Fundamentals of the Nervous System and Nervous Tissue

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; 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 brings information to the CNS; motor (efferent) division carries commands from the CNS to effectors.

• Neurons: Classified as sensory neurons, interneurons, and motor neurons. Sensory neurons detect stimuli; interneurons process information; motor neurons elicit responses.

Neural Pathways and Information Flow

Information flows through the nervous system via specialized neural pathways, allowing for rapid communication and response to stimuli.

• Pathway: Sensory input → Integration (CNS) → Motor output

• Association Neurons: Interneurons within the CNS that process and relay information.

• Examples: Reflex arcs, sensory-motor integration.

Divisions of the Nervous System

The nervous system is further divided based on function and anatomy.

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

• Autonomic Nervous System: Regulates involuntary functions (smooth muscle, cardiac muscle, glands).

• Sympathetic Division: Prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Promotes 'rest and digest' activities.

Neuron Structure and Classification

Neurons are specialized cells with distinct structures and functions. They can be classified by shape and function.

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

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

• Unipolar Neurons: Single process; primarily sensory neurons in PNS.

• Generalized Neuron: Consists of dendrites, cell body (soma), axon, and axon terminals.

Neuroglial Cells

Neuroglia support and protect neurons. There are five main types, each with specific functions.

• Astrocytes: Maintain blood-brain barrier, support neurons.

• Microglia: Act as immune cells in CNS.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

• Oligodendrocytes: Form myelin in CNS.

• Schwann Cells: Form myelin in PNS.

Membrane Potentials and Action Potentials

Neurons communicate via electrical signals called action potentials, which depend on changes in membrane potential.

• Resting Membrane Potential: The voltage across the membrane at rest, typically -70 mV.

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Membrane potential returns to resting value.

• Hyperpolarization: Membrane potential becomes more negative than resting.

• Key Ions: Na+, K+; movement through channels creates electrical changes.

Equation:

Generation and Propagation of Action Potentials

Action potentials are generated when threshold is reached, leading to rapid depolarization and repolarization.

• All-or-None Principle: Action potentials either occur fully or not at all.

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger stimulus can initiate another action potential.

• Propagation: Action potentials travel along axons via local currents and voltage-gated channels.

Synaptic Transmission

Neurons communicate at synapses, where neurotransmitters are released to transmit signals to other cells.

• Chemical Synapses: Use neurotransmitters to relay signals.

• Electrical Synapses: Direct flow of ions between cells.

• Excitatory Synapses: Increase likelihood of action potential in postsynaptic cell.

• Inhibitory Synapses: Decrease likelihood of action potential.

Neurotransmitters and Their Effects

Neurotransmitters are chemicals that transmit signals across synapses. They can be classified by structure and function.

• Acetylcholine: Involved in muscle activation.

• Dopamine: Regulates mood, movement; imbalances linked to Parkinson's disease.

• Serotonin: Affects mood, sleep; imbalances linked to depression.

• Classes: Amines, amino acids, peptides, purines.

Effects of Drugs and Imbalances

Certain drugs and imbalances in neurotransmitter levels can affect synaptic transmission and are linked to neurological disorders.

• Stimulants (e.g., caffeine, amphetamines): Increase neurotransmitter release or action.

• Depressants (e.g., barbiturates, alcohol): Decrease neural activity.

• Parkinson's Disease: Caused by dopamine deficiency.

• Schizophrenia: Linked to dopamine imbalance.

• Depression: Associated with serotonin imbalance.

Summary Table: Major Neurotransmitters

Additional info:

• Neural coding refers to how the nervous system represents different types of sensory stimuli, often through frequency and pattern of action potentials.

• Synaptic plasticity is the ability of synapses to strengthen or weaken over time, crucial for learning and memory.