Nerve Cells and Electrical Signaling: Structure and Function in Human Physiology

Chapter 7: Nerve Cells and Electrical Signaling

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and processing sensory information. It is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord; integrates and processes information.

• Peripheral Nervous System (PNS): Composed of nerves outside the CNS; transmits sensory input and motor output.

• Cells of the Nervous System: Includes neurons (signal transmission) and glial cells (support and protection).

Example: Touching a hot surface activates sensory neurons in the PNS, which send signals to the CNS for processing and response.

Organization of the Peripheral Nervous System

The PNS is further divided based on function and type of information processed.

• Afferent Division (Input): Transmits sensory information to the CNS.

• Efferent Division (Output): Carries motor commands from the CNS to effectors (muscles, glands).

• Somatic Senses: Skin, muscles, joints.

• Special Senses: Vision, hearing, equilibrium.

• Visceral Senses: Internal organs (e.g., fullness of stomach).

• Somatic Motor Division: Controls voluntary movements (skeletal muscle).

• Autonomic Motor Division: Regulates involuntary functions (cardiac muscle, smooth muscle, glands).

• Autonomic Subdivisions: Sympathetic ("fight or flight"), Parasympathetic ("rest and digest"), and Enteric Nervous System (digestive tract).

Example: The sympathetic division increases heart rate during stress, while the parasympathetic division slows it during relaxation.

Relationship Between Central & Peripheral Nervous Systems

The CNS and PNS work together to process sensory input and generate motor output. Sensory (afferent) neurons carry information from the periphery to the CNS, while motor (efferent) neurons transmit commands from the CNS to muscles and glands.

• Afferent Pathways: Sensory division of PNS to CNS.

• Efferent Pathways: Motor division of CNS to PNS effectors.

Example: Cranial and spinal nerves connect the brain and spinal cord to the rest of the body.

Structure and Classification of Neurons

Structure of a Typical Neuron

Neurons are specialized cells for transmitting electrical signals. They have distinct structural components:

• Dendrites: Receive incoming information; usually numerous.

• Cell Body (Soma): Contains nucleus and organelles; integrates signals.

• Axon Hillock: Site where action potentials are generated.

• Axon: Conducts action potentials; can be short or long (1 mm to 1 m).

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

Example: A motor neuron transmits signals from the spinal cord to a muscle, causing contraction.

Structural Classification of Neurons

Neurons are classified based on the number and arrangement of their processes:

• Unipolar Neurons: Single process (axon); rare in humans.

• Pseudo-unipolar Neurons: Peripheral axon originates in periphery, central axon extends to CNS; typical of sensory neurons.

• Multipolar Neurons: Multiple dendrites and one axon from cell body; most common type in CNS.

Example: Multipolar neurons are predominant in the brain and spinal cord.

Establishment of the Resting Membrane Potential

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential difference across the cell membrane when the neuron is not actively sending signals. It is typically around -70 mV, with the inside of the cell more negative than the outside.

• Key Factors: Ion concentration gradients (mainly Na+ and K+), membrane permeability.

• Leak Channels: More K+ leak channels than Na+, making the cell more permeable to K+.

• Equilibrium Potentials: K+ equilibrium potential ≈ -94 mV; Na+ equilibrium potential ≈ +60 mV.

• Steady-State: RMP is maintained when the net flow of ions is balanced.

Equation:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Example: The sodium-potassium pump helps maintain RMP by moving 3 Na+ out and 2 K+ in.

Electrical Signaling Through Changes in Membrane Potential

Action Potentials and Graded Potentials

Neurons communicate via changes in membrane potential. These changes can be graded potentials (small, variable changes) or action potentials (large, all-or-none signals).

• Graded Potentials: Vary in size; occur in dendrites and cell body; can summate to reach threshold.

• Action Potentials: Triggered when membrane potential reaches threshold (about -55 mV); propagate along axon.

• Phases of Action Potential:

  1. Depolarization: Voltage-gated Na+ channels open; membrane potential rises to +30 mV.

  2. Repolarization: Na+ channels close, K+ channels open; membrane potential returns toward -70 mV.

  3. Hyperpolarization: K+ channels remain open briefly; membrane potential becomes more negative than RMP.

Equation:

$I = g(V_m - E_{ion})$

Example: A stimulus strong enough to depolarize the membrane to -55 mV triggers an action potential.

Propagation of Action Potentials

Action potentials travel along axons by sequentially opening voltage-gated channels. The speed of propagation depends on axon diameter and myelination.

• Myelination: Myelin sheaths (formed by oligodendrocytes in CNS, Schwann cells in PNS) increase conduction velocity.

• Saltatory Conduction: Action potentials "jump" between nodes of Ranvier in myelinated axons.

• Clinical Note: Multiple sclerosis is a disease where myelin is damaged, impairing signal transmission.

Example: Myelinated axons conduct signals much faster than unmyelinated axons.

Frequency Coding

The intensity of a stimulus is encoded by the frequency of action potentials, not their magnitude.

• Sub-threshold Stimulus: Does not trigger an action potential.

• Suprathreshold Stimulus: Triggers action potentials at higher frequency.

Example: Stronger pressure on the skin increases the frequency of action potentials in sensory neurons.

Table: Comparison of Neuron Structural Types

Type	Processes	Location/Function
Unipolar	Single process (axon)	Rare in humans
Pseudo-unipolar	Peripheral and central axon	Sensory neurons (PNS)
Multipolar	One axon, multiple dendrites	Most common in CNS