Nervous System: Organization and Function

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body activities by transmitting signals to and from different parts of the body. It is essential for sensation, movement, cognition, and homeostasis.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and is the main control center.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. It connects the CNS to limbs and organs.

• Enteric Nervous System: A specialized network in the digestive tract, functioning both autonomously and as part of the autonomic nervous system.

Key Functions:

• Organization of the nervous system

• Movement of electrical signals (action potentials and graded potentials)

• Electrical signaling for communication

• Factors affecting conduction of action potentials

• Synaptic communication

Functional Divisions of the PNS

• Sensory (Afferent) Neurons: Transmit sensory information to the CNS.

• Efferent Neurons: Carry commands from the CNS to effectors (muscles, glands).

• Somatic Motor Division: Controls skeletal muscle.

• Autonomic Division: Regulates involuntary functions (e.g., heart rate, digestion). Subdivided into:

  • Sympathetic

  • Parasympathetic

Cell Types in the Nervous System

Neurons

Neurons are the primary functional units of the nervous system, specialized for electrical signaling.

• Structure: Consist of a cell body (soma), dendrites (receive signals), and axon (transmits signals).

• Types by Shape: Unipolar, bipolar, multipolar, and pseudounipolar.

• Types by Function: Afferent (sensory), efferent (motor), interneurons.

• Pre- and Post-synaptic Neurons: Refer to the direction of signal transmission at synapses.

Glial Cells (Neuroglia)

Glial cells provide support, protection, and nutrition to neurons. They do not conduct electrical impulses.

Electrical Signaling in Neurons

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the neuronal membrane when the cell is not transmitting a signal. It is typically around -70 mV (inside negative relative to outside).

• Maintained by: Sodium-potassium ATPase pump and selective permeability of the membrane.

• Key ions: Na+, K+, Cl-

Equation:

(Nernst equation for equilibrium potential of an ion)

Action Potentials

An action potential is a rapid, transient change in membrane potential that propagates along the axon, allowing neurons to transmit signals over long distances.

• Phases: Resting, depolarization (Na+ influx), repolarization (K+ efflux), hyperpolarization, return to resting.

• "Domino effect": Once initiated, the action potential propagates without decrement.

• All-or-none principle: Action potentials occur fully or not at all.

Graded Potentials

Graded potentials are local changes in membrane potential that vary in size and decay with distance. They are important for initiating action potentials.

• Triggered by: Chemical or mechanical stimuli.

• Characteristics: Travel short distances, signal strength decreases with distance, can be summed.

Comparison: Graded vs. Action Potentials

Sodium-Potassium ATPase Pump

This pump maintains the resting membrane potential by actively transporting 3 Na+ ions out and 2 K+ ions into the neuron, using ATP.

Neuronal Communication

Electrical and Chemical Signaling

Neurons communicate via electrical signals (action potentials) and chemical signals (neurotransmitters at synapses).

• Electrical synapses: Direct passage of ions via gap junctions.

• Chemical synapses: Neurotransmitter release into synaptic cleft, binding to receptors on postsynaptic cell.

Synaptic Transmission: Main Steps

1. Action potential arrives at axon terminal.

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

3. Vesicles release neurotransmitter into synaptic cleft.

4. Neurotransmitter binds to receptors on postsynaptic membrane.

5. Postsynaptic response (excitation or inhibition).

Factors Affecting Conduction of Action Potentials

• Axon diameter: Larger diameter = faster conduction.

• Myelination: Myelinated axons conduct faster due to saltatory conduction (jumping between nodes of Ranvier).

• Temperature: Higher temperature increases conduction speed (within physiological limits).

Types of Axonal Transport

• Fast axonal transport: Moves organelles and vesicles rapidly along microtubules.

• Slow axonal transport: Moves cytoskeletal elements and soluble proteins.

Summary Table: Glial Cells and Their Functions

Key Terms and Definitions

• Neuron: Electrically excitable cell that transmits information.

• Glial cell: Non-neuronal cell providing support and protection.

• Action potential: Rapid, all-or-none electrical signal along the axon.

• Graded potential: Local change in membrane potential, variable in size.

• Synapse: Junction between two neurons for signal transmission.

• Myelin sheath: Insulating layer around axons, increases conduction speed.

• Node of Ranvier: Gaps in myelin sheath where action potentials are regenerated.

Example: Application

Damage to myelinated axons, as seen in multiple sclerosis, slows or blocks action potential conduction, leading to muscle weakness and sensory disturbances.

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard Anatomy & Physiology content.