Core Concepts of Neural Anatomy and Signaling

Posted on Nov 5, 2025 in Medicine

Nervous System Structure and Function

The Nervous System (NS) maintains body temperature, monitors, integrates, and responds to information in the environment.

Divisions of the Nervous System

  • Central Nervous System (CNS): Includes the brain and spinal cord.
  • Peripheral Nervous System (PNS): Includes cranial and spinal nerves.

PNS Divisions

  1. Sensory (Afferent) Division: Conveys impulses to the CNS.
  2. Motor (Efferent) Division: Conveys impulses from the CNS.
    • Somatic (Voluntary) System: Serves skeletal muscles.
    • Autonomic (Involuntary) System: Innervates smooth and cardiac muscle and glands.

Neurons and Supporting Neuroglia

Neuroglia segregate, insulate, and assist neurons.

Neuroglia Types

  • CNS Neuroglia: Astrocytes, microglial cells, ependymal cells, and oligodendrocytes.
  • PNS Neuroglia: Schwann cells and satellite cells.

Neuron Anatomy

  • The cell body is the biosynthetic center of the neuron.
  • A collection of cell bodies is called a nucleus in the CNS and a ganglion in the PNS.
  • Most neurons have many dendrites and one axon, which generates and conducts nerve impulses away from the cell body.
  • Axon terminals release neurotransmitters.
  • A long axon is called a nerve fiber.
  • A bundle of nerve fibers is called a tract in the CNS and a nerve in the PNS.

Axonal Transport and Myelination

Bidirectional transport along axons uses ATP-dependent motor proteins moving along microtubule tracks. Large nerve fibers are myelinated. The myelin sheath is formed in the PNS by Schwann cells and in the CNS by oligodendrocytes. Nonmyelinated fibers are surrounded by supporting cells.

Functional Classification of Neurons

  • Sensory Neurons: Conduct impulses toward the CNS.
  • Motor Neurons: Conduct impulses away from the CNS.
  • Interneurons: Lie between sensory and motor neurons in the neural pathway.

Membrane Potentials and Action Potentials (AP)

The resting membrane potential depends on differences in ion concentration and permeability.

Electrical Principles

  • Voltage: Measure of potential energy.
  • Current (I): The flow of electrical charge from one point to another (Ions provide electric charges).
  • Resistance (R): Hindrance to current flow.

Changes in Membrane Potential

  • Depolarization: A reduction in membrane potential (the inside becomes less negative).
  • Hyperpolarization: An increase in membrane potential (the inside becomes more negative).

Four Steps of the Action Potential

  1. Resting State: Only leakage channels are open, maintaining the resting membrane potential. No ions move through voltage-gated channels. Each Na+ channel has two gates: a voltage-sensitive activation gate (closed at rest) and an inactivation gate (blocks the channel once it is open).
  2. Depolarization: Caused by Na+ flowing into the cell. The voltage-gated sodium channels open, and Na+ rushes into the cell. This influx of positive charge depolarizes the local patch of membrane further, opening more Na+ channels.
  3. Repolarization: Caused by K+ flowing out of the cell. This phase is self-limiting because the inactivation gates of the Na+ channels begin to close. The membrane permeability to Na+ declines to resting levels, and the net influx of Na+ stops completely. Consequently, the AP spike stops rising.
  4. Hyperpolarization: Caused by K+ continuing to leave the cell. The period of increased K+ permeability typically lasts longer than needed to restore the resting state. The excessive K+ efflux before the potassium channels close results in a slight dip following the spike on the AP curve.

Synaptic Transmission and Neurotransmitter Classes

Graded potentials are small, brief, local changes in membrane potential that act as short-distance signals. An Action Potential (AP) is a large but brief depolarization signal that underlies long-distance neural communication. AP begins and ends at the resting membrane potential.

Synapses

A synapse is a functional junction between neurons. The information-transmitting neuron is the presynaptic neuron, and the information-receiving neuron is the postsynaptic neuron.

Types of Synapses

  • Electrical Synapses: Allow ions to flow directly from one neuron to another.
  • Chemical Synapses: Are sites of neurotransmitter release and binding.

Neurotransmitters

Major classes of neurotransmitters based on chemical structure include:

  • Acetylcholine
  • Biogenic amines
  • Amino acids
  • Peptides
  • Purines
  • Dissolved gases
  • Lipids

Neurotransmitters are classified as inhibitory or excitatory, and direct or indirect.

Reflex Arcs and Muscle Contraction

Reflexes

Reflexes are mediated over neural pathways called reflex arcs. The five essential components of a reflex arc are:

  1. Receptor
  2. Sensory neuron
  3. Integration center
  4. Motor neuron
  5. Effector

Muscle Contraction Process

  1. Depolarization and calcium ion release.
  2. Actin and myosin cross-bridge formation.
  3. Sliding mechanism of actin and myosin filaments.
  4. Sarcomere shortening (muscle contraction).

Central Nervous System (CNS) Anatomy

Brain Development and Structure

The brain develops from three primary vesicles:

  • Prosencephalon: Develops into the cerebral hemispheres and diencephalon.
  • Mesencephalon: Develops into the midbrain.
  • Rhombencephalon: Develops into the pons, medulla oblongata, and cerebellum.

