Physiology of the Neuron

Introduction

The neuron is the fundamental unit of the nervous system, responsible for receiving, propagating, and transmitting electrical impulses throughout the body. Understanding the physiology of neurons is essential for comprehending how the nervous system functions at both cellular and systemic levels.

Special Properties of Neurons

Unique Features of Neurons

• Extreme Longevity: Neurons can survive for over 100 years if adequately nourished, making them some of the longest-living cells in the body.

• Amitotic Nature: Most neurons do not undergo mitosis after differentiation. This means that if neurons are damaged, they are generally not replaced, which has significant implications for nervous system injuries.

• High Metabolic Rate: Neurons require continuous and abundant supplies of oxygen and glucose to function properly, reflecting their high metabolic activity.

Example: The human brain contains approximately 100 billion neurons, with more possible interconnections than atoms in the universe.

Structural Regions of the Neuron

Main Regions and Their Functions

• Receptive Region: Includes dendrites and the cell body (soma). Responsible for receiving signals from other neurons.

• Conducting Region: The axon, which generates and propagates action potentials away from the cell body.

• Secretory Region: The axon terminals (synaptic boutons), which release neurotransmitters to communicate with other neurons or effector cells.

Neuron Cell Body (Soma)

• Contains a spherical nucleus and granular cytoplasm.

• Acts as the biosynthetic center, with extensive rough endoplasmic reticulum (Nissl bodies), Golgi apparatus, and mitochondria.

• Integrates incoming signals and initiates action potentials.

Neuron Processes

• Dendrites: Short, tapering, highly branched extensions that provide a large surface area for receiving signals. Conduct impulses toward the cell body as graded potentials.

• Axon: A single, long process that arises from the axon hillock. Conducts action potentials away from the cell body. Axon diameter affects conduction speed; larger diameters conduct faster.

• Axon Terminals: Branches at the end of the axon that form synapses with other neurons or effector cells.

Additional Terms

• Tract: Bundle of nerve processes in the central nervous system (CNS).

• Nerve: Bundle of nerve processes in the peripheral nervous system (PNS).

Electrical Properties of Neurons

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential difference across the plasma membrane of a resting neuron, typically between -40 and -90 mV. This potential is due to the differential distribution of ions, mainly sodium (Na+) and potassium (K+), across the membrane.

• Na+: Primarily found outside the cell (extracellular fluid).

• K+: Primarily found inside the cell (cytoplasm).

• Na+/K+ ATPase Pump: Maintains the RMP by moving 3 Na+ out and 2 K+ in for each ATP hydrolyzed.

Equation:

Ion Channels

• Leakage (Passive) Channels: Always open; allow ions to move according to their electrochemical gradients.

• Gated (Active) Channels: Require a signal to open or close. Types include:

  • Chemically-gated: Open in response to neurotransmitters or hormones.

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

Electrochemical Gradient

The combined effect of the concentration gradient and electrical gradient that drives ion movement across the membrane.

Graded Potentials and Action Potentials

Graded Potentials

• Occur in the receptive region (dendrites and cell body).

• Can be depolarizations (make the inside less negative) or hyperpolarizations (make the inside more negative).

• Magnitude decreases with distance (decremental).

• Strength determined by the strength of the stimulus.

Action Potentials (APs)

• Brief reversal of membrane potential (from -70 mV to +30 mV).

• Generated at the axon hillock and propagated along the axon.

• Do not decrease in amplitude with distance (non-decremental).

• All-or-none events: only occur if threshold is reached.

Phases of the Action Potential:

1. Resting State: Voltage-gated Na+ and K+ channels are closed.

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

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

4. Hyperpolarization: K+ channels remain open longer, causing the membrane potential to become more negative than RMP.

Refractory Periods

• Absolute Refractory Period: No new action potential can be generated, regardless of stimulus strength (Na+ channels open or inactivated).

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential (Na+ channels reset, K+ channels still open).

Propagation of Action Potentials

Factors Influencing Propagation

• Axon Diameter: Larger diameter axons conduct impulses faster due to lower resistance.

• Myelination: Myelin sheaths (formed by Schwann cells in the PNS and oligodendrocytes in the CNS) insulate axons and increase conduction speed via saltatory conduction (impulse jumps between nodes of Ranvier).

Clinical Note: Multiple Sclerosis (MS)

• Autoimmune destruction of myelin sheaths in the CNS.

• Symptoms: Visual disturbances, muscle weakness, clumsiness, urinary incontinence.

• Treatment: Reduce inflammation, manage symptoms, promote myelin repair.

Synaptic Transmission

Types of Synapses

• Electrical Synapses: Direct current flow via gap junctions; rapid transmission; found in some brain regions and muscle tissue.

• Chemical Synapses: Most common; involve neurotransmitter release from presynaptic neuron and binding to receptors on postsynaptic neuron.

Mechanism 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.

4. Neurotransmitter binds to postsynaptic receptors, opening ion channels.

5. Postsynaptic potential generated (EPSP or IPSP).

Termination of Neurotransmitter Action:

• Enzymatic degradation (e.g., acetylcholine by acetylcholinesterase).

• Reuptake by presynaptic terminal (e.g., norepinephrine).

• Diffusion away from synaptic cleft (e.g., nitric oxide).

Synaptic Delay

• The time required for neurotransmitter release, diffusion, and receptor binding (0.3–5 ms); the slowest step in neurotransmission.

Postsynaptic Potentials and Integration

Types of Postsynaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Local depolarizations that increase the likelihood of action potential generation.

• Inhibitory Postsynaptic Potentials (IPSPs): Local hyperpolarizations that decrease the likelihood of action potential generation.

Summation

• Temporal Summation: Multiple EPSPs/IPSPs from a single synapse in rapid succession.

• Spatial Summation: EPSPs/IPSPs from multiple synapses at the same time.

• Axon Hillock: Acts as the neural integrator, summing all incoming signals to determine if threshold is reached for action potential initiation.

Summary Table: Key Differences in Synaptic Types

Additional info: Some details, such as the exact mechanisms of graded and action potentials, have been expanded for clarity and completeness based on standard academic sources.