Nervous System Overview: CNS and PNS

Overview of the Nervous System

• The nervous system is vital for understanding the connection between the brain and behavior.
• It consists of neurons (nerve cells) and glial cells (support cells).
• There are approximately 86 billion neurons in the brain and 100–150 billion in the entire nervous system.
• This system creates intricate networks connecting to every muscle and organ.
• Its primary roles include collecting sensory input and governing behavior through motor control.

Major Divisions of the Nervous System

• The nervous system includes the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS)

• Includes the brain and spinal cord.
• It processes sensory data and organizes responses.

Peripheral Nervous System (PNS)

• Includes nerves and sensory organs across the body.
• The PNS relays communication between the CNS and the rest of the body.

Peripheral Nervous System Nerves

• Cranial nerves connect directly to the brain.
• Spinal nerves connect to the spinal cord.
• Neural circuits in the PNS transmit sensory data to the CNS and motor commands from the CNS to muscles and glands.

Functional Neuron Types

• Neurons transmit electrical and chemical signals.
• There are three main types: sensory neurons, interneurons, and motor neurons.

Sensory Neurons

• These detect environmental changes and send data to the CNS.
• Photoreceptors in the eye are an example, detecting light for visual processing.

Interneurons

• Interneurons connect sensory and motor neurons within the CNS only.
• They integrate and process sensory information before transmitting responses to motor neurons.

Motor Neurons

• These control muscles and glands, which leads to motor behavior.
• For example, they send signals to arm muscles to lift a glass of water.

Neural Circuit Example

• Neurons work together in circuits to facilitate movement and perception.
• Sensory neurons detect a stimulus and send the information to the CNS.
• There interneurons process information.
• Motor neurons send signals to muscles for movement.

Summary of Key Concepts

• The nervous system handles sensory input, processing, and motor control.
• It has two main divisions: the CNS (brain and spinal cord) and the PNS (nerves and sensory organs).
• Sensory neurons detect stimuli, interneurons process and relay information, and motor neurons execute movement.
• Neural circuits integrate sensory input with motor output.

Neuron Overview

• Neurons (brain or nerve cells) transmit information.
• They work in networks that enable behavior and cognition.
• Understanding neuron structure is key to understanding neuronal communication.
• There are four main parts: dendrites, axons, terminal buttons, and soma.

Dendrites – The Receivers

• Dendrites receive chemical messages from other neurons.
• These tree-like extensions increase surface area for signal reception.
• Neurons communicate across synapses.
• A presynaptic neuron releases neurotransmitters, which a postsynaptic neuron detects via its dendrites.

Axon – The Message Sender

• The axon transmits electrochemical messages (action potentials).
• It’s a long, tube-like structure extending from the soma to terminal buttons.
• Action potentials travel along the axon surface.
• The myelin sheath insulates the axon to increase conduction speed and prevent interference.

Axon Length

• Sensory and motor neurons have long axons.
• Interneurons have short axons.

Axoplasmic Transport – Internal Transport System

• This system moves substances inside the neuron using microtubules.
• Anterograde transport moves substances from the soma to terminal buttons quickly.
• Retrograde transport moves substances from terminal buttons back to the soma slowly.

Terminal Buttons – The Message Transmitters

• Terminal buttons release neurotransmitters into the synapse.
• Action potentials trigger neurotransmitter release into the synaptic gap.
• Neurotransmitters bind to receptors on the next neuron’s dendrites.
• This triggers an excitatory or inhibitory response.
• Excitatory signals increase the chance of an action potential; inhibitory signals decrease it.

Soma – The Cell Body of a Neuron

• The soma maintains the neuron and produces proteins and chemicals.
• It contains the nucleus (with chromosomes and genetic instructions), cytoplasm, and organelles.
• Mitochondria produce ATP, and the cytoskeleton (protein strands) gives the neuron shape and assists in transport.

The Neuron Membrane – The Protective Barrier

• The neuron membrane controls what enters and exits, made of a lipid bilayer with embedded proteins.
• Proteins act as substance detectors and gatekeepers and transport substances across the membrane.

Recap: The Journey of a Message Through a Neuron

• Dendrites receive chemical signals.
• A strong signal generates an action potential.
• The axon transmits the electrochemical signal to terminal buttons.
• Terminal buttons release neurotransmitters into the synapse.
• Neurotransmitters bind to receptors on the next neuron’s dendrites.
• The cycle continues, allowing neural communication.

Summary of Key Concepts

• Neurons are specialized cells for communication.
• Main components include dendrites (receive), axon (sends signals), terminal buttons (transmit), and soma (maintains cell).
• Axoplasmic transport moves substances inside the axon.
• Neural signals can be excitatory or inhibitory.
• Membrane proteins control substance entry and communication.

Glial Cells Overview

• Half the nervous system is made up of glial cells.
• Neurons need support due to high energy demands and the need for protection.
• Glial cells supply nutrients, maintain structure, control the chemical environment, provide insulation, and clean up dead neurons.

Types of Glial Cells

• Glial cells are divided into those in the CNS and those in the PNS.

Glial Cells in the CNS

• This includes astrocytes, oligodendrocytes, and microglia.

Astrocytes (The Structural & Chemical Regulators)

• These cells regulate chemicals, provide structural support, supply nutrients, and control damage.
• They maintain extracellular fluid composition and ensure neurotransmitters remain contained.
• They act as "neuron glue" and deliver glucose.
• Astrocytes also engulf and remove dead neurons.

Oligodendrocytes (The Myelin Producers of the CNS)

• Oligodendrocytes produce myelin sheath around axons.
• They create insulation to speed up neural signals.
• Nodes of Ranvier help with rapid signal transmission.

Microglia (The Immune Cells of the Brain)

• Microglia engulf and remove dead neurons and are active during brain injury or inflammation.
• They increase activity to reduce inflammation after injury.

Glial Cells in the PNS

• Schwann cells are the main glial cell in the PNS.

Schwann Cells (The Myelin Producers of the PNS)

• Schwann cells produce myelin sheath around PNS axons.
• Each Schwann cell wraps around only one segment of an axon.
• Myelin insulates axons and increases action potential transmission speed.

Summary of Key Concepts for Glial Cells

• Astrocytes in the CNS provide structural support, regulate chemicals, supply nutrients, and control damage.
• Oligodendrocytes in the CNS produce myelin, wrapping multiple axons.
• Microglia in the CNS provide immune response and cleanup.
• Schwann cells in the PNS produce myelin, each wrapping one axon segment.

Recap: Why Are Glial Cells Important?

• Neurons rely on glial cells for energy, insulation, immune protection, and cleanup.
• Key takeaways include that astrocytes handle balance and cleanup, oligodendrocytes produce CNS myelin, microglia defend and clean, and Schwann cells produce PNS myelin.

Understanding Multiple Sclerosis

• MS affects the central nervous system (brain, spinal cord, optic nerve).
• The CNS is essential for bodily functions, controlled by neurons.

The Role of Myelin in Neural Communication

• Neurons communicate via axons insulated by myelin.
• Myelin ensures smooth and fast signal transmission.

How MS Disrupts the Nervous System

• MS involves the immune system attacking myelin, leading to demyelination and scar tissue.
• This disrupts neural signals and slows or blocks communication.

Symptoms of MS

• Symptoms include muscle weakness, decreased coordination, fatigue, vision problems, numbness, and paralysis.

Diagnosis of MS

• MS is diagnosed using MRI scans to detect lesions and scarring in the CNS, alongside symptom analysis.

Types and Progression of MS

• RRMS involves relapses followed by recovery.
• PPMS involves gradual worsening without relapses.
• SPMS starts as RRMS and becomes steadily progressive.

Causes and Research on MS

• The exact cause is unknown, but factors include genetic predisposition, environmental factors, and viral infections.
• There is no cure but treatments aim to alleviate symptoms and slow progression.

The Importance of Awareness and Research

• It is important to raise MS awareness.
• Raising awareness supports research and improves treatments.

Overview of the Blood-Brain Barrier

• The Blood-Brain Barrier (BBB) is a selective barrier between blood and the brain.
• It consists of tightly packed cells lining brain blood vessels.
• The BBB prevents harmful substances from entering the brain while allowing essential molecules to pass.

Functions of the Blood-Brain Barrier

• The BBB protects the brain from bacteria and toxins and regulates the chemical environment around neurons.
• Maintaining the chemical environment keeps ion and neurotransmitter levels stable.

How the Blood-Brain Barrier Works

• Endothelial cells are tightly packed, forming a physical barrier with tight junctions.
• Fat-soluble molecules pass easily, while other molecules need transport proteins.
• Examples of molecules that pass easily include oxygen, carbon dioxide, alcohol, and nicotine.
• Glucose, is transported via protein transporters.

Exceptions to the Blood-Brain Barrier

• Not all brain areas have the same level of protection.
• Some regions have a more permeable BBB for specific functions.

Example: The Area Postrema (Vomiting Centre)

• The area postrema is in the hindbrain.
• It detects toxic chemicals and triggers vomiting to remove harmful substances.
• The increased permeability allows quicker reaction to potential threats.

Summary of Key Concepts for the Blood-Brain Barrier

• The Blood-Brain Barrier (BBB) is a barrier protecting the brain.
• Tightly packed cells prevents pathogens and toxins from entering.
• Selective permeability allows lipid-soluble molecules to pass while others need transport proteins.
• The area postrema in the hindbrain helps detect toxins and induce vomiting.

Recap: Why is the Blood-Brain Barrier Important?

• The BBB protects the brain, prevents infections, and maintains brain function.
• There is selective transport of nutrients and adaptability in the area postrema for toxin detection.