Nervous Tissue and the Nervous System

Core Nervous Tissue Structures

The nervous system is composed of specialized cells and structures that coordinate body functions through electrical and chemical signaling.

• Neuron: The primary signaling cell of the nervous system, responsible for transmitting electrical impulses.

• Neuroglia (Glial cells): Supportive cells that protect, nourish, and insulate neurons.

• Nerve: A bundle of axons in the peripheral nervous system (PNS).

• Ganglion: A cluster of neuron cell bodies in the PNS.

• Tract: A bundle of axons in the central nervous system (CNS).

• Gray matter: Regions containing neuron cell bodies, dendrites, and unmyelinated axons; involved in processing and integration.

• White matter: Regions composed mainly of myelinated axons; responsible for transmission of signals.

• Nucleus (CNS): A cluster of neuron cell bodies within the CNS.

Key Comparisons:

• Nerve vs Tract: Nerves are bundles of axons in the PNS; tracts are bundles of axons in the CNS.

• Ganglion vs Nucleus: Ganglia are clusters of neuron cell bodies in the PNS; nuclei are clusters in the CNS.

• Gray vs White Matter: Gray matter contains cell bodies and is involved in processing; white matter contains myelinated axons and is involved in signal transmission.

Neuroglial Cells

Types and Functions

Neuroglia provide structural and functional support to neurons. They differ between the CNS and PNS.

• CNS Neuroglia:

  • Astrocytes: Maintain the blood-brain barrier, provide nutrients, and regulate the extracellular environment.

  • Microglial cells: Act as immune cells, removing debris and pathogens.

  • Ependymal cells: Line ventricles and produce/circulate cerebrospinal fluid (CSF).

  • Oligodendrocytes: Form myelin sheaths around CNS axons.

• PNS Neuroglia:

  • Satellite cells: Support neuron cell bodies in ganglia.

  • Schwann cells: Form myelin sheaths around PNS axons.

Neuroglia Function Check:

• Myelin in CNS: Oligodendrocytes

• Myelin in PNS: Schwann cells

• Debris removal: Microglial cells

• Line fluid-filled spaces: Ependymal cells

Neuron Classification

Structural Classification

Neurons are classified by the number and arrangement of their processes (axons and dendrites).

• Multipolar: One axon, multiple dendrites; most common type, found in CNS.

• Bipolar: One axon, one dendrite; found in special sensory organs (e.g., retina).

• Unipolar (Pseudounipolar): Single process that splits into two branches; primarily sensory neurons in PNS.

Structural Check:

• Most common: Multipolar

• Sensory pathways: Unipolar

Functional Classification

Neurons are also classified by the direction in which they transmit impulses.

• Sensory (Afferent): Carry signals to the CNS from sensory receptors.

• Motor (Efferent): Carry signals from the CNS to effectors (muscles/glands).

• Interneurons (Integrative): Connect sensory and motor neurons; found only in the CNS.

Functional Check:

• Carries signals TO CNS: Sensory (Afferent)

• Carries signals FROM CNS: Motor (Efferent)

• Found only in CNS: Interneurons

Structure of a Nerve

Nerves are organized into layers of connective tissue that protect and bundle axons.

• Axon: The long, slender projection of a neuron that conducts impulses.

• Myelin sheath: Insulating layer around axons, increases signal speed.

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds bundles of axons (fascicles).

• Fascicle: Bundle of axons within a nerve.

• Epineurium: Surrounds the entire nerve.

Nerve Organization Check:

• Surrounds a single axon: Endoneurium

• Surrounds a fascicle: Perineurium

• Surrounds the entire nerve: Epineurium

Signal Conduction and Myelin

• Myelin increases the speed of nerve impulse conduction via saltatory conduction.

• Damage to myelin (e.g., in multiple sclerosis) slows or blocks signal transmission.

• Cells affected: Oligodendrocytes (CNS), Schwann cells (PNS).

Histology of Nervous Tissue

Microscopic examination reveals the structure and organization of neurons and neuroglia.

• Neuron: Large cell body, prominent nucleus, dendrites, and axon.

• Neuroglia: Smaller, more numerous support cells.

• Myelinated vs Unmyelinated: Myelinated axons appear as white, insulated fibers; unmyelinated lack this sheath.

• Motor nerve ending: Axon terminal forms a neuromuscular junction with muscle fibers.

Histology Check:

• Distinguish neuron vs neuroglia by size and structure.

• Identify direction of signal flow: dendrites → cell body → axon → axon terminal.

• Recognize myelinated (faster conduction) vs unmyelinated axons.

Nervous System Organization

Structural Divisions

• Central Nervous System (CNS): Brain and spinal cord; integration and control center.

• Peripheral Nervous System (PNS): Cranial and spinal nerves; connects CNS to the body.

Functional Divisions of the PNS

• Sensory (Afferent) Division: Transmits sensory information to the CNS.

  • Somatic sensory: From skin, muscles, joints.

  • Visceral sensory: From organs.

• Motor (Efferent) Division: Transmits commands from CNS to effectors.

  • Somatic motor: Voluntary control of skeletal muscles.

  • Autonomic motor: Involuntary control of smooth muscle, cardiac muscle, glands.

Organization Check:

• Division carrying signals TO CNS: Sensory (Afferent)

• Division controlling involuntary function: Autonomic motor

Brain: Major Structures

Cerebrum

• Hemispheres: Right and left halves of the brain.

• Gyrus (gyri): Elevated ridges.

• Sulcus (sulci): Shallow grooves.

• Longitudinal fissure: Separates hemispheres.

• Transverse fissure: Separates cerebrum from cerebellum.

• Lobes: Frontal, Parietal, Temporal, Occipital, Insula.

• Key Landmarks: Central sulcus, Lateral sulcus, Precentral gyrus (primary motor cortex), Postcentral gyrus (primary somatosensory cortex).

• Internal Structures: Cerebral cortex, Corpus callosum, Lateral ventricle, Fornix.

Diencephalon

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis, endocrine functions.

• Infundibulum: Connects hypothalamus to pituitary gland.

• Pituitary gland: Master endocrine gland.

• Epithalamus (pineal gland): Secretes melatonin.

• Olfactory bulb/tract: Involved in sense of smell.

• Optic chiasma: Crossing of optic nerves.

• Interventricular foramen: Connects lateral and third ventricles.

• Third ventricle: Fluid-filled cavity.

Brain Stem

• Midbrain: Visual and auditory reflexes.

• Pons: Relays information, regulates breathing.

• Medulla oblongata: Controls vital functions (heart rate, breathing).

• Superior colliculi: Visual reflex centers.

• Inferior colliculi: Auditory reflex centers.

• Cerebral aqueduct: Connects third and fourth ventricles.

• Fourth ventricle: Fluid-filled cavity.

Cerebellum

• Hemispheres: Right and left.

• Vermis: Connects hemispheres.

• Arbor vitae: Tree-like arrangement of white matter.

Brain Function Check:

• Precentral gyrus: Primary motor cortex (controls voluntary movement)

• Postcentral gyrus: Primary somatosensory cortex (receives sensory input)

• Cerebellum: Coordinates movement and balance

Cranial Nerves

There are 12 pairs of cranial nerves, each with specific sensory, motor, or mixed functions.

Examples:

• CN I: Olfactory → Sensory (smell)

• CN V: Trigeminal → Both (facial sensation, chewing)

• CN X: Vagus → Both (autonomic control of heart, lungs, digestive tract)

Classification: Cranial nerves can be sensory, motor, or mixed (both).

System Integration and Reflex Pathways

The nervous system integrates sensory input, processes information, and produces responses.

• Neuron → nerve → CNS → response: Sensory input is transmitted to the CNS, processed, and a motor response is generated.

• Structure supports signaling: Myelination, synapses, and organization of neurons enable rapid and precise communication.

• Simple reflex pathway: Sensory neuron → interneuron (in CNS) → motor neuron → effector.

Clinical Example: Loss of facial sensation may indicate damage to the Trigeminal nerve (CN V).

Summary Table: Nervous System Organization

Key Terms and Definitions

• Neuron: Nerve cell specialized for communication.

• Neuroglia: Support cells in nervous tissue.

• Myelin: Fatty sheath that insulates axons.

• Ganglion: Cluster of neuron cell bodies in PNS.

• Nucleus (CNS): Cluster of neuron cell bodies in CNS.

• Tract: Bundle of axons in CNS.

• Nerve: Bundle of axons in PNS.

• Reflex: Automatic response to a stimulus.

Additional info: This guide expands on checklist points with academic context, definitions, and examples for comprehensive exam preparation.