Nervous System Overview

Background and Location

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.

• Where is nervous tissue found? Nervous tissue is primarily found in the brain, spinal cord (central nervous system, CNS), and peripheral nerves (peripheral nervous system, PNS).

• Function of nervous tissue: Nervous tissue is specialized for communication, processing sensory input, and controlling motor output.

• Cells in nervous tissue: The main cell types are neurons (signal-conducting cells) and neuroglia (supporting cells).

• Histology: Neurons can be identified on slides by their large cell bodies, prominent nuclei, and long processes (axons and dendrites).

Anatomical and Functional Divisions of the Nervous System

Main Divisions

The nervous system is divided both anatomically and functionally to efficiently process and respond to information.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integrating and processing information.

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

• Functional Divisions:

  • Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

  • Motor (Efferent) Division: Transmits commands from the CNS to effectors (muscles and glands).

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

  • Autonomic Nervous System: Controls involuntary functions (smooth muscle, cardiac muscle, glands); subdivided into sympathetic and parasympathetic divisions.

  • Interneurons: Connect neurons within the CNS and are involved in processing information.

Neurons and Neuroglia

Structure and Function

Neurons and neuroglia are the two main types of cells in nervous tissue, each with distinct roles.

• Neurons: Specialized for transmitting electrical impulses. Key structures include the cell body (soma), dendrites (receive signals), and axon (sends signals).

• Neuroglia (Glial Cells): Support, protect, and nourish neurons. Types include astrocytes, oligodendrocytes, microglia, ependymal cells (CNS), and Schwann cells, satellite cells (PNS).

• Myelin: A fatty substance produced by oligodendrocytes (CNS) and Schwann cells (PNS) that insulates axons, increasing the speed of nerve impulse conduction.

Comparison Table: Neurons vs. Neuroglia

Key Terms in the Nervous System

• Central NS (CNS): Brain and spinal cord.

• Peripheral NS (PNS): All neural tissue outside CNS.

• Sensory (Afferent): Carries information to CNS.

• Motor (Efferent): Carries commands from CNS.

• Somatic: Voluntary control (skeletal muscle).

• Autonomic: Involuntary control (organs, glands).

• Interneuron: Connects neurons within CNS.

Membrane Potentials and Action Potentials

Membrane Potential

The membrane potential is the electrical potential difference across a cell's plasma membrane, crucial for nerve impulse transmission.

• Resting membrane potential: Typically about -70 mV in neurons, maintained by ion gradients (mainly Na+ and K+).

• Key ions: Sodium (Na+), potassium (K+), chloride (Cl-), and calcium (Ca2+).

• Ion channels: Voltage-gated, ligand-gated, and leak channels regulate ion movement.

Action Potential

• Threshold: The minimum membrane potential required to trigger an action potential (usually around -55 mV).

• All-or-none principle: Once threshold is reached, an action potential occurs fully; if not, it does not occur.

• Phases:

  • Depolarization: Na+ channels open, Na+ enters the cell, membrane potential becomes more positive.

  • Repolarization: K+ channels open, K+ leaves the cell, membrane potential returns to negative.

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

Equation for Nernst Potential (for a single ion):

Propagation of Action Potentials

• Continuous propagation: Occurs in unmyelinated axons; action potential moves along every segment of the membrane.

• Saltatory propagation: Occurs in myelinated axons; action potential jumps between nodes of Ranvier, increasing speed.

• Disruption: Factors such as demyelination (e.g., multiple sclerosis) can impair action potential conduction.

Synapses and Neurotransmitters

Structure and Function of Synapses

A synapse is the junction between two neurons or between a neuron and an effector cell, where nerve impulses are transmitted.

• Components: Presynaptic terminal, synaptic cleft, postsynaptic membrane.

• Transmission: Action potential triggers release of neurotransmitters from presynaptic neuron, which bind to receptors on postsynaptic cell, causing a response.

Neurotransmitters

• Definition: Chemical messengers that transmit signals across a synapse.

• Types: Can be excitatory (promote action potentials) or inhibitory (suppress action potentials).

• Examples: Acetylcholine (ACh), dopamine, serotonin, norepinephrine, GABA, glutamate.

• Determination: The effect (excitatory/inhibitory) depends on the type of receptor on the postsynaptic cell.

Neuronal Pools

Neuronal pools are groups of interconnected neurons that process specific types of information and coordinate responses.

Additional info: Some explanations and neurotransmitter examples were inferred for completeness and clarity.