Nervous Tissue and Neural Physiology: Structure and Function

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is composed of two main types of cells: neurons and neuroglia (glial cells). Each plays a distinct role in the structure and function of the nervous system.

• Neurons: Specialized for intercellular communication, transmitting electrical and chemical signals throughout the body.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons. They outnumber neurons and help maintain the structure of nervous tissue.

Additional info: Neuroglia are essential for homeostasis, forming myelin, and providing support and protection for neurons in both the central and peripheral nervous systems.

Anatomical Divisions of the Nervous System

Central and Peripheral Nervous Systems

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integrating, processing, and coordinating sensory data and motor commands.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Divided into sensory (afferent) and motor (efferent) divisions.

Motor Division: Transmits commands from the CNS to effector organs (muscles and glands).

Sensory Division: Brings information from sensory receptors to the CNS.

Neuron Structure

Parts of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Dendrites: Branch-like extensions that receive information from other neurons and carry it toward the soma.

• Axon: Long, slender projection that conducts electrical impulses away from the soma.

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

• Axolemma: Plasma membrane of the axon.

• Axoplasm: Cytoplasm within the axon.

• Axon Terminals: Endings where the neuron communicates with other cells.

• Collaterals: Branches of the axon.

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

• Nissl Bodies: Rough endoplasmic reticulum in neurons, involved in protein synthesis.

Types of Neuroglia

Glial Cells in the CNS and PNS

• Ependymal Cells: Line the central canal of the spinal cord and ventricles of the brain; produce and circulate cerebrospinal fluid (CSF).

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

• Oligodendrocytes: Produce myelin in the CNS, insulating axons to increase the speed of electrical impulses.

• Microglia: Smallest and least numerous neuroglia; act as phagocytes, removing cellular debris and pathogens.

• Satellite Cells: Surround neuron cell bodies in ganglia; regulate the environment around neurons.

• Schwann Cells (Neurolemmocytes): Form myelin sheaths around axons in the PNS; the outer layer is called the neurolemma.

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

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

Classification of Neurons

Structural Types

• 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 in between.

• Unipolar Neurons: Have a single process extending from the cell body; found in sensory neurons of the PNS.

• Multipolar Neurons: Most common type; have many dendrites and one axon; control skeletal muscles.

Functional Types

• Somatic Sensory Neurons: Monitor the external environment.

• Visceral Sensory Neurons: Monitor the internal environment.

• Interneurons: Connect sensory and motor neurons; coordinate activity within the nervous system; most abundant type.

• Motor Neurons: Carry instructions from the CNS to effectors (muscles and glands).

Sensory Receptors

Types of Sensory Receptors

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

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

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

Neural Physiology

Membrane Potentials

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

• Electrochemical Gradient: The 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.

Key Ion Concentrations:

• Sodium (Na+): $E_{Na} \approx +66\ \mathrm{mV}$

• Potassium (K+): $E_{K} \approx -90\ \mathrm{mV}$

Ion Channels

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

• Active (Gated) Channels: Open or close in response to stimuli; require energy.

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

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

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative (more positive); usually due to Na+ influx.

• Repolarization: Return to resting membrane potential after depolarization; typically involves K+ efflux.

• Hyperpolarization: Membrane potential becomes more negative than resting; often due to continued K+ efflux.

Graded Potentials vs. Action Potentials

• Graded Potential: Temporary, localized change in membrane potential; decreases with distance from the stimulus.

• Action Potential: Large, rapid depolarization that propagates along the axon without diminishing in strength.

Action Potential Phases

1. Threshold: The minimum membrane potential required to trigger an action potential (typically $-60$ to $-55\ \mathrm{mV}$).

2. Rapid Depolarization: Voltage-gated Na+ channels open, Na+ enters the cell.

3. Repolarization: At about $+30\ \mathrm{mV}$, Na+ channels close, K+ channels open, K+ leaves the cell.

4. Hyperpolarization: K+ channels close slowly, causing the membrane potential to become more negative than resting.

5. Return to Resting Potential: Ion concentrations are restored to resting levels.

All-or-None Principle: An action potential either occurs fully or not at all; if threshold is not reached, no action potential is generated.

Refractory Periods

• Absolute Refractory Period: The membrane cannot respond to another stimulus, regardless of strength.

• Relative Refractory Period: The membrane can respond to a stronger-than-normal stimulus during hyperpolarization.

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated axons; action potential moves along every segment of the membrane.

• Saltatory Propagation: Occurs in myelinated axons; action potential jumps from node to node (nodes of Ranvier), increasing speed.

Types of Axon Fibers

Type	Myelination	Diameter	Speed	Function
Type A	Myelinated	Large	Up to 120 m/s	Rapid transmission to/from CNS
Type B	Myelinated	Medium	Up to 18 m/s	Intermediate speed
Type C	Unmyelinated	Small	Slowest	Involuntary functions

Synapses and Neurotransmitters

Types of Synapses

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

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

Presynaptic Cell: Neuron sending the message.

Postsynaptic Cell: Cell receiving the message.

Synaptic Delay: Time required for neurotransmitter release and binding.

Synaptic Fatigue: Occurs when neurotransmitters cannot be synthesized quickly enough to meet demand during intense stimulation.

Neurotransmitter Effects

• Excitatory Neurotransmitters: Cause depolarization of the postsynaptic membrane, promoting action potentials.

• Inhibitory Neurotransmitters: Cause hyperpolarization, suppressing action potentials.

Examples:

• Acetylcholine (ACh): Can be excitatory or inhibitory depending on the receptor; released at cholinergic synapses.

• Norepinephrine (NE): Released at adrenergic synapses; generally excitatory and depolarizing.

Summary Table: Neuron Types and Functions

Neuron Type	Structure	Location	Function
Anaxonic	No clear axon/dendrite distinction	Brain, special sense organs	Unknown, possibly integration
Bipolar	One dendrite, one axon	Special sense organs	Sensory (sight, smell, hearing)
Unipolar	Single process, cell body off to side	PNS sensory neurons	Sensory information to CNS
Multipolar	Many dendrites, one axon	CNS, motor neurons	Motor control, integration

Additional info: This guide covers the structure and function of nervous tissue, neuron types, neuroglia, membrane potentials, action potentials, synaptic transmission, and neurotransmitter effects, providing a comprehensive overview for exam preparation in anatomy and physiology.