Central Nervous System Processing & Reflex Arcs

Central Nervous System (CNS)

• Anatomical limits:
  • Brain + spinal cord only (boxed region on diagram).
  • Contains interneurons for information processing.
• Functional role:
  • Integrates (processes) all incoming sensory information.
  • Generates outgoing motor commands.
  • Analogy: computer “central processing unit” (CPU)—runs constant background tasks.
• Conscious vs. unconscious processing:
  • Conscious thought → cerebrum (higher brain).
  • Unconscious/automatic processing can occur in spinal cord or brain-stem (still part of CNS).

What Is a Reflex?

• Common misconception: “Reflex = involuntary.”
• Preferred definition: Processing occurs below the level of conscious awareness (below cerebrum).
• Key idea: Effector may be skeletal muscle (which is normally voluntary), yet the action is still a reflex because cerebrum is bypassed.
• Everyday examples:
  • Touching a hot stove → instant withdrawal before you "think" about heat.
  • Doctor taps patellar tendon → leg kicks without conscious intent.

Classic 5-Step Reflex Arc

1. Stimulus at a receptor (e.g., heat, stretch, pressure).
2. Sensory/Afferent neuron carries impulse into CNS.
3. Integration/processing inside CNS (may be one or many synapses).
4. Motor/Efferent neuron exits CNS.
5. Effector receives command (skeletal muscle, smooth muscle, cardiac muscle, or gland).

Types of Reflexes (by Site of Integration & Effector)

Important comparative notes:

• Somatic = skeletal muscle; can be activated reflexively even though normally “voluntary.”
• Visceral/Autonomic = cardiac, smooth muscle, glands; maintains homeostasis (BP, heart rate, breathing, etc.).

Key Vocabulary & Concepts

• Monosynaptic: sensory neuron synapses directly with motor neuron (→ one synapse).
• Polysynaptic: ≥2 synapses; involves at least one interneuron.
• Ipsilateral: sensory and motor activity occur on same side of spinal cord.
• Contralateral: response generated on opposite side from stimulus.
• Reciprocal innervation: simultaneous excitation of one muscle group & inhibition of the antagonist (e.g., hamstrings flex while quadriceps relax).
• Baroreceptor: pressure receptor that detects arterial wall stretch (blood pressure).

Spinal Reflex Examples

1. Stretch Reflex (Patellar/Knee-Jerk)

• Location in text: p. 573 (labelling figure), similar to figure p. 571.
• Receptor: Muscle spindle embedded in quadriceps.
• Mechanism:
  1. Tap patellar tendon → tendon/quad briefly stretches.
  2. Spindle fires → sensory impulse enters dorsal horn.
  3. Monosynaptic pathway: sensory neuron → motor neuron.
  4. Motor neuron exits ventral root → quadriceps contracts.
  5. Lower leg “kicks.”
• Features:
  • $\text{1 synapse} \Rightarrow \text{fast response}$.
  • Also inhibits antagonistic hamstrings (reciprocal innervation).

2. Flexor & Crossed-Extensor Reflex (Withdrawal Reflex)

• Location: p. 575.
• Scenario: Step on sharp object while walking on beach.
• Two coordinated parts:
  • Flexor/Withdrawal (ipsilateral): injured leg flexes (hamstrings contract, quadriceps inhibited) to pull foot away.
  • Crossed-Extensor (contralateral): opposite leg extends to support body weight and maintain balance.
• Pathway characteristics:
  • Polysynaptic—multiple interneurons involved.
  • Includes reciprocal innervation on both sides.
• Sensory awareness: cerebrum receives the “pain” message only after spinal cord has already executed protective movements.

Visceral (Cranial) Reflex Example — Baroreceptor Blood-Pressure Reflex

• Receptors: Baroreceptors in carotid sinus & aortic arch measure arterial stretch.
• Pathway:
  1. Stand up quickly → $BP \downarrow$ in upper body.
  2. Baroreceptors send afferent signals via glossopharyngeal & vagus nerves to medulla oblongata.
  3. Medulla integrates info (brain-stem reflex center).
  4. Efferent sympathetic output → peripheral vasoconstriction + ↑ heart rate.
  5. Result: $BP \uparrow$, preventing fainting.
• Significance: operates continuously without conscious thought; failure leads to dizziness or syncope.

Connections to Previous & Future Topics

• Higher vs. lower brain centers: builds on earlier lectures distinguishing cerebrum (conscious) from diencephalon, brain-stem, spinal cord (unconscious control).
• Somatic vs. Autonomic pathways: dovetails with autonomic nervous system chapters (sympathetic vs. parasympathetic).
• Homeostasis theme: reflex arcs exemplify negative feedback loops covered in physiology basics.
• Clinical relevance:
  • Absence of patellar reflex may indicate spinal cord (L2–L4) damage.
  • Exaggerated reflexes can signal upper motor neuron lesions.
  • Orthostatic hypotension relates to baroreceptor reflex impairment.

Exam Pointers & Frequently Tested Details

• Be able to list the 5 components of a reflex arc in correct order.
• Know integration center & effectors for:
  • Spinal (somatic) vs. cranial (visceral) reflexes.
• “Involuntary” is not the best descriptor—emphasize “below conscious (cerebral) processing.”
• Cerebrum is never part of a basic reflex arc (possible multiple-choice distractor).
• Terminology: ipsilateral / contralateral, monosynaptic / polysynaptic, reciprocal innervation.
• Real-life examples reinforce understanding and may appear as applied questions (hot stove, tendon tap, stepping on nail, standing up quickly).