Nervous Tissue & System Part 1

Divisions and Subdivisions of the Nervous System

• Central Nervous System (CNS)

  • Brain: Positioned within the cranium (skull); serves as the primary integrating and coordinating center for processing sensory input and generating administrative responses.

  • Spinal Cord: Protected inside the vertebral column; functions as a physical bridge and major conduction pathway connecting peripheral nerves to the brain.

  • Central Processing Mechanism: Sensory input entering the spinal cord ascends to specific brain regions capable of deciphering touch, pain, temperature, pressure, stretching, or light touch localized to precise anatomical regions (e.g., skin of the right index finger). Following interpretation, motor instructions descend through the spinal cord to execute target responses.

• Peripheral Nervous System (PNS)

  • Composition: Composed of cranial and spinal nerves distributed throughout the entire body.

  • Function: Connects the brain and spinal cord to peripheral structures including skin, skeletal muscles, glands, blood vessels, and internal organs.

• Subdivisions of the Peripheral Nervous System

  • Somatic Nervous System: Controls voluntary bodily activities; specifically innervates skeletal muscles attached to bones.

  • Autonomic Nervous System (ANS):

    • An involuntary, self-regulatory division controlling cardiac muscle, smooth muscle in blood vessels and internal organs, and various glands.

    • Sympathetic Nervous System: Governs the "fight-or-flight" response during stress, mediated primarily by epinephrine and norepinephrine (adrenaline and noradrenaline).

    • Parasympathetic Nervous System: Governs the "rest-and-digest" or housekeeping activities; enhances digestive functions during feeding and conserves energy during inactivity.

    • Both autonomic divisions function via excitatory or inhibitory actions depending on target tissue receptors.

Structural and Functional Organization of Neurons

• Functional Definition of Neurons

  • Neurons are specialized, excitable nerve cells responsible for receiving physical or chemical stimuli, generating electrical impulses, and transmitting signals across the nervous system.

• Anatomical Structure of a Neuron

  • Dendrites: Branching, tree-like receptive processes that receive incoming stimuli and chemical signals from adjacent cells or sensory receptors.

  • Soma (Cell Body): The central metabolic hub of the cell containing the nucleus; processes incoming graded signals from dendrites.

  • Axon: A single, long cylindrical filamentous projection extending from the soma that conducts outgoing electrical action potentials.

  • Myelin Sheath: A lipid-based, waxy, whitish insulating layer covering many axons; speeds up conduction velocity along the axon.

  • Axon Terminals (Terminal Boutons / Synaptic End Bulbs): Distal, branched ends of the axon containing synaptic vesicles loaded with chemical signaling molecules known as neurotransmitters.

• Types of Neurons

  • Sensory (Afferent) Neurons: Conduct sensory impulses away from the periphery (skin, muscles, organs) toward the CNS. Enter the spinal cord via the dorsal (posterior) aspect/ganglion.

  • Motor (Efferent) Neurons: Conduct motor instructions away from the CNS toward effector targets (muscles, glands, organs). Exit the spinal cord via the ventral (anterior) aspect.

  • Interneurons: Localized inside the CNS between sensory and motor pathways; receive incoming afferent information and pass signals along to efferent pathways or higher processing centers.

Structural Classes of Neurons

• Multipolar Neurons

  • Characterized by multiple dendritic trees extending directly from the soma and a single myelinated axon ending in dendritic terminals.

  • Represent the vast majority of neurons in the human body, predominantly operating as motor (efferent) neurons.

• Bipolar Neurons

  • Feature a centrally located soma with two distinct processes: one dendritic process at one end and one axon at the opposite end.

  • Unmyelinated; specialized for short-distance, local communication (acting similar to interneurons).

  • Rare in the body; exclusively restricted to special sensory organs, specifically inside the eyes (retina) and ears.

• Unipolar (Pseudo-unipolar) Neurons

  • Feature a cell body offset from a continuous axon split into a peripheral process (extending to peripheral receptors) and a central process (entering the spinal cord).

  • Typically myelinated; function as long-distance sensory (afferent) receptor neurons conveying touch, pressure, and pain stimuli directly into the CNS.

Neuroglial Support Cells in the CNS and PNS

• Functional Definition of Neuroglia

  • Non-conducting support cells surrounding neurons that do not produce or transmit electrical impulses. They perform structural support, metabolic regulation, electrical insulation, secretion, and immune protection.

• Neuroglia of the Central Nervous System (CNS)

  • Oligodendrocytes: Light-blue colored cells responsible for synthesizing and wrapping the insulating lipid myelin sheath around axons within the brain and spinal cord.

