Neuroanatomy #3

3 Types of Neuroglia:

1. Myelinating 2. Signaling/Cleaning/Nourishing 3. Defending

The inside of the neuron contains more negative charges than the outside. This electrochemical gradient is maintained by the following:

1. Negatively charged molecules trapped inside the neuron because they are too large to discuss through the channels. 2. Passive diffusion of ions through leak channels in the cell membrane. 3. A sodium-potassium (Na+-K+) pump.

These types of electrical potentials in neurons are essential for transmission of information:

1. Resting membrane potential 2. Local potential 3. Action potential

Typical Neuron has 4 main components:

1. Soma (cell body) 2. Dendrites 3. Axon 4. Presynaptic terminals

Carry sensory information from the outer body toward the central nervous system (CNS).

Afferent Neurons

Cellular mechanism that transports substances along an axon

Axoplasmic Transport

• Rare and act as specialized sensory processing • Eyes and nose

Bipolar cells

Dendrites:

Convey incoming messages (electrical signals) toward the cell body.

• Branch-like extensions that serve as the main input sites for the cell. • Project from the soma. • Specialized to receive information from other cells.

Dendrites

excitatory and means the neuron is more likely to generate a transmittable electrical signal.

Depolarization

Demyelination: PNS Disorders - Peripheral Neuropathy

Destruction of myelin surrounding largest most myelinated sensory and motor fibers resulting in decreased proprioception and weakness.

Relay commands from the CNS to smooth and striated muscles and to glands.

Efferent Neurons

Axons

Generate nerve impulses and conduct them away from the cell body • Neurons may have hundreds of dendrites but only one axon.

means the potential has become more negative and this inhibits and decreases the ability of the neuron to generate an electrical signal

Hyperpolarization

• The largest class of neurons. • Act throughout the nervous system. • They process information locally and convey information short distances. • Local reflexes in the Spinal Cord are controlled by?

Interneurons

Membrane Channels

Leak channels allow diffusion of a small number of ions through the membrane at a slow continuous rate. • All channels serve as openings through the membrane.

Anterograde transport:

Moves neurotransmitters and other substances from the soma down the axon toward the presynaptic terminal

Retrograde transport

Moves substances from the synapse back to the soma.

• Multiple dendrites from many regions of the cell body and a single axon. • Most common type of cell in the human nervous system. Variety of shapes and organizations. • Specialized to receive huge amounts of synaptic input to their dendrites.

Multipolar Cells

• Multiple processes extending from cell body • Most common structural type • All motor and association neurons are this type

Multipolar Neuron (Based on the number of processes extending from the cell body)

Ions that diffuse through the openings:

Na+, K+, Cl-, Ca2+

• "Nerve Glue" • Many types of cells that support, insulate, and protect neurons. • Also called glia or glial cells.

Neuroglia

Transmit messages (nerve impulses) from one part of the body to another. • Receive information. Process it. Generate an output

Neurons (Nerve cells)

• They bring information from the body into the spinal cord. • The peripheral axon conducts sensory information from the periphery to the cell body. • The central axon conducts information from the cell body to the spinal cord. • One soma supports both axons.

Pseudounipolar cells

• Metabolic center of the neuron. • Transparent nucleus has a nucleolus. • Cytoplasm surrounds the nucleus and contains organelles.

Soma

Synthesizes large quantity and variety of proteins used as neurotransmitters.

Soma (cell body)

• Projects from the spinal cord to innervate skeletal muscle fibers. • Typical motor cell receives 8,000 synapses on its dendrites and 2000 synapses on the cell body itself. • Purkinje cells (multipolar cells in the cerebellum) receive 150,000 synapse on their expansive dendritic trees.

Spinal motor neurons are an example

Types of Neurons • Four general components of neurons (soma, dendrites, axons, and presynaptic terminal) all stay the same. • Organization of the parts is what varies depending on the type of neuron.

Two groups: 1. Bipolar Cells 2. Multipolar Cells Classification is based on the number of processes that arise from the cell body.

Axoplasmic Transport occurs in two directions:

anterograde and retrograde

All axons branch at their terminal end forming hundreds to thousands of __________________.

axon terminals

Neurotransmitters are released into the _______________________.

extracellular space

Axon terminals contain vesicles that contain chemicals called __________________.

neurotransmitters

Ligand-gated

open in response to a neurotransmitter binding to the surface of a channel receptor on a postsynaptic cell membrane. A local potential is generated when these gates are open and electrically charged ions are free to flow between extracellular and intracellular environments.

Voltage-gated

open in response to changes in electrical potential across the cell membrane. Open and close almost instantaneously. Important for action potential and release of neurotransmitters to adjacent cells.

