Cells of the Nervous System and Neuron Function
Cells of the Nervous System
Introduction to Nervous System
The nervous system consists of two main types of cells: Neurons and Glia.
Historical Context
Before the late 1800s, microscopy was limited and could not reveal much detail about the nervous system.
Nobel Prize in Physiology or Medicine (1906): Ramón Cajal and Camillo Golgi were awarded for their discovery concerning the structure of neurons.
Cajal utilized the chemical/staining techniques developed by Golgi to reveal that a small gap separates the tip of a neuron's fiber from the surface of the next neuron.
This finding emphasized that the brain, similar to other parts of the body, is composed of individual cells.
Neuron Population in the Human Brain
Neuron Count
The human brain contains approximately 86 billion neurons.
Each neuron receives signals from about 1,000 other neurons, leading to a vast network of connections.
This results in far more synapses in the brain than there are stars in the Milky Way, estimated at 400 billion stars.
Neurons: Basic Structure and Function
Characteristics of Neurons
Neurons are distinguished from other body cells by their long branching extensions.
Each neuron typically has:
Soma (Cell Body): Contains the nucleus and other organelles.
Dendrites: Receive information from other neurons.
Axon: Sends information to other neurons, organs, or muscles.
Presynaptic Terminals: Release neurotransmitters to communicate with neighboring cells.
Structure of a Neuron
Dendrites:
Branching fibers that are narrower at their ends.
Lined with synaptic receptors that receive information from other neurons.
Sensory receptors respond to various stimuli:
Light (vision)
Mechanical deformation (touch, pain)
Pressure changes in the air (hearing)
Molecules in the air (smell)
Molecules in liquid (taste)
These receptors convert environmental energy into electrical energy, a process called transduction.
Example in Vision: Light enters the eye through the pupil, activating photoreceptors in the retina, and the electrical signals travel through approximately 1 million axons that constitute the optic nerve.
Axon:
A thin fiber with a consistent diameter that conveys impulses toward other neurons, organs, or muscles.
Axons can exceed 1 meter in length (e.g., from the spinal cord to feet).
Neurons can have multiple dendrites but only one axon, which may have branches.
At the end of each branch is the presynaptic terminal, which releases chemicals across the synaptic junction.
Soma (Cell Body):
Contains the nucleus, ribosomes, and mitochondria.
Membrane:
Neurons are surrounded by a cell membrane, which separates the internal cell environment from the external environment.
Nucleus: Houses chromosomes.
Mitochondria: Responsible for metabolic activities and energy production.
Ribosomes: Sites for protein synthesis, providing building materials for the cell.
Glia Cells: Supportive Roles in the Nervous System
Types of Glia Cells
Glia (Neuroglia): Constitute the other significant component of the nervous system. Key types include:
Astrocytes: Star-shaped cells that wrap around synapses of related axons to protect them from outside chemicals. They also facilitate chemical exchanges between neurons.
Microglia: Small cells that serve as part of the immune system by removing viruses and fungi from the brain and proliferating in response to brain damage to eliminate dead or damaged neurons.
Radial Glia: Guide neuron migration and axon/dendrite growth during embryonic development.
Oligodendrocytes: Located in the brain and spinal cord to form myelin sheaths around certain axons.
Schwann Cells: Perform a similar function as oligodendrocytes but are located in the peripheral nervous system.
Nerve Impulses: Resting Potential
Electrical Gradient and Resting Potential
The neuron membrane is approximately 8 nanometers (nm) thick, separating the internal cellular environment from the external.
At rest, the membrane maintains an electrical gradient, also known as polarization—a difference in electrical charge between the inside and outside of the cell.
The typical resting potential is around –70 millivolts (mV), indicating a negative charge inside relative to the exterior.
Microelectrodes are utilized to record the electrical signals of individual neurons.
Understanding the Resting Potential and Chemical Concentration
Ion Movement and Membrane Permeability
Charged ions do not flow freely across the membrane due to selective permeability, meaning some ions cross more freely than others:
Oxygen, carbon dioxide, urea, and water: Can freely cross through always-open channels.
Notably, sodium (Na) ions and potassium (K) ions pass through channels that open and close in response to stimuli.
Sodium-Potassium Pump
The sodium-potassium pump is a protein complex that actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell.
This pump is a crucial mechanism for maintaining resting potential:
Sodium ions are more concentrated outside the membrane, while potassium ions are concentrated inside.
The body uses significant energy to operate this mechanism, preparing the neuron for rapid response.
Stimulation of a neuron permits the influx of sodium, resulting in an action potential due to the change in internal charge.
