12.1 Nervous System Physiology

12.1 Nervous System Physiology

Quick Answer: Nervous physiology explains how the wired system works: reflex arcs, sensory (afferent) vs motor (efferent) pathways, sympathetic vs parasympathetic organ effects, neurotransmitters (especially acetylcholine and norepinephrine), brain region functions (cerebrum, cerebellum, brainstem), and myelin speeding conduction. Anatomy named the parts; this section turns them on for NEX Human Physiology (~20% of scored Science).

Nerve signals move in milliseconds, making the nervous system the body’s fastest coordinator of sensation, movement, and short-term internal adjustments. Nursing entrance items often ask which pathway is sensory, what a reflex includes, how fight-or-flight changes the heart or pupils, or which brain region handles cognition versus vital signs. Build from pathway direction → reflex logic → autonomic effects → brain jobs → myelin speed.

Sensory vs Motor Pathways

Direction relative to the central nervous system (CNS) is the first sorting rule.

Mixed nerves contain both sensory and motor axons, but individual fibers are one or the other. In the spinal cord region, dorsal roots are sensory and ventral roots are motor—physiology inherits that anatomy.

Somatic vs Autonomic Motor Outflow

Autonomic motor pathways typically use a two-neuron chain (preganglionic → ganglion → postganglionic). Somatic motor pathways use a single neuron from CNS to skeletal muscle. That structural difference explains why autonomic drugs and injuries often act at ganglia or long effector nerves differently from skeletal paralysis.

Reflex Arcs: Fast Automatic Circuits

A reflex is a rapid, predictable, involuntary response to a stimulus. The anatomical–physiological unit is the reflex arc.

Example — stretch (knee-jerk) reflex: tapping the patellar tendon stretches the quadriceps → muscle spindle fires → sensory neuron → synapse on motor neuron in spinal cord → quadriceps contracts → leg extends. The brain may be informed, but the arc itself does not require conscious decision-making.

Example — withdrawal reflex: stepping on a sharp object activates pain receptors → sensory neuron → interneurons in the cord → motor neurons flex the limb away. Reciprocal inhibition often relaxes opposing muscles at the same time—an integrated spinal pattern.

Reflexes can be somatic (skeletal muscle) or autonomic (e.g., pupil light reflex, baroreceptor adjustments). Intro exams emphasize the five-part arc and that many spinal reflexes work without waiting for the cerebrum to “decide.”

Autonomic Divisions: Fight-or-Flight vs Rest-and-Digest

The ANS has two major divisions with complementary functional patterns.

Both divisions are active to some degree most of the time (autonomic tone); the balance shifts with need. Many organs receive dual innervation with opposite effects—exam stems love pairing an organ with the correct division’s direction of change.

Functional Story, Not Just Labels

In a “fight or flight” scenario, sympathetic activation prepares the body to act: faster heart delivery of oxygenated blood, open airways for airflow, less energy spent on digestion, and dilated pupils for a wider visual field. In “rest and digest,” parasympathetic dominance slows the heart, supports GI processing of a meal, and constricts pupils in bright, calm settings. Memorize the organ table, then attach the story so application items are easier.

Neurotransmitters: Acetylcholine and Norepinephrine

At chemical synapses, the presynaptic neuron releases a neurotransmitter into the cleft; the postsynaptic cell responds if it has matching receptors.

Epinephrine (adrenaline) is mainly a hormone from the adrenal medulla (see endocrine physiology) but acts on similar receptors as NE and amplifies fight-or-flight. For nervous physiology, lock: ACh at the neuromuscular junction and NE as the typical sympathetic effector transmitter.

Receptors determine the effect: the same messenger can excite or inhibit depending on receptor type (e.g., different adrenergic receptor subtypes on heart vs bronchi). Intro NEX needs the messenger names and division pairing more than receptor subtype lists.

Brain Region Functions (Physiology Labels)

Anatomy placed the regions; physiology assigns jobs at overview depth.

Clinical anchors: cortical stroke may impair speech, sensation, or voluntary movement on the opposite body side; cerebellar injury yields ataxia (uncoordinated gait); brainstem injury can threaten breathing and cardiovascular stability because vital centers live there.

The hypothalamus (diencephalon) links nervous and endocrine control—appetite, temperature, and pituitary regulation—bridging this chapter to endocrine physiology. Thalamic relays and limbic emotion/memory circuits appear in deeper courses; NEX prioritizes cerebrum–cerebellum–brainstem contrasts.

Myelin and Conduction Speed

Myelin is a lipid-rich insulating sheath wrapped around many axons (Schwann cells in the PNS; oligodendrocytes in the CNS). Gaps called nodes of Ranvier expose membrane between segments.

Myelin lets urgent signals—motor commands to limbs, some sensory lines—arrive quickly. Demyelinating disease slows or blocks conduction, producing weakness, sensory loss, or incoordination—clinical motivation for knowing why myelin matters, without needing membrane-equation physics.

Action potential (concept only): a brief, all-or-none electrical change travels along the axon when threshold is reached at the trigger zone. Synapses convert that electrical arrival into chemical release. Chapter 10 covers contraction and signaling basics; here, emphasize pathway direction, reflexes, autonomic effects, and myelin speed as physiology applied to whole-system behavior.

Putting Pathways Together

1. Receptor detects stimulus → sensory neuron → CNS.
2. CNS may process consciously (cerebrum) or automatically (reflex center, brainstem).
3. Motor output leaves via somatic or autonomic neurons.
4. Autonomic balance (sympathetic vs parasympathetic) sets organ tone using ACh/NE patterns.
5. Myelinated pathways deliver time-critical commands faster.

Clinical and Nursing Anchors

• Reflex testing (patellar, plantar, pupil light) checks sensory–integration–motor integrity.
• Sympathetic surge in acute stress: tachycardia, dilated pupils, pale cool skin from redistributed blood flow.
• Parasympathetic excess or drugs can slow heart rate and increase GI activity/secretions.
• Spinal cord injury may interrupt ascending sensory and descending motor tracts below the level of injury.
• Neuromuscular junction failure (e.g., myasthenia-related weakness) is an ACh-receptor problem at skeletal muscle.

Exam Traps

• Afferent = sensory (in); efferent = motor (out) — do not reverse.
• Reflexes can occur without conscious thought; that does not mean the brain never receives information.
• Sympathetic dilates pupils and bronchi and raises heart rate; parasympathetic does the opposite for those effectors.
• ACh is not “only parasympathetic”—it also drives skeletal muscle and all ANS preganglionic synapses.
• Cerebellum coordinates; cerebrum cognates/initiates voluntary plans; brainstem runs vitals.
• Myelin speeds conduction; it does not create the neurotransmitter.

Study Map for NEX

1. Draw a five-part reflex arc and label each step once without notes.
2. Recite the sympathetic vs parasympathetic table for heart, bronchi, GI, and pupils.
3. Pair ACh and NE with at least one correct synapse each.
4. Give one sentence each for cerebrum, cerebellum, and brainstem function.
5. Explain saltatory conduction in one sentence linking myelin and nodes of Ranvier.

Mastering these functional rules lets anatomy from Chapter 8 “run” as physiology—and prepares endocrine contrast: nerves are fast and brief; hormones are slower and longer-lasting.