CNS & Reflex Arc Essentials

Consists solely of brain and spinal cord.
Primary role: integrate incoming sensory information and generate outgoing motor commands.
Functions analogously to a computer’s "central processing unit".

A reflex = rapid, integrated response that occurs below conscious (cerebral) level.
Sequence of events (in words to avoid numbers):
• Stimulus detected by a sensory receptor.
• Afferent (sensory) neuron conveys impulse into CNS.
• Integration occurs within CNS (may be mono- or polysynaptic).
• Efferent (motor) neuron carries command outward.
• Effector (muscle or gland) performs the response.

Integration site: spinal cord gray matter.
Effector: skeletal muscle (voluntary muscle acting without conscious thought).
Examples discussed:
• Stretch (knee-jerk) reflex – monosynaptic link from muscle spindle to motor neuron; tendon tap stretches quadriceps → immediate contraction.
• Flexor / withdrawal reflex – painful stimulus in a limb → ipsilateral flexor activation.
• Crossed-extensor component – simultaneously activates contralateral extensors to maintain balance.
Phenomenon of reciprocal innervation: agonist contracts while antagonist is inhibited during the reflex.

Integration site: brain-stem nuclei (medulla, pons, midbrain).
Effectors: cardiac muscle, smooth muscle, glands.
Classic example: baroreceptor reflex — drop in arterial pressure sensed by stretch receptors; medulla triggers vasoconstriction and heart-rate change to restore pressure.

Afferent = sensory toward CNS; Efferent = motor away from CNS.
Ipsilateral: same side; Contralateral: opposite side.
Monosynaptic: single synapse (sensory → motor).
Polysynaptic: one or more interneurons present.
Reflex ≠ "involuntary" by definition; it simply bypasses conscious cerebral processing.