Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). It is responsible for coordinating body activities and processing sensory information.

• CNS: Composed of the brain and spinal cord; responsible for integrating sensory data and coordinating motor output.

• PNS: Consists of nerves and ganglia outside the CNS; connects the CNS to limbs and organs.

• Neurons: The functional units of the nervous system, specialized for communication.

• Neuroglia: Supporting cells that protect, nourish, and insulate neurons.

Neurons: Structure and Function

Neurons are highly specialized cells that transmit electrical signals throughout the body.

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

• Dendrites: Branching extensions that receive signals from other neurons.

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

• Axon Terminals: Endings where neurotransmitters are released to communicate with other cells.

• Myelin Sheath: Insulating layer that increases the speed of impulse transmission.

Example: Motor neurons transmit signals from the CNS to muscles, causing movement.

Neuroglia (Glial Cells)

Neuroglia are non-neuronal cells that provide support and protection for neurons.

• Astrocytes: Regulate the ionic composition of CNS extracellular fluid and support neurons.

• Ependymal Cells: Line brain cavities and help circulate cerebrospinal fluid.

• Microglial Cells: Act as phagocytes, removing debris and pathogens.

• Oligodendrocytes: Myelinate axons in the CNS.

• Schwann Cells: Myelinate axons in the PNS.

• Satellite Cells: Surround neuron cell bodies in ganglia, providing support.

Electrical Signals in Neurons

Neurons communicate via electrical impulses known as action potentials and graded potentials.

• Resting Membrane Potential: The voltage difference across the membrane of a resting neuron, typically around -70 mV.

• Action Potential: A rapid change in membrane potential that travels along the axon.

• Graded Potential: Local changes in membrane potential that decrease in strength with distance.

• Depolarization: Occurs when sodium (Na+) channels open and Na+ enters the cell.

• Repolarization: Occurs when potassium (K+) channels open and K+ leaves the cell.

• Hyperpolarization: Membrane potential becomes more negative than resting potential.

Equation:

Example: During the action potential, Na+ influx causes depolarization, followed by K+ efflux for repolarization.

Synaptic Transmission and Neurotransmitters

Neurons communicate at synapses using chemical messengers called neurotransmitters.

• Presynaptic Neuron: Releases neurotransmitter into the synaptic cleft.

• Postsynaptic Neuron: Receives the neurotransmitter and responds.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing the likelihood of an action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of an action potential.

• Neurotransmitters: Chemical messengers such as acetylcholine, dopamine, serotonin, norepinephrine.

Example: Acetylcholine is released at neuromuscular junctions to stimulate muscle contraction.

Types of Neurotransmitters

Neurotransmitters are classified based on their chemical structure and function.

Additional info: Biogenic amines include dopamine and serotonin, which are important for mood regulation and are implicated in various psychiatric disorders.

Neuronal Circuits

Neuronal circuits are patterns of synaptic connections that determine how information is processed.

• Converging Circuit: Many inputs converge on a single neuron.

• Diverging Circuit: One input spreads to multiple neurons.

• Parallel After-Discharge Circuit: Inputs travel along several pathways to a common output.

• Reverberating Circuit: Neurons stimulate each other in a loop, maintaining activity.

Example: Reflex arcs often use diverging circuits to activate multiple effectors.

Myelination and Nerve Fiber Classification

Myelination increases the speed of nerve impulse conduction.

• Myelinated Fibers: Conduct impulses rapidly; found in both CNS and PNS.

• Unmyelinated Fibers: Conduct impulses slowly.

• Large-Diameter Fibers: Conduct impulses faster than small-diameter fibers.

Example: Motor neurons controlling skeletal muscles are typically large-diameter, myelinated fibers.

Supporting Cells Table

Action Potential Sequence

The generation of an action potential involves a series of steps:

1. Resting state: All voltage-gated channels are closed.

2. Depolarization: Na+ channels open, Na+ enters the cell.

3. Repolarization: Na+ channels close, K+ channels open, K+ leaves the cell.

4. Hyperpolarization: K+ channels remain open briefly, membrane potential becomes more negative.

5. Return to resting state: Ion concentrations restored by Na+/K+ pump.

Equation:

Where is the ionic current, is conductance, is membrane potential, and is equilibrium potential.

Synaptic Integration

Neurons integrate multiple synaptic inputs to determine whether to fire an action potential.

• Spatial Summation: Multiple synapses at different locations add together.

• Temporal Summation: Rapid, repeated inputs at the same synapse add together.

Example: If EPSPs outweigh IPSPs, the neuron may reach threshold and fire an action potential.

Functional Types of Neurons

Neurons are classified by their function:

• Multipolar: Many dendrites, one axon; most common type in CNS.

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

• Unipolar: Single process; sensory neurons in PNS.

Example: Sensory neurons in the skin are unipolar and transmit touch information to the CNS.

Summary Table: Neuron Types and Functions

Additional info:

• Graded potentials travel shorter distances than action potentials, which propagate along the entire length of the axon.

• Biogenic amines, such as dopamine and serotonin, are involved in mood, arousal, and cognition.

• Acetylcholinesterase breaks down acetylcholine in the synaptic cleft, terminating its action.