Background of the Nervous System

Location and Function of Nervous Tissue

The nervous system is a complex network responsible for coordinating body activities and responding to internal and external stimuli. Nervous tissue is primarily found in the brain, spinal cord, and peripheral nerves.

• Nervous tissue is specialized for communication via electrical and chemical signals.

• Main functions: Sensory input, integration, and motor output.

Cell Types in Nervous Tissue

• Neurons: The primary signaling cells, responsible for transmitting nerve impulses.

• Neuroglia (glial cells): Support, protect, and nourish neurons. Types include astrocytes, oligodendrocytes, microglia, Schwann cells, and ependymal cells.

• Role of neuroglia: Maintain homeostasis, form myelin, and provide support and protection for neurons.

Example: On a histology slide, neurons appear as large cells with prominent nuclei, while neuroglia are smaller and more numerous.

Anatomical and Functional Divisions of the Nervous System

Main Divisions

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; connects the CNS to limbs and organs.

Functional Divisions

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

• Motor (Efferent) Division: Carries commands from the CNS to effectors (muscles and glands).

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion); subdivided into sympathetic and parasympathetic divisions.

• Interneurons: Found within the CNS, responsible for integration and processing of information.

Additional info: The efferent division is further classified into somatic (voluntary) and autonomic (involuntary) systems.

Neurons vs. Neuroglia: Structure and Function

Structural Differences

• Neurons: Have a cell body (soma), dendrites, and an axon. Specialized for rapid communication.

• Neuroglia: Lack axons and dendrites; provide support and insulation.

Functional Differences

• Neurons: Generate and conduct electrical impulses.

• Neuroglia: Support neuronal function, form myelin, maintain the environment, and defend against pathogens.

General Structure of a Neuron

• Cell body (soma): Contains the nucleus and organelles.

• Dendrites: Receive signals from other neurons.

• Axon: Transmits impulses away from the cell body.

• Axon terminals: Release neurotransmitters to communicate with other cells.

Myelin and Its Importance

• Myelin: A fatty substance produced by oligodendrocytes (CNS) and Schwann cells (PNS) that insulates axons and increases the speed of impulse transmission.

• Function: Enables rapid and efficient signal conduction.

Key Terms Associated with the Nervous System

• Central NS

• Peripheral NS

• Sensory

• Motor

• Interneuron

• Somatic

• Autonomic

• Afferent

• Efferent

Additional info: These terms are essential for understanding the organization and function of the nervous system.

Membrane Potential and Action Potentials

Membrane Potential

The membrane potential is the electrical difference across a cell membrane due to the distribution of ions.

• Resting membrane potential: Typically around -70 mV in neurons.

• Key ions: Sodium (Na+), potassium (K+), chloride (Cl-), and calcium (Ca2+).

Ion Channels

• Voltage-gated channels: Open in response to changes in membrane potential.

• Ligand-gated channels: Open in response to binding of a chemical messenger.

Threshold and All-or-None Principle

• Threshold: The minimum membrane potential required to trigger an action potential (usually around -55 mV).

• All-or-none principle: Once threshold is reached, an action potential is generated and propagated without decrement.

Phases of Action Potential

• Polarization: Resting state; inside of neuron is negative relative to outside.

• Depolarization: Na+ channels open, Na+ enters cell, membrane potential becomes positive.

• Repolarization: K+ channels open, K+ exits cell, membrane potential returns to negative.

Movement of ions: Na+ influx during depolarization, K+ efflux during repolarization.

Equation:

Additional info: Numbers: Resting potential (-70 mV), threshold (-55 mV), peak depolarization (+30 mV).

Propagation of Action Potentials

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

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

Disruption of Action Potential

• Possible causes: Toxins, injury, demyelinating diseases (e.g., multiple sclerosis).

• Importance: Disruption can lead to loss of function or neurological deficits.

Synapses and Neurotransmitters

Structure of a Synapse

A synapse is the junction between two neurons or between a neuron and an effector cell.

• Presynaptic neuron: Releases neurotransmitters.

• Synaptic cleft: The gap between neurons.

• Postsynaptic neuron: Receives the signal.

Nerve Impulse Transmission at a Synapse

• Action potential arrives at axon terminal.

• Neurotransmitters are released into the synaptic cleft.

• Neurotransmitters bind to receptors on the postsynaptic membrane, causing ion channels to open and generating a new action potential.

Neurotransmitters

• Definition: Chemical messengers that transmit signals across a synapse.

• Excitatory neurotransmitters: Promote action potentials (e.g., glutamate).

• Inhibitory neurotransmitters: Suppress action potentials (e.g., GABA).

• Examples: Acetylcholine, dopamine, serotonin, norepinephrine.

Neuronal Pools

• Neuronal pools: Groups of interconnected neurons that process specific types of information.

Summary Table: Nervous System Divisions

Key Equations

• Membrane potential:

• Nernst equation (for equilibrium potential):

Conclusion

The nervous system is essential for communication, coordination, and control within the body. Understanding its structure, divisions, cellular components, and physiological processes is fundamental for students of anatomy and physiology.