The neurotransmitter diffuses from one side of the cleft to the other, and it binds to the receptors on the

postsynaptic neuron, muscle cell, or gland.

Modality-gated

specific to sensory neurons and open in response to mechanical forces (stretch, touch, pressure), temperature, or chemicals.

The _________________ is the functional junction between neurons (they never actually touch each other).

synapse

Axon terminals are separated from the next neuron by a _____________________.

synaptic cleft

Neurons transmit information about their activity via the release of chemicals called neurotransmitters from the presynaptic terminal into the ___________________.

synaptic cleft

synaptic cleft

the space between a synaptic terminal of one neuron and the target cell of another neuron (postsynaptic neuron). This is where interneuronal communication occurs.

Pseudounipolar Cells

• A subclass of bipolar cells, appear to have a single projection from the cell body that divides into axonal roots. • These cells have two axons and no true dendrites.

Signaling/Cleaning/Nourishing: Astrocytes

• Abundant and star-shaped. • Account for half of the neural tissue. • Anchor and brace neurons to their nutrient supply, the blood capillaries. • Form barrier between capillaries and neurons. • Protect neurons from harmful substances that could be in blood. • Recapture released neurotransmitters as needed to control chemical environment in brain.

Demyelination: PNS Disorders - Guillain-Bare Syndrome

• Acute demyelination of sensory and motor fibers. Autoimmune condition where antibodies are generated to attack Schwann cells. • Symptoms include acute onset decreased sensation and skeletal muscle paralysis, progressing distal to proximal, with plateau, then gradual recovery.

Microglia and Astrocytes excessive activity, causing lose of physiologic function and release of toxic components in the neural environment is associated with the following diseases/disorders:

• Alzheimer's Disease • Parkinson's Disease • MS • ALS • CVA • HIV/AIDS

Two forces act on ions to determine their distribution across the plasma membrane:

• Concentration Gradient • Electrical Gradient • These forces control the movement of ions. Equilibrium of distribution of a specific ion is reached when there is no net movement of that ion across the membrane.

Local Potentials

• Electrical potentials within each neuron conduct information in a predictable and consistent direction • The initial change in membrane potential is called the local potential because it spreads only a short distance across the membrane. • Categorized as receptor potentials or synaptic potentials, depending on whether they are generated at a peripheral receptor (sensory neuron) or at a postsynaptic membrane (motor and interneuron). • Spread only passively and confined to a small area of the membrane.

Axon

• Extends from the soma. • Reaches from the cell body to target cells. • Output unit of the cell specialized to send information to other neurons, muscle cells, or glands. • Most neurons have a single axon that arises from a specialized region of the cell (axon hillock). • 1 mm to 1 m long. • End in the presynaptic terminal.

Myelinating: Oligodendrocytes (CNS)

• Glia that wrap their flat extensions tightly around the nerve fibers. • Produce fatty insulating coverings called Myelin Sheaths.

Bipolar Cells

• Have two primary processes that extend from the cell body: 1. Dendritic Root (divides into multiple dendritic branches) 2. Axon (projects to form its presynaptic terminals)

Demyelination

• If an action potential reaches an axon with damage or loss of myelin, there is a resistance to conduction of the signal. The current slows and eventually may stop before it reaches the next site of conduction.

Action Potentials

• If the local potential activates a significant enough charge in the neuron, an action potential is generated. • Brief, large depolarization in electrical potential that is repeatedly regenerated along the length of an axon. • Essential for rapid movement of information over long distances. • Regeneration allows an action potential to actively spread long distances, transmitting information down the axon to presynaptic chemical release sites of the presynaptic terminal. • Unlike local input signals (receptor or synaptic signals), action potentials do NOT vary in amplitude. It is all or none. • This means that every time even minimally sufficient stimuli are provided, an action potential will be produced. • Example: pushing a key on a keyboard. Hard or lightly, as long as a sufficient amount of pressure is achieved, the shape of the letter is the same.

Demyelination: CNS Disorders - Multiple Sclerosis

• Immune system produces antibodies that attack oligodendrocytes. • Produces patches of demyelination called plaques in the white matter of the CNS. • Results in slowed and blocked transmission of signals. • Weakness, lack of coordination, impaired vision, impaired sensation, slurred speech, disruption of memory and emotions may occur.

