Fundamentals of the Nervous System and Nervous Tissue

The Nervous System: Overview and Functions

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to rapidly and specifically coordinate bodily functions, often resulting in immediate responses.

• Sensory Input: Information is gathered by sensory receptors about internal and external changes.

• Integration: The processing and interpretation of sensory input, deciding on a response.

• Motor Output: Activation of effectors (muscles and glands) produces a response.

Organization of the Nervous System

The nervous system is divided into two principal parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each has distinct structures and functions.

• CNS: Consists of the brain and spinal cord; serves as the integration and control center.

• PNS: Consists mainly of nerves extending from the CNS; includes spinal and cranial nerves, and the enteric nervous system.

Functional Divisions of the PNS

• Sensory (Afferent) Division: Somatic sensory fibers convey impulses from skin, skeletal muscles, and joints; visceral sensory fibers convey impulses from visceral organs.

• Motor (Efferent) Division: Transmits impulses from CNS to effectors; includes the somatic (voluntary) and autonomic (involuntary) nervous systems.

• Autonomic Nervous System (ANS): Regulates smooth muscle, cardiac muscle, and glands; subdivided into sympathetic and parasympathetic divisions.

Neuroglia and Neurons

Neuroglia (Glial Cells)

Neuroglia are supporting cells in nervous tissue, essential for neuron function and health. They differ between CNS and PNS.

• CNS Neuroglia: Astrocytes, microglial cells, ependymal cells, oligodendrocytes.

• PNS Neuroglia: Satellite cells (support neuron cell bodies), Schwann cells (form myelin sheaths).

Neurons: Structure and Function

Neurons are the structural units of the nervous system, specialized for conducting impulses. They exhibit extreme longevity, are mostly amitotic, and have high metabolic rates.

• Cell Body (Soma): Contains nucleus, nucleolus, and cytoplasm; biosynthetic and metabolic center.

• Processes: Dendrites (receptive/input regions) and axons (conducting/output regions).

Dendrites

• Short, highly branched processes; receive signals and convey them toward the cell body as graded potentials.

Axons

• Single, long process; generates and transmits nerve impulses away from the cell body.

• Axon terminals release neurotransmitters to communicate with other cells.

• Axonal transport moves molecules and organelles in both directions (anterograde and retrograde).

Myelin Sheath

• White, fatty covering that insulates axons and increases impulse speed.

• Formed by Schwann cells in PNS and oligodendrocytes in CNS.

• Myelin sheath gaps (nodes of Ranvier) are sites for impulse propagation.

Classification of Neurons

Structural Classification

Neurons are classified by the number of processes extending from the cell body.

Functional Classification

• Sensory (Afferent) Neurons: Transmit impulses toward CNS; mostly unipolar.

• Motor (Efferent) Neurons: Carry impulses from CNS to effectors; multipolar.

• Interneurons: Shuttle signals within CNS; most abundant type.

Membrane Potentials and Electrical Properties

Basic Principles of Electricity

Neurons use electrical signals for communication. Key concepts include voltage, current, and resistance, governed by Ohm's law:

• Voltage (V): Potential energy from separated charges.

• Current (I): Flow of electrical charge (ions).

• Resistance (R): Hindrance to charge flow.

Ohm's Law:

Resting Membrane Potential

Neurons have a resting membrane potential, typically around -70 mV, due to differences in ion concentrations and membrane permeability.

• Potassium (K+) and sodium (Na+) are key ions.

• Sodium-potassium pump maintains gradients: 3 Na+ out, 2 K+ in.

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative; increases likelihood of action potential.

• Hyperpolarization: Membrane potential becomes more negative; decreases likelihood of action potential.

Graded Potentials and Action Potentials

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential, essential for initiating action potentials.

• Triggered by stimulus opening gated ion channels.

• Spread and decay quickly; act as signals over short distances.

Action Potentials

Action potentials are brief, long-distance signals generated by neurons and muscle cells. They do not decay over distance.

• Four main steps: resting state, depolarization, repolarization, hyperpolarization.

• Involves opening and closing of voltage-gated Na+ and K+ channels.

Threshold and All-or-None Phenomenon

• Action potential occurs only if depolarization reaches threshold.

• All-or-none: AP either happens completely or not at all.

Propagation and Conduction Velocity

• AP propagates along axon, self-propagating in one direction.

• Myelinated axons conduct impulses faster (saltatory conduction).

Synapses and Neurotransmitters

Synapses

Synapses are junctions that mediate information transfer between neurons or from neurons to effector cells. Two main types: electrical and chemical synapses.

• Electrical Synapses: Direct, rapid communication via gap junctions.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

Chemical Synapse Transmission

Transmission across chemical synapses involves six steps: AP arrival, Ca2+ influx, neurotransmitter release, diffusion, receptor binding, and termination.

Postsynaptic Potentials and Neural Integration

Postsynaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize membrane, increase likelihood of AP.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize membrane, decrease likelihood of AP.

Summation and Integration

• Temporal Summation: Rapid-fire impulses from one neuron add together.

• Spatial Summation: Multiple neurons stimulate simultaneously, effects add together.

Neurotransmitter Classification and Function

Chemical Structure Classification

• Acetylcholine: Released at neuromuscular junctions; degraded by acetylcholinesterase.

• Biogenic Amines: Includes dopamine, norepinephrine, serotonin; involved in mood and biological clock.

• Amino Acids: Glutamate, aspartate, glycine, GABA.

• Peptides: Substance P (pain), endorphins (natural opiates).

• Purines: ATP, adenosine.

• Gases and Lipids: Nitric oxide, carbon monoxide, endocannabinoids.

Functional Classification

• Effects: Excitatory (depolarizing) or inhibitory (hyperpolarizing), depending on receptor.

• Actions: Direct (fast, ion channel opening) or indirect (slow, second messenger pathways).

• Neuromodulators: Chemicals that modulate synaptic transmission strength.

Neurotransmitter Receptors

Channel-Linked Receptors

• Ligand-gated ion channels; immediate and brief action.

• Excitatory: Na+ influx; Inhibitory: Cl- influx.

G Protein–Linked Receptors

• Indirect, complex, slow, and prolonged responses.

• Activate second messengers (e.g., cyclic AMP) to modulate cell activity.

Neural Integration and Circuits

Patterns of Neural Processing

• Serial Processing: Input travels along one pathway; produces specific response (e.g., reflex arc).

• Parallel Processing: Input travels along several pathways; promotes higher-level functions.

Types of Neural Circuits

• Diverging: One input, many outputs.

• Converging: Many inputs, one output.

• Reverberating: Chain of neurons with feedback.

• Parallel After-Discharge: Several pathways to one output.

Developmental Aspects of Neurons

The nervous system originates from the neural tube and neural crest. Neurons undergo proliferation, migration, and connection to form functional circuits. Synaptic pruning and apoptosis shape the mature nervous system.

• Neurons are mostly amitotic after birth, with exceptions in olfactory neurons and hippocampus.

• Learning reinforces certain synapses and removes others.

Additional info: This study guide covers the core concepts of Chapter 11: Fundamentals of the Nervous System and Nervous Tissue, suitable for college-level Anatomy & Physiology students. All images included are directly relevant to the adjacent content and reinforce key concepts.