Nerve Impulses: Action Potential
Definition and Activation
Disturbance of the resting potential initiates the transmission of information via neurons, referred to as action potentials.
When stimulation exceeds a certain threshold, a significant depolarization of the membrane occurs:
Threshold: The point at which sodium channels in the neuron membrane open, allowing sodium ions to flow into the cell.
All-or-None Principle
Any depolarization meeting or surpassing the threshold results in an action potential, adhering to the all-or-none law:
In any given axon, the amplitude and velocity of an action potential remain constant, independent of stimulus intensity, as long as the threshold is reached.
Note: Amplitude, velocity, and shape of action potentials can differ from one neuron to another.
Thicker axons transmit action potentials at greater velocities and can signal more action potentials per second.
Graded Potentials vs. Action Potentials
The system distinguishes between weak and strong stimuli through graded potentials, where the potential varies according to stimulus strength instead of following the all-or-none principle.
Communication Between Neurons
Transmission of Information
Upon reaching the end of a neuron, the electrical impulse prompts a chemical process transmitting signals across the synapse to subsequent neurons.
Synaptic vesicles open and release neurotransmitters, which bridge the gap between neurons.
Most neurons possess a synaptic space separating an axon’s end from the next neuron.
Afferent and Efferent Neurons
Afferent Neurons: Convey information into another neuron or structure (e.g., sensory neurons that bring information to the nervous system).
Efferent Neurons: Carry information away from a neuron or structure (e.g., motor neurons that transmit commands from the nervous system).
Interneurons: Neurons where both dendrites and axon are contained within a single structure (e.g., intrinsic neuron of the thalamus).
Properties of Neurons
Lateral Inhibition and Receptive Fields
Lateral Inhibition: The process by which an excited neuron diminishes the activity of neighboring neurons.
Receptive Field: The spatial area where a stimulus activates a neuron, causing sensitivity to stimuli.
Ganglion Cells: Neurons in the retina that receive input from others and whose axons form the optic nerve:
On-centre/off-surround cells: Excited by light in the center and inhibited by light in the surrounding field.
Off-centre/on-surround cells: Inhibited by light in the center and excited by surrounding light.
Example: Hermann Grid Illusion
The response pattern of ganglion cells can lead to visual illusions (e.g., black dots appearing in a grid) caused by lateral inhibition during stimulation.
Cool Neurobiology Insights
Biological Neurons and Machine Learning
Biological neurons amalgamate multiple signals via synapses on dendrites and output a single stream of action potentials through their axon.
This conceptual framework inspired models for artificial neural networks and deep learning systems, where multiple inputs combine to produce one output signal.
Propagation of the Action Potential
Mechanism of Action Potential Propagation
Rather than conducting an electrical impulse uniformly, the axon regenerates the action potential at various nodes along its length.
Myelin's Role in Action Potential Propagation
Importance of Myelin
In the thinnest axons, action potentials may travel at a velocity of less than 1 m/s, while axons with increased diameter can transmit impulses up to 10 m/s.
For example, an impulse between a giraffe's spinal cord and foot reaches the destination in about half a second.
The evolutionary development of myelin—a lipid-rich insulating sheath—enhances conduction speed:
Myelin sheaths wrap around axons, interspersed with nodes of Ranvier.
Saltatory Conduction
In myelinated axons, the action potential begins at the initial nodes of Ranvier. Once an action potential occurs at one node, sodium ions diffuse and push a positive charge to the next node, allowing faster transmission compared to the sequential regeneration of action potentials.
This rapid jumping mechanism of action potentials is termed saltatory conduction (Latin: saltare = “to jump”).
Clinical Relevance: Multiple Sclerosis
In the case of multiple sclerosis, immune system attacks lead to damage of myelin sheaths. Consequently, many action potentials fail to propagate between nodes, resulting in various neurological impairments (e.g., visual problems, poor muscle coordination).
White Matter in the Central Nervous System
Composition of White Matter
The central nervous system comprises white matter and gray matter.
White Matter: Contains bundles of myelinated axons, also known as white matter tracts.
The white appearance is due to the fatty myelin that envelops the axons.
White matter is typically situated in deeper brain regions and serves as the communication network connecting different gray matter areas, facilitating coordinated brain functions.
Relevant Neuroscientific Technique
The organization and structural characteristics of white matter can be analyzed using Diffusion-weighted Imaging via MRI technology.
Bibliography and Reading Material
Kalat, J. W. (2017). Biological Psychology (13th ed.). Cengage. Chapter 1.
Next Lecture: Synapses
Introduce the topic of synapses in the following session.