Exam 3: Lesson 12

What are the three fundamental steps of nervous system function?

• Sensory input: Receptors detect stimuli and send information to the CNS.

• Integration: CNS processes and interprets input to determine a response.

• Motor output: CNS sends commands through motor neurons to effectors (muscles or glands) to carry out the response.

How do the nervous and endocrine systems maintain internal coordination differently?

• Endocrine: Communicates via hormones in the bloodstream—slower and longer lasting.

• Nervous: Communicates via electrical impulses and neurotransmitters—rapid and specific.
  Both maintain homeostasis, but the nervous system allows immediate response and fine control.

Central Nervous System

Brain and spinal cord; integration and control center

Peripheral Nervous System

All nerves and ganglia; connects CNS to organs and tissues

Nerve detailed

bundle of axons (nerve fibers) wrapped in connective tissue in the PNS

Ganglion

Knot-like swelling in a neve containing neurons cell bodies in PNS

Sensory (afferent) division of PNS

Carries information TO CNS.

• Somatic sensory: From skin, muscles, bones, joints.

• Visceral sensory: From organs (heart, lungs, stomach, bladder).

Motor (efferent) division of PNS

Carries information FROM CNS.

• Somatic motor: To skeletal muscles (voluntary + reflexes).

• Visceral motor (autonomic): To glands, cardiac and smooth muscle (involuntary).

What are the three subdivisions of the autonomic nervous system (ANS)?

• Sympathetic: “Fight or flight” – increases alertness, heart rate, respiration, etc.

• Parasympathetic: “Rest and digest” – slows heart, stimulates digestion.

• Enteric: Network in digestive tract wall that coordinates motility and secretion independent of CNS control.

Tract

bundle of axons in the CNS

Nerve

bundle of axons in the PNS

Nucleus

cluster of cell bodies in the CNS

Ganglion

cluster of cell bodies in the PNS

What are the three universal properties of neurons?

• Excitability (irritability): Ability to respond to stimuli.

• Conductivity: Ability to transmit electrical signals along the membrane.

• Secretion: Ability to release neurotransmitters when the signal reaches the axon terminal.

What are the three functional classes of neurons?

• Interneurons: Process and integrate information within CNS (90% of neurons).

• Sensory (afferent) neurons: Detect stimuli and send signals to CNS.

• Motor (efferent) neurons: Send commands from CNS to effectors (muscles or glands).

Which type of neuron is most abundant in the human body and why?

Interneurons, because they link sensory and motor pathways and perform all the information processing in the CNS.

Describe the structure and main features of the neuron’s cell body (soma)

• Contains the nucleus and organelles.

• Cytoskeleton: microtubules + neurofibrils.

• Nissl bodies (chromatophilic substance): rough ER + ribosomes; indicate high protein synthesis.

• No centrioles → no mitosis after adolescence (mature neurons are in G₀).

• Exceptions: Neural stem cells in olfactory epithelium and hippocampus can divide.

What is the axon (nerve fiber) and its structural regions?

• Long, cylindrical projection specialized for rapid conduction.

• Originates at axon hillock, ends in terminal arborization.

• Each terminal branch ends in a terminal bouton (axon terminal) that forms a synapse with the next cell.

• Membrane: axolemma; cytoplasm: axoplasm.

• A neuron has only one axon, but it may branch into collaterals.

Multipolar neuron

one axon, many dendrites (most common; motor neurons, interneurons)

Bipolar Neurons

one axon, one dendrite (retina, olfactory mucosa)

Unipolar Neuron

single process dividing into peripheral and central branches (sensory neurons).

Anaxonic Neurons

many dendrites, no axon (brain, retina, adrenal medulla); do not generate action potentials

What is axonal transport, and why is it necessary?

Movement of materials between the soma and axon terminals because proteins and organelles are made in the soma and must be delivered to the axon.

Anterograde transport

Soma→ terminal (via kinesin)

Retrograde transport

Terminal → soma (via dynein)

Clinical note: Rabies and herpes viruses use retrograde transport to invade the CNS.

General functions of glial cells

Support, insulate, and protect neurons, maintain homeostasis; form myelin; guide developing neurons, prevent synaptic “cross talk”

Name and describe the four types of glia in the CNS.

• Oligodendrocytes: Form myelin sheaths in CNS.

• Ependymal cells: Line ventricles and central canal; secrete and circulate cerebrospinal fluid (CSF).

• Microglia: Phagocytic macrophages that remove debris and pathogens.

• Astrocytes: Most abundant; form blood–brain barrier, provide nutrients (lactate), regulate ions, secrete growth factors, and form scar tissue (astrocytosis/sclerosis) after injury.

Name and describe the two types of glia in the PNS

• Schwann cells (neurolemmocytes): Form myelin sheath around axons; assist in regeneration after injury.

• Satellite cells: Surround soma in ganglia; regulate chemical environment and provide insulation.

Why can glial cells form brain tumors but neurons cannot?

Mature neurons are amitotic (cannot divide), but glial cells retain mitotic ability, so gliomas (glial cell tumors) can arise and grow rapidly.

What is myelin and what is its function?

A multilayered insulating sheath composed of 80% lipid and 20% protein that increases conduction speed and electrical efficiency of nerve fibers.

Myelination in the PNS

Schwann cell wraps around one segment of a single axon; outermost layer = neurilemma (contains nucleus).

Myelination in the CNS

Oligodendrocyte extends processes to multiple axons; myelin layers added inward.

Nodes of Ranvier

Gaps between myelinated segments where ion exchange occurs.

Internodes

Myelinated segments between nodes of ranvier.

Trigger zone

Axon hillock + initial segment where action potential is generated.

Describe the cause and symptoms of multiple sclerosis (MS). (disease of myelin sheath)

Autoimmune destruction of CNS myelin → scar tissue replaces myelin → disrupted nerve conduction → symptoms like double vision, tremors, numbness, speech defects. Usually develops between ages 20–40; progressive and often fatal within decades.

Describe the mechanism of Tay–Sachs disease.

(usually in jewish ancestry) Inherited lysosomal enzyme deficiency → causes accumulation of GM₂ ganglioside in myelin → disrupted conduction of nerve signals.

symptoms: blindness, loss of coordination, dementia; fatal before age 4.

How are unmyelinated axons organized in the PNS?

Multiple small axons lie in surface grooves of a Schwann cell; the cell wraps once around each (not multiple times) and shares one neurilemma.

Under what conditions can a damaged nerve fiber regenerate?

Only if the soma is intact and part of the neurilemma remains (so this occurs only in the PNS, not CNS).

Outline the steps of PNS nerve regeneration.

• Degeneration: Distal axon and myelin disintegrate; macrophages clean debris.

• Soma reaction: Cell body swells; nucleus moves off-center.

• Sprouting: Axon stump forms growth cones.

• Regeneration tube: Schwann cells + endoneurium + basal lamina form path.

• Reconnection: Axon grows through tube to target cell and reestablishes synapses.

• Restoration: Soma returns to normal; target muscle regains function.

Why can’t CNS axons regenerate?

Because oligodendrocytes and astrocytes form scar tissue and release inhibitory molecules that block axon regrowth. CNS lacks neurilemma and supportive environment for regeneration.

How long does PNS nerve regeneration take and is it perfect?

It is slow (up to 2 years) and often incomplete—some fibers die or reconnect incorrectly.

Flow of info through the nervous

Stimulus → Sensory receptor → Sensory neuron → CNS interneurons → Motor neuron → Effector (muscle/gland).

What feature gives precision to neuronal communication?

Neuroglia insulate neurons so signals travel along defined paths without interference—this maintains accurate communication between specific neurons.