Nervous Tissue and Neural Physiology: Structure, Function, and Communication

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is composed of two main types of cells: neurons and neuroglia (glial cells). Neurons are specialized for intercellular communication, while neuroglia support and protect neurons, outnumbering them and preserving the structure of nervous tissue.

• Neurons: Cells specialized for transmitting electrical impulses and communication.

• Neuroglia (Glial Cells): Support, protect, and insulate neurons; maintain the environment of nervous tissue.

Anatomical Divisions of the Nervous System

The nervous system is divided into two main anatomical regions:

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; responsible for sensory input and motor output.

Structure of a Neuron

Parts of a Neuron

Neurons have specialized structures for receiving, processing, and transmitting information.

• Dendrites: Branches that carry information into the cell body (soma).

• Soma (Cell Body): Contains the nucleus and organelles; site of metabolic activity.

• Perikaryon: Cytoplasm of the soma, containing neurotubules, neurofibrils, and neurofilaments.

• Nissl Bodies: Regions of rough endoplasmic reticulum involved in protein synthesis.

• Axon: Long process that carries electrical impulses away from the soma.

• Axolemma: Plasma membrane of the axon.

• Axoplasm: Cytoplasm within the axon.

• Axon Hillock: Region where the axon attaches to the soma; site of action potential initiation.

• Collaterals: Branches of the axon.

• Axon Terminals: Endings of the axon where neurotransmitters are released.

Neuroglia (Glial Cells)

Types of Glial Cells

There are six main types of glial cells, each with specific functions:

• Ependymal Cells: Line the central canal and ventricles of the brain; produce and circulate cerebrospinal fluid.

• Astrocytes: Maintain the blood-brain barrier, provide structural support, regulate ion and nutrient concentrations.

• Oligodendrocytes: Provide insulation for neurons in the CNS by forming myelin sheaths; enable fast and strong electrical impulses.

• Microglia: Smallest and least numerous; migrate through nervous tissue and clean up cellular debris and pathogens by phagocytosis.

• Satellite Cells: Regulate interstitial fluid around neurons in ganglia.

• Schwann Cells (Neurolemmocytes): Insulate axons in the PNS by forming myelin; outer layer is called the neurolemma.

Myelination and Nervous Tissue Regions

• Myelin: Protein that insulates axons, increasing the speed of electrical impulse transmission.

• Nodes (Nodes of Ranvier): Gaps in the myelin sheath where ion channels are concentrated.

• Internodes: Myelinated segments of the axon.

• White Matter: Regions of the CNS with many myelinated axons.

• Gray Matter: Regions of the CNS containing neuron cell bodies and unmyelinated axons.

Classification of Neurons

Types of Neurons

• Anaxonic Neurons: Found in the brain and special sense organs; cannot distinguish axons from dendrites.

• Bipolar Neurons: Found in special sense organs; have one dendrite and one axon with the soma between them; carry receptor information to other neurons.

• Unipolar Neurons: Connect PNS to CNS; dendrites and axons are continuous, with the cell body off to the side; responsible for sensory information.

• Multipolar Neurons: Most common; many dendrites and one axon; control skeletal muscles and carry efferent (motor) information.

Functional Classification

• Somatic Sensory Neurons: Monitor external environment.

• Visceral Sensory Neurons: Monitor internal environment.

• Sensory Receptors: Specialized cells that detect changes in the environment.

• Interoceptors: Monitor internal systems (e.g., digestive, respiratory).

• Exteroceptors: Monitor external environment (touch, temperature, smell, sight).

• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

• Somatic Motor Neurons: Innervate skeletal muscles.

• Visceral Motor Neurons: Innervate peripheral effectors (e.g., glands, smooth muscle).

• Interneurons: Connect sensory and motor neurons; coordinate both systems; most abundant type.

Neural Physiology

Membrane Potentials

Neurons communicate via electrical signals generated by changes in membrane potential.

• Graded Potential: Temporary, local change in charge at the original segment; decreases with distance.

• Action Potential: Electrical impulse carried along the entire axon; does not diminish over length.