Conduction originates with local potentials at the receiving sites of the neuron:

• In sensory neurons = sensory receptor • In motor and interneurons = postsynaptic membrane

Four types of membrane channels allow ions to flow across the membrane:

• Leak channels • Modality-gated channels • Ligand-gated channels • Voltage-gated channels

Local Potentials: Motor Neuron + Interneuron

• Local synaptic potentials are generated when they are stimulated by input from other neurons. • When a presynaptic neuron releases its neurotransmitter, the chemical travels across the synaptic clefts and interacts with the chemical receptor sites on the membrane of the postsynaptic cell. • Binding of the neurotransmitter opens ligand-gated ion channels, which changes the resting membrane potential of the cell. • Graded in both amplitude and duration: if the neurotransmitter is available in large amounts and for longer time, the synaptic potential will be larger and longer lasting.

Saltatory Conduction

• Myelinated axons have small patches that lack myelin called nodes of Ranvier. • Rapid propagation of an action potential along a myelinated axon, because the action potential quickly jumps from node to node. • Nodes are Ranvier are the only site in myelinated axons where ion exchange across the membrane occurs.

Propagation of Information by Neurons

• Neurons function by undergoing rapid changes in electrical potential across the cell membrane. • An electrical potential across a membrane exists when the distribution of ions creates a difference in electrical charge on each side of the cell membrane.

Direction of Information Flow in Neurons

• Normally, information within a neuron is propagated in only one direction. • Depending on its role in the direction of information transfer, a neuron falls into one of three functional groups: 1. Afferent Neurons 2. Efferent Neurons 3. Interneurons

Myelinating: Schwann Cells (PNS)

• Only support cells in the PNS. • Form the Myelin Sheaths around nerve fibers in PNS. • Also can act as phagocytes when the cell is inflamed (cells that ingest and destroy bacteria). • Can also provide factors for repair when peripheral nerves are damaged.

A rapid change in electrical charge across the cell membrane transmits information along the length of an axon and elicits the release of chemical transmitters to:

• Other neurons • The electrically excitable membrane of a muscle or gland

Local Potentials: Sensory Neurons

• Peripheral receptors (sensory) have modalitygated channels. Potential is generated when the peripheral receptors of the sensory neuron are stretched, compressed, deformed, or exposed to thermal or chemical agents. • This causes modality-gated channels to open, encoding the sensory information into a flow of ionic current. • Sensory receptor potentials can be depolarizing (excitatory) or hyperpolarizing (inhibitory).

Divergence

• Process whereby a single axon may have many branches that terminate on multiple cells. • Pinprick activates end-receptors of a sensory neuron that transmits message to multiple neurons in the spinal cord to elicit a motor response to move many body parts.

Microglia are beneficial when they:

• Remove debris • Produce neurotrophic factors that support axonal regeneration and remyelination • Mobilize astrocytes to reseal blood-brain barrier

Neuroinflammation

• Response of the CNS to infection, disease, and injury. • Mediated by reactive microglia and astrocyte. • Can kill neurons and oligodendrocytes and inhibit neural regeneration.

Changes from Resting Membrane Potential

• Results from the flow of electrically charged ions through voltagegated channels. • The membrane is depolarized when the potential becomes less negative than the resting potential (-70mV to -55 mV).

Propagation of Action Potentials

• Some axons are specialized for faster action potential propagation. • Structural Adaptations: 1. Increased Diameter of the Axon 2. Myelination • Presence of a sheath of protein and fats surrounding the axon. • Increases the speed of action potential propagation and the distance the current can passively spread. - Thicker myelin leads to faster conduction.

Defending: Microglial Cells

• Spider Like phagocytes. • Act as the immune system of the CNS. • Dispose of debris. • Clean up dead brain cells and bacteria. • Active during development and following injury, infection, or disease.

Convergence

• The process by which multiple inputs from a variety of cells terminate on a single neuron. • Neural input from sensory association areas in the cerebral cortex, where information from hearing, vision, and touch are integrated.

Resting Membrane Potential

• The value of the electrical potential across the membrane when a neuron is not transmitting the information. • A steady-state condition with no net flow of ions across the membrane. • There is no net charge in the total distribution of ions across the two sides, even though some individual ions may continually move across the membrane through leak channels. • When the neuron is resting the cell membrane serves as a capacitor, separating the electrical charges on either side of the plasma membrane. • An unequal distribution of charge across the membrane is essential for the neuron to become excitable. • In a resting neuron the membrane potential is the difference in voltage between the interior and exterior of the neuron. • Typical resting membrane potential is -70 mV.

The Na+-K+ Pump

• Uses Adenosine Triphosphate (ATP) to actively move ions across the membrane against their electrochemical gradient. • The pump carries two K+ ions into the cell and three Na+ ions out of the cell with each cycle. • Leaves an associated negative charge inside of the ion as long as the cell has ATP.