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Introduction to the Nervous System and Nervous Tissue – Study Notes

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Chapter 11: Introduction to the Nervous System and Nervous Tissue

Module 11.1 Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is divided into central and peripheral components, each with specialized structures and functions.

  • Major Functions of the Nervous System:

    • Sensory Function: Detects changes in the internal and external environment (stimuli) and transmits this information to the central nervous system (CNS).

    • Integrative Function: Processes and interprets sensory input and determines an appropriate response.

    • Motor Function: Initiates responses by activating effectors such as muscles and glands.

  • Central Nervous System (CNS):

    • Composed of the brain and spinal cord.

    • Responsible for integration and processing of information.

  • Peripheral Nervous System (PNS):

    • Consists of nerves (cranial and spinal) and ganglia outside the CNS.

    • Divided into sensory (afferent) and motor (efferent) divisions.

    • Motor division further subdivided into somatic (voluntary control of skeletal muscles) and autonomic (involuntary control of smooth muscle, cardiac muscle, and glands) systems.

  • Functional Divisions of the PNS:

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

    • Autonomic Nervous System (ANS): Regulates involuntary functions; subdivided into sympathetic and parasympathetic divisions.

Module 11.2 Nervous Tissue

Nervous tissue is specialized for communication and consists of two main cell types: neurons and neuroglia. Each has distinct structures and functions that support the overall activity of the nervous system.

  • Neurons:

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

    • Dendrites: Short, branched processes that receive signals from other neurons.

    • Axon: Long process that transmits electrical impulses away from the cell body.

    • Axon Terminals: Endings that form synapses with other cells.

  • Types of Neurons (by structure):

    • Multipolar: Many dendrites, one axon (most common in CNS).

    • Bipolar: One dendrite, one axon (found in sensory organs).

    • Unipolar (Pseudounipolar): Single process that splits into two branches (sensory neurons in PNS).

  • Neuroglial Cells:

    • CNS Neuroglia:

      • Astrocytes: Support neurons, maintain blood-brain barrier, regulate environment.

      • Oligodendrocytes: Form myelin sheaths in CNS.

      • Microglia: Act as phagocytes, removing debris and pathogens.

      • Ependymal Cells: Line ventricles, produce and circulate cerebrospinal fluid (CSF).

    • PNS Neuroglia:

      • Schwann Cells: Form myelin sheaths in PNS.

      • Satellite Cells: Support neuron cell bodies in ganglia.

Module 11.3 Electrophysiology of Neurons

Neurons communicate via electrical signals generated by the movement of ions across their membranes. This process involves resting membrane potential, action potentials, and the properties of ion channels.

  • Resting Membrane Potential:

    • Established by the unequal distribution of ions (mainly Na+ and K+) across the membrane and selective permeability.

    • Maintained by the sodium-potassium pump (Na+/K+ ATPase).

    • Typical value: (inside negative relative to outside).

  • Ion Channels:

    • Leak Channels: Always open; contribute to resting potential.

    • Voltage-Gated Channels: Open in response to changes in membrane potential; essential for action potentials.

  • Action Potential:

    • Rapid, temporary change in membrane potential that propagates along the axon.

    • Phases:

      • Depolarization: Membrane potential becomes less negative (Na+ influx).

      • Repolarization: Return to resting potential (K+ efflux).

      • Hyperpolarization: Membrane potential becomes more negative than resting.

    • Graph interpretation: Voltage vs. time graphs show these phases clearly.

  • Refractory Periods:

    • Absolute Refractory Period: No new action potential can be initiated.

    • Relative Refractory Period: Stronger stimulus required for action potential.

  • Conduction:

    • Continuous Conduction: Occurs in unmyelinated axons; slower.

    • Saltatory Conduction: Occurs in myelinated axons; action potential jumps between nodes of Ranvier; faster.

    • Factors Affecting Velocity: Larger axon diameter and myelination increase conduction speed.

Module 11.4 Neuronal Synapses

Synapses are specialized junctions where neurons communicate with other neurons, muscles, or glands. They can be electrical or chemical, each with distinct mechanisms and structures.

  • Types of Synapses:

    • Electrical Synapses: Direct passage of ions via gap junctions; rapid, bidirectional communication.

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

  • Structures of a Chemical Synapse:

    • Presynaptic neuron (axon terminal)

    • Synaptic cleft

    • Postsynaptic neuron (receptor region)

  • Neurotransmitter and Receptor Relationship:

    • Neurotransmitters bind to specific receptors, triggering a response in the postsynaptic cell.

  • Events of Chemical Synaptic Transmission:

    1. Action potential arrives at axon terminal.

    2. Voltage-gated Ca2+ channels open; Ca2+ enters terminal.

    3. Neurotransmitter released into synaptic cleft via exocytosis.

    4. Neurotransmitter binds to receptors on postsynaptic membrane.

    5. Postsynaptic potential generated (EPSP or IPSP).

  • Postsynaptic Potentials:

    • Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, increases likelihood of action potential.

    • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, decreases likelihood of action potential.

    • Voltage vs. time graphs illustrate these changes.

  • Summation:

    • Temporal Summation: Multiple signals from one neuron in rapid succession.

    • Spatial Summation: Simultaneous signals from multiple neurons.

Module 11.5 Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses. Their effects depend on the type of receptor they bind to and the location of the synapse.

  • Excitatory vs. Inhibitory Effects:

    • The same neurotransmitter can be excitatory at one synapse and inhibitory at another, depending on the receptor type.

  • Major Classes of Neurotransmitters:

    • Amino Acids: e.g., glutamate (excitatory), GABA (inhibitory).

    • Monoamines: e.g., dopamine, norepinephrine, serotonin.

    • Peptides: e.g., substance P, endorphins.

    • Others: e.g., acetylcholine (ACh).

  • Common Neurotransmitters in the CNS:

    • Excitatory: Glutamate

    • Inhibitory: GABA, glycine

Module 11.6 Functional Groups of Neurons

Neurons are organized into functional groups that process specific types of information. These groups form circuits that determine the flow of information in the CNS.

  • Neuronal Pool:

    • A group of interconnected neurons that process specific types of information.

    • Purpose: Integration and distribution of signals to various parts of the nervous system.

  • Types of Neural Circuits:

    • Diverging Circuits: One neuron sends signals to multiple downstream neurons (amplifies signal).

    • Converging Circuits: Multiple neurons send signals to a single neuron (integration of input).

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