Neurons Chapter 8: Cellular and Network Properties

Neurons: Cellular and Network Properties

Organization of the Nervous System

• Nervous tissue is composed of neurons and glial cells
• The central nervous system (CNS) consists of the brain and spinal cord
• The peripheral nervous system (PNS) consists of nerves outside the CNS and brain
• The PNS has two divisions: sensory (afferent) and motor (efferent)

Cells of the Nervous System

• Neurons are the functional units of the nervous system, carrying electrical signals
• Glial cells provide support and structure, outnumbering neurons 5:1
• There are four types of glial cells in the CNS: astrocytes, oligodendrocytes, microglia, and ependymal cells
• Glial cells have different functions, such as wrapping around axons to form myelin sheaths and providing support to neurons

Neuron Anatomy

• A neuron consists of a cell body (soma), dendrites, and an axon
• Dendrites receive incoming signals, while axons carry outgoing signals
• Axons are covered by a myelin sheath, which insulates and speeds up signal transmission
• Multipolar neurons have multiple dendrites, while unipolar and bipolar neurons have one or two dendrites, respectively

Synapses

• A synapse is the region where an axonal terminal meets a target cell
• Synapses can be chemical or electrical, and are not fixed for life
• Chemical synapses involve the release of neurotransmitters from the presynaptic cell to the postsynaptic cell

Axonal Transport

• Axonal transport is the