contraction, lifting limb away
• Opposite limb receives crossed extension reflex
  • Contralateral extensor muscles contract for balance support

Contralateral Reflex Arc Steps

1. Nociceptor → sensory neuron → dorsal horn
2. Multiple interneurons generate EPSPs & IPSPs
3. Ipsilateral motor neurons excite flexors & inhibit extensors
4. Contralateral motor neurons excite extensors & inhibit flexors

Developmentally Dynamic (Infant) Reflexes

• Present early; disappear when cortical inhibition matures
  • Palmar grasp: gone by $5\text{–}6\ \text{months}$
  • Moro (startle): gone by $\approx3\text{–}4\ \text{months}$
  • Rooting: gone by $\approx4\ \text{months}$
  • Sucking, Stepping, Plantar/Babinski, Parachute (appears $6\text{–}7\ \text{months}$)
• In adults, re-emergence as frontal release signs indicates frontal-lobe pathology

Ascending (Sensory) Spinal Tracts

• Carry information upward through a chain of $3$ neurons:
  1. First-order: receptor → spinal cord or brainstem
  2. Second-order: cord/brainstem → thalamus (major relay/gate)
  3. Third-order: thalamus → cerebral cortex (primary sensory areas)
• Major modalities
  1. Proprioception (leg/trunk & arm) → dorsal columns (gracile/cuneate fasciculi)
  2. Muscle feedback for balance → spinocerebellar tracts
  3. Light touch, pain, temperature, pressure → anterolateral system (spinothalamic, spinoreticular)

Decussation Sites Example

• Cuneate fasciculus: crosses in medulla
• Spinothalamic: crosses in spinal cord segment
• Posterior spinocerebellar: does not decussate (ipsilateral to cerebellum)

Descending (Motor) Spinal Tracts

• Deliver motor commands from cortex/brainstem → (\alpha) & (\gamma) motor neurons
• Typical organization = $2$ neurons (upper- and lower-motor)
• Functional groupings
  1. Fine limb movement: corticospinal (lateral & anterior)
  2. & 4. Limb/posture control: reticulospinal (med & lat)
  3. Head-turning reflex: tectospinal (from superior colliculus)
  4. Balance/anti-tilt: vestibulospinal (medial & lateral)

Decussation & Origin Quick Chart

• Lateral corticospinal: originates cortex; crosses in medulla
• Anterior corticospinal: cortex; crosses in spinal cord
• Tectospinal: superior colliculus (midbrain); crosses immediately
• Reticulospinal: reticular formation; no decussation (mostly ipsilateral)
• Medial vestibulospinal: vestibular nuclei; partial decussation in medulla
• Lateral vestibulospinal: vestibular nuclei; no crossing

Clinical Correlation & “Stop-and-Think” Scenarios

• Case: Post-MVA patient lacks position sense in left arm/hand
  • Likely damage to cuneate fasciculus (dorsal column) on left side above decussation or right medial lemniscus/thalamic relay after crossing
  • Cortex not receiving = right post-central gyrus (primary somatosensory)
• Temperature sensation pathway
  • Lateral spinothalamic tract of the anterolateral system
• Motor command for withdrawing hand from flame
  • Lateral corticospinal tract (fine distal limb movement)

Review Questions (self-test)

• Purpose of (\gamma) motor fibers?
  • Maintain spindle tautness & sensitivity during active contraction, preventing “slack.”
• Sequential components of a reflex arc?
  • Stimulus → receptor → sensory neuron → integration center → motor neuron → effector.
• Why some reflexes disappear with age?
  • Maturation of cortical (particularly frontal) inhibition suppresses primitive brainstem/cord circuits.

Practical / Ethical Considerations

• Reflex testing (e.g., Babinski) is a non-invasive diagnostic of myelin integrity & cortical control; essential in neonatal & neurological exams.
• Understanding tract organization informs localization of spinal cord injuries, guiding prognosis & therapy.