• Resting Membrane Potential (RMP): The charge across the membrane at rest, typically $-70\,\text{mV}$.

• Electrochemical Gradient: Sum of chemical and electrical forces acting on an ion across the membrane.

• Current: Movement of charges to eliminate a potential difference.

• Resistance: Restriction of ion movement across the membrane.

Ion Channels

• Leak Channels: Always open; allow passive movement of ions.

• Active Channels: Require energy; gated ion channels open and close in response to stimuli.

• Voltage-Gated Channels: Open in response to changes in membrane potential; found on axons.

• Chemically-Gated Channels: Open when they bind specific chemicals (e.g., acetylcholine); found on cell bodies and dendrites.

Phases of Action Potential

• Depolarization: Change to a more positive charge; caused by sodium influx.

• Repolarization: Return to resting membrane potential after depolarization; potassium channels open.

• Hyperpolarization: Charge becomes more negative than resting membrane potential; caused by potassium efflux.

• Threshold: Minimum charge required to trigger an action potential, typically $-60$ to $-55\,\text{mV}$.

• All-or-None Principle: An action potential is either triggered or not; if threshold is not reached, no action potential occurs.

Propagation of Action Potentials

• Continuous Propagation: Occurs on unmyelinated axons; action potential moves stepwise along the entire membrane.

• Saltatory Propagation: Occurs on myelinated axons; action potential jumps from node to node, increasing speed.

Refractory Periods

• Absolute Refractory Period: Membrane will not respond to additional depolarizing stimuli.

• Relative Refractory Period: Membrane can respond, but only to a stronger stimulus; occurs during hyperpolarization.

Types of Axon Fibers

Type	Myelination	Diameter	Speed	Function
Type A	Myelinated	Large	120 m/sec	Rapid transmission to/from CNS
Type B	Myelinated	Medium	18 m/sec	Intermediate transmission
Type C	Unmyelinated	Small	Slow	Slow transmission

Synapses and Neurotransmitters

Synapse Structure and Function

• Synapse: Site where a neuron communicates with another cell.

• Presynaptic Cell: Neuron sending the message.

• Postsynaptic Cell: Cell receiving the message.

Types of Synapses

• Electrical Synapses: Direct physical contact between cells; rapid transmission; mostly found in the brain.

• Chemical Synapses: Signal transmitted across a gap (synaptic cleft) using neurotransmitters; most common type.

Synaptic Transmission

• Exocytosis: Release of neurotransmitters from presynaptic cell.

• Synaptic Delay: Time required for neurotransmitter release and binding.

• Synaptic Fatigue: Weakened synapse response when neurotransmitters cannot be synthesized fast enough for intense stimuli.

Neurotransmitter Effects

• Excitatory Neurotransmitters: Cause depolarization of postsynaptic membranes; promote action potential generation.

• Inhibitory Neurotransmitters: Cause hyperpolarization of postsynaptic membranes; suppress action potential generation.

Major Neurotransmitters

• Acetylcholine (ACh): Released at many synapses; can be excitatory or inhibitory.

• Biogenic Amines: Includes norepinephrine (NE), which is released at adrenergic synapses and has excitatory, depolarizing effects.

Key Equations and Values

• Resting Membrane Potential: $-70\,\text{mV}$

• Threshold for Action Potential: $-60$ to $-55\,\text{mV}$

• Potassium Equilibrium Potential: $-90\,\text{mV}$

• Sodium Equilibrium Potential: $+66\,\text{mV}$

Summary Table: Neuron Types

Type	Structure	Location	Function
Anaxonic	Cannot distinguish axons/dendrites	Brain, special sense organs	Unknown, integration
Bipolar	One dendrite, one axon	Special sense organs	Sensory transmission
Unipolar	Continuous dendrite/axon, cell body off to side	PNS	Sensory information
Multipolar	Many dendrites, one axon	Most common, CNS	Motor control

Example: A multipolar neuron in the spinal cord transmits motor commands to skeletal muscles, enabling voluntary movement.

Additional info: Academic context was added to clarify the functions and structure of glial cells, neuron types, and synaptic transmission, as well as to organize fragmented notes into a coherent study guide.