HAP_12e_Lecture_Ch 11_Accessible

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Importance of Understanding Neurotransmitters

• Understanding neurotransmitter function is crucial for awareness of how drugs affect a patient's nervous system.

The Nervous System Overview

• Nervous System Functions:

  • Receives, integrates, and responds to information.

  • Acts as the master controlling and communicating system of the body.

  • Utilizes electrical and chemical signals for rapid and specific communication leading to immediate responses.

Functions of the Nervous System

1. Sensory Input:

   • Gathers information from sensory receptors about internal and external changes.

2. Integration:

   • Processes and interprets sensory input and decides on a response.

3. Motor Output:

   • Activation of effectors (muscles and glands) to produce a response.

Structure of the Nervous System

• Divided into two principal parts:

  • Central Nervous System (CNS):

    • Composed of the brain and spinal cord; serves as the integration and control center.

    • Interprets sensory input and dictates motor output.

  • Peripheral Nervous System (PNS):

    • Comprises nerves that extend from the brain and spinal cord:

      • Spinal nerves to and from the spinal cord.

      • Cranial nerves to and from the brain.

      • Contains the enteric nervous system within the walls of the gastrointestinal tract.

Functional Divisions of the PNS

• Sensory (Afferent) Division:

  • Somatic sensory fibers convey impulses from skin, skeletal muscles, and joints to CNS.

  • Visceral sensory fibers convey impulses from visceral organs to CNS.

• Motor (Efferent) Division:

  • Transmits impulses from CNS to muscles and glands.

  • Further divided into the somatic nervous system (voluntary control) and autonomic nervous system (involuntary control).

Neuroglia and Neurons

• Nervous tissue consists of two main types of cells:

  1. Neuroglia (Glial Cells):

     • Support and protect neurons.

  2. Neurons:

     • Conduct electrical impulses.

Types of Neuroglia in CNS

1. Astrocytes:

   • Most abundant and versatile glial cells; support and brace neurons, control chemical environment.

2. Microglial Cells:

   • Defensive cells that monitor neurons and remove debris through phagocytosis.

3. Ependymal Cells:

   • Line brain cavities; involved in CSF circulation.

4. Oligodendrocytes:

   • Form myelin sheath around CNS axons.

Neuroglia in PNS

• Satellite Cells:

  • Surround neuron cell bodies in PNS; similar function to CNS astrocytes.

• Schwann Cells:

  • Surround peripheral nerve fibers and form myelin sheaths; crucial for regeneration of damaged nerve fibers.

Structure and Function of Neurons

• Neurons are structural units of the nervous system capable of conducting impulses.

• Key characteristics: extreme longevity, amitotic (most cannot divide), high metabolic rate requiring continuous oxygen and glucose supply.

• Neurons consist of a cell body (soma) and processes (dendrites and axon).

Neuron Types by Structure

1. Multipolar Neurons:

   • Many processes (1 axon, multiple dendrites); predominant type in CNS.

2. Bipolar Neurons:

   • Two processes (1 axon, 1 fused dendrite); rare, found in sensory organs.

3. Unipolar Neurons:

   • One process that branches into central (CNS) and peripheral (sensory receptor) branches; primarily in PNS ganglia.

Membrane Potentials

• Neurons have a resting membrane potential, allowing them to rapidly change this potential.

• Graded Potentials:

  • Short-lived changes in membrane potential that can lead to action potentials.

• Action Potentials (AP):

  • Brief reversal of membrane potential; used for long-distance signal transmission.

Action Potential Stages

1. Resting State:

   • All voltage-gated channels are closed.

2. Depolarization:

   • Voltage-gated Na+ channels open, causing large influx of Na+.

3. Repolarization:

   • Na+ channels inactivate, K+ channels open allowing K+ to exit.

4. Hyperpolarization:

   • Some K+ channels remain open, causing cell interior to become more negative than resting potential.

Propagation of Action Potentials

• Action potentials propagate along axons via local currents.

• Two types of conduction:

  1. Continuous conduction in nonmyelinated axons.

  2. Saltatory conduction in myelinated axons, where APs jump from node to node, increasing conduction speed.

Synapses

• Chemical Synapses:

  • Most common type; neurotransmitter releases across synaptic cleft to transmit signals.

• Electrical Synapses:

  • Connect neurons via gap junctions, allowing rapid communication.

Postsynaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs):

  • Graded potentials that depolarize and can lead to action potentials.

• Inhibitory Postsynaptic Potentials (IPSPs):

  • Hyperpolarize and reduce likelihood of action potentials.

Summation of Synaptic Inputs

• EPSPs and IPSPs can sum to determine whether a postsynaptic neuron will fire an action potential.

• Temporal Summation:

  • Rapid succession of impulses from a single presynaptic neuron.

• Spatial Summation:

  • Simultaneous impulses from multiple presynaptic neurons.

Neurotransmitters

• More than 50 neurotransmitters identified with various functions.

• Classes of Neurotransmitters:

  • Acetylcholine (ACh): First identified, involved in muscle stimulation.

  • Biogenic Amines: Include serotonin, dopamine, etc., involved in mood and behavior.

  • Amino Acids: Include glutamate and GABA, major neurotransmitters in brain and spinal cord.

  • Peptides: Include endorphins and substance P, involved in pain perception.

  • Gases: Include nitric oxide, involved in various physiological functions.

Clinical Correlations

• Multiple Sclerosis (MS): Autoimmune disorder affecting myelin in CNS, causing muscle weakness and motor control issues.

• Opioid Use Disorder: Long-term opiate use leads to dependency and withdrawal challenges.

Neural Integration

• Neural Integration involves the coordination of signals and functions within the nervous system, critical for complex processing tasks such as memory and learning.

• Patterns of neural processing include serial processing (reflexes) and parallel processing (simultaneous responses to one stimulus).

Developmental Aspects of Neurons

• The nervous system develops from the neural tube and neural crest, with various growth factors guiding neuron growth and synapse formation.

• Learning and experiences further refine neuronal connections throughout childhood.