  • Microglia: Phagocytic immune cells derived from macrophages and dendritic cells; actively ingest cellular debris, damaged tissue, and invading foreign pathogens.

  • Astrocytes: Star-shaped, green-hued supporting cells whose processes envelop vascular capillaries. Regulate neuronal metabolism, sugar transport, and nutrient distribution.

  • Ependymal Cells: Pink-hued epithelial-like cells forming a continuous layer lining the four ventricles of the brain and choroid plexus; produce and secrete cerebrospinal fluid (CSF) to cushion and protect CNS structures.

• Neuroglia of the Peripheral Nervous System (PNS)

  • Schwann Cells: Functional PNS equivalents of oligodendrocytes; synthesize and wrap the myelin sheath around peripheral neuronal axons to increase conduction speed.

  • Satellite Cells: Envelop and cushion the cell bodies (soma) of peripheral neurons; regulate local microenvironments, protect soma structures, and provide nutritional support.

Resting Neuron/Membrane Potential and Ion Distribution

• Membrane Potential ($E_m$)

  • Defined as the difference or distribution of electrical charges across the neuronal plasma membrane, quantified in millivolts ($\text{mV}$).

• Ion Distribution at Rest

  • Extracellular Fluid: High concentration of sodium ions ($\text{Na}^+$), chloride ions ($\text{Cl}^-$), and calcium ions ($\text{Ca}^{2+}$). Low potassium ($\text{K}^+$) concentration.

  • Intracellular Fluid: High concentration of potassium ions ($\text{K}^+$) and negatively charged internal proteins/organic anions. Low concentration of $\text{Na}^+$, $\text{Cl}^-$, and $\text{Ca}^{2+}$.

• Resting State Parameters

  • The resting membrane potential ($E_m$) of an unstimulated neuron is precisely $-70\,\text{mV}$.

  • The negative value indicates that the internal face of the membrane is more negatively charged relative to the external fluid, established by charge distributions, intracellular protein anions, and selective membrane permeability.

• Plasma Membrane Transport Proteins in Neurons

  • Passive Leaky Channels: Always open; permit continuous background ion diffusion according to concentration gradients. Neuronal membranes possess significantly more passive leaky $\text{K}^+$ channels than passive leaky $\text{Na}^+$ channels.

  • Gated Ion Channels: Open or close only in response to specific triggers:

    • Voltage-Gated Channels: Respond directly to changes in membrane potential ($E_m$).

    • Ligand-Gated Channels: Respond to chemical binding (e.g., neurotransmitter attachment).

    • Mechanically-Gated Channels: Respond to physical pressure, stretching, or tactile movement.

  • Sodium-Potassium Active Pump ($\text{Na}^+/\text{K}^+$ ATPase Antiporter): Driven by ATP hydrolysis; actively exports $3\,\text{Na}^+$ out of the cell and imports $2\,\text{K}^+$ into the cell against concentration gradients to maintain resting ionic balance.

Stages of the Action Potential and Electrical Conduction

• Phase 1: Polarization (Resting State)

  • Membrane potential is static at $-70\,\text{mV}$.

  • Gated $\text{Na}^+$ and $\text{K}^+$ channels are closed; passive leaky channels and the $\text{Na}^+/\text{K}^+$ ATPase pump maintain resting conditions.

• Phase 2: Depolarization

  • A mechanical, chemical, or electrical stimulus disturbs the dendrites or soma, causing gated $\text{Na}^+$ channels to open.

  • $\text{Na}^+$ rapidly rushes into the cell down its electrochemical gradient, driving the membrane potential toward positive values.

  • Threshold Potential: Must reach $-55\,\text{mV}$ to trigger a full action potential. If $-55\,\text{mV}$ is reached, voltage-gated $\text{Na}^+$ channels open sequentially along the axon, driving internal membrane potential up to positive values such as $+30\,\text{mV}$, $+45\,\text{mV}$, or $+50\,\text{mV}$.

• Phase 3: Repolarization

  • Voltage-gated $\text{Na}^+$ channels close at peak positive voltage, halting $\text{Na}^+$ influx.

  • Voltage-gated $\text{K}^+$ channels open completely, causing $\text{K}^+$ to rush out of the cell.

  • Loss of positive ions restores internal negativity back toward resting levels.

• Phase 4: Hyperpolarization

  • Voltage-gated $\text{K}^+$ channels close slowly, allowing excess $\text{K}^+$ outflow.

  • Membrane potential transiently drops below the resting potential, reaching values between $-75\,\text{mV}$, $-80\,\text{mV}$, or $-85\,\text{mV}$.

  • The $$ ext{Na}^+/\text