The two cerebral hemispheres exhibit gyri, sulci, and fissures. Each hemisphere consists of the cerebral cortex, cerebral white matter, and basal nuclei. Each hemisphere receives sensory impulses from, and dispatches motor impulses to, the opposite side of the body.

Functional Areas of the Cerebrum

  • Motor Areas: Primary motor and premotor cortex of the frontal lobe, the frontal eye field, and Broca’s area (in the frontal lobe of one hemisphere).
  • Sensory Areas: Primary somatosensory cortex and somatosensory association cortex (parietal lobe), visual areas (occipital lobe), olfactory and auditory areas (temporal lobe), gustatory, visceral, and vestibular areas (insula).
  • Association Areas: Anterior association area (frontal lobe), and posterior and limbic association areas spanning several lobes.

Brain Stem and Diencephalon

  • Cerebellum: Connected to the brain stem by superior, middle, and inferior peduncles. It processes and interprets impulses from the motor cortex and sensory pathways and coordinates motor activity for smooth movement.
  • Diencephalon:
    • Thalamus: Major relay station for sensory impulses ascending to the sensory cortex, and inputs from subcortical motor nuclei and the cerebellum traveling to association cortices from lower centers.
    • Hypothalamus: Important control center of the Autonomic Nervous System (ANS). It maintains water balance and regulates thirst, eating behavior, gastrointestinal activity, and body temperature.
    • Epithalamus: Includes the pineal gland, which secretes the hormone melatonin.
  • Midbrain: Contains the corpora quadrigemina, red nucleus, and the substantia nigra.
  • Pons: A conduction area; its nuclei contribute to regulating respiration and cranial nerves.
  • Medulla Oblongata: The pyramids form the ventral face of the medulla oblongata.

Cranial Nerve Functions (A Summary)

Note: The following functional descriptions are based on the source text.

Olfactory
Eyelid & eyeball movement.
Optic
Innervates superior oblique, turns eye downward & laterally.
Oculomotor
Chewing, face & mouth touch & pain.
Trochlear
Turns eye laterally.
Trigeminal
Controls most facial expression, secretion of tears & saliva, taste.
Vestibulocochlear
Hearing, equilibrium sensation.
Glossopharyngeal
Taste, senses carotid blood pressure.
Vagus
Senses aortic blood pressure, slows heart rate, stimulates digestive organs, taste.
Spinal Accessory
Controls trapezius & sternocleidomastoid, controls swallowing movements.
Hypoglossal
Controls tongue movements.

Sensory Receptors and Integration

Sensory receptors are specialized to respond to environmental changes. These include pain, touch, pressure, and temperature receptors in the skin, as well as receptors in the skeletal muscle, tendons, and visceral organs.

Sensation and Perception

  • Sensation: Awareness of internal and external stimuli.
  • Perception: Conscious interpretation of those stimuli.

Levels of Sensory Integration

Sensory receptors transduce stimulus energy via the receptor level.

  1. Receptor Level: Sensory receptors transduce stimulus energy.
  2. Circuit Level: Consists of the ascending pathways, including the axons of the first, second, and third-order sensory neurons.
  3. Perceptual Level: The internal, conscious image of the stimulus that serves as the basis for response.

Motor Endings

  • Motor endings of somatic nerve fibers form neuromuscular junctions with skeletal muscle cells.
  • Autonomic motor endings are functionally similar but structurally simpler, beaded terminals that innervate smooth muscle and glands.

The Autonomic Nervous System (ANS)

The ANS is the motor division of the PNS that controls visceral activities, aiming to maintain internal homeostasis.

ANS Divisions

  • Parasympathetic Division: Conserves body energy and maintains body activities at basal levels.
  • Sympathetic Division: Prepares the body for activity (fight or flight).

The ANS releases two major neurotransmitters: acetylcholine and norepinephrine.

ANS Control Levels

ANS function is controlled by several levels:

  • Spinal cord and brain stem.
  • Hypothalamic integration centers interact with higher and lower centers to orchestrate autonomic responses.
  • Cortical centers influence autonomic function via connections with the limbic system.

Nervous System Homeostatic Relationships

Endocrine System
The sympathetic division of the ANS activates the adrenal medulla. The hypothalamus helps regulate the activity of the anterior pituitary gland and produces the two posterior pituitary hormones. Hormones influence neuronal metabolism.
Cardiovascular System
The ANS helps regulate heart rate and blood pressure. The cardiovascular system provides blood containing oxygen and nutrients to the nervous system and carries away wastes.
Lymphatic System
Nerves innervate lymphoid organs, and the brain plays a role in regulating immune function.
Respiratory System
The nervous system initiates and regulates respiratory rhythm and depth.
Digestive System
The ANS regulates digestive motility and glandular activity. The digestive system provides nutrients needed for neuronal health.
Urinary System
The ANS regulates bladder emptying and renal blood pressure.
Integumentary System
The sympathetic division of the ANS regulates true sweat glands and blood vessels of the skin.
Skeletal System
Nerves innervate bones and joints, providing for pain and joint sense.
Muscular System
The somatic division of the nervous system activates skeletal muscles and maintains muscle health.