BIOS5130 Week 9 Lecture Part 1 Slides

Central Nervous System Overview

Presented by Dr. Emma Hargreaves, Week 9Content based on Silverthorn, Human Physiology

1. Organization of the Nervous System

1.1 Components

• Central Nervous System (CNS): Comprises the brain and spinal cord, serving as the main control center for processing information and directing responses.

• Peripheral Nervous System (PNS): Acts as a communicator between the CNS and the rest of the body, divided into two primary divisions:

  • Sensory Division: Sends information to the CNS via afferent neurons, facilitating the perception of stimuli from the environment.

  • Efferent Division: Transmits commands from the CNS to target cells, signaling muscles and glands to act upon stimuli.

1.2 Structure

• The CNS is protected by:

  • Bone: The cranium encases the brain and the vertebral column surrounds the spinal cord, providing structural support and protection against trauma.

  • Meninges: Comprising three protective membranes - the dura mater (outer), arachnoid (middle), and pia mater (inner), these layers safeguard the CNS.

  • Cerebrospinal Fluid (CSF): A clear fluid that cushions the CNS, providing physical protection, reducing shock, and regulating the ionic environment essential for neuronal function.

  • Blood-Brain Barrier: A selective permeability barrier that protects the brain from potential toxins while allowing essential nutrients such as glucose and amino acids to pass through.

2. Neuron Structure and Function

2.1 Neuron Composition

• Synapse: The contact point between neurons or between a neuron and muscle, crucial for signal transmission.

  • Presynaptic Neuron: The neuron that sends the signal, releasing neurotransmitters into the synaptic cleft.

  • Postsynaptic Neuron: The neuron that receives the signal, with receptors that bind to neurotransmitters, initiating or inhibiting a response.

2.2 Glial Cells

• Glial cells support neurons and significantly outnumber them by a ratio of 10 to 50:1. They play vital roles in maintaining homeostasis, forming myelin, and providing support and protection for neurons, essential for the normal functioning of the nervous system.

3. Membrane Potential

3.1 Basics

• Electrical potential across the cell membrane is defined as the difference in charge, crucial for the generation and transmission of action potentials.

• Resting Membrane Potential: The state of a neuron when it is not transmitting signals, typically around -70 mV, indicating that the inside of the neuron is negatively charged relative to the outside.

3.2 Ion Composition

• Extracellular Fluid: Rich in sodium (Na+) and chloride ions (Cl-), contributing to the positive charge outside the neuron.

• Intracellular Fluid: Abundant in potassium (K+), phosphate ions, and negatively charged proteins, maintaining the negative charge within the neuron.

4. Gated Ion Channels

4.1 Types

• Voltage-gated Channels: Open in response to changes in membrane potential, playing a key role in initiating action potentials.

• Ligand-gated Channels: Open in response to the binding of neurotransmitters, mediating synaptic transmission.

• Mechanically-gated Channels: Respond to physical stimuli, such as stretch or vibration, important for sensory modalities like touch and hearing.

5. NA+/K+ ATPase Function

5.1 Role

• The Na+/K+ ATPase pump maintains the ionic composition of the neuron by pumping three Na+ ions out and two K+ ions in per ATP hydrolyzed. This activity is crucial for creating and maintaining the resting membrane potential, ensuring that a neuron can respond to stimuli effectively.

6. Action Potentials

6.1 Characteristics

• All-or-None Phenomenon: Action potentials occur only if the threshold is reached, triggering a full depolarization.

• Frequency: The frequency of action potentials encodes stimulus strength; stronger stimuli produce more rapid firing of action potentials, allowing the CNS to interpret the intensity of stimuli.

• Distinction from Graded Potentials: Unlike graded potentials, which vary in magnitude, action potentials are uniform and propagate without diminishing.

6.2 Conduction

• Saltatory Conduction: In myelinated axons, action potentials jump between nodes of Ranvier, enhancing conduction speed significantly, allowing efficient signal transmission.

• Conduction Speed: Myelinated axons conduct signals at speeds of up to 120 m/sec, whereas unmyelinated axons conduct at speeds of approximately 2 m/sec.

7. CNS Anatomy and Function

7.1 Basic Structure

• Brain: The brain is primarily dominated by the cerebrum, which is responsible for higher cognitive functions, sensory perception, and motor control. The cerebellum, located below the cerebrum, coordinates movement and balance, while the brainstem connects the brain to the spinal cord and controls basic life functions.

• Cerebrospinal Fluid (CSF): Produced by the choroid plexus within the ventricles of the brain, it cushions and protects the brain from trauma while facilitating the removal of waste products.

7.2 Protective Structures

• Meninges: Three major layers surround the brain and spinal cord, acting to protect and contain CSF.

  • Dura Mater: The tough outer layer that provides a durable protective sheath.

  • Arachnoid Membrane: The middle layer, which is web-like in structure.

  • Pia Mater: The thin inner layer that closely follows the contours of the brain and spinal cord.

• CSF Functions: It provides physical and chemical protection, maintains a stable ionic composition, and facilitates nutrient transport and waste removal.

8. Blood-Brain Barrier (BBB)

8.1 Importance

• The brain requires 15% of the body's total blood output, demonstrating its high oxygen and glucose requirement for metabolism and function.

• The BBB provides a shield against harmful toxins while allowing essential nutrients to pass through, maintaining homeostasis within the CNS.

8.2 Structural Components

• Tight Junctions: Located between the endothelial cells of capillaries, they create a selective barrier that restricts passage of substances.

• Astrocytes: Glial cells that induce tight junction formation and facilitate nutrient transport across the BBB, playing a vital role in maintaining CNS health.

9. Brain Structure and Functional Regions

9.1 Major Divisions

• Cerebrum: Responsible for sensory perception, voluntary motor control, higher cognitive functions, and memory.

• Cerebellum: Essential for coordinating movements, balance, and fine motor skills.

• Diencephalon: Integrates sensory and motor information and plays a vital role in regulating autonomic functions.

9.2 Functional Areas of the Cerebral Cortex

• Various functional areas for sensory perception, motor control, and higher cognitive functions are distributed across the lobes, allowing for the complex integration of information and decision-making.

10. Spinal Cord Anatomy

• Functional Tracts: The spinal cord contains ascending tracts that carry sensory information to the brain and descending tracts that convey motor commands from the brain to peripheral targets.

• Sensory nerves enter through the dorsal side of the spinal cord, while motor nerves exit via the ventral horn, creating a structured pathway for communication within the nervous system.

11. Imaging Techniques

11.1 Methods

• Electroencephalography (EEG): A non-invasive method that measures electrical activity in the brain, useful for diagnosing conditions such as epilepsy and sleep disorders.

• Functional MRI (fMRI): This imaging technique assesses blood flow linked to neuronal activity, enabling visualization of brain function during various tasks, providing insights into brain connectivity and activity.

12. Learning Outcomes

• Review Chapter 9 in Silverthorn: Focus on understanding the organization, protection, and anatomy of the CNS, the functions across different brain regions, mechanisms of myelination, and explore the relevant sections in the Moodle book to reinforce learning and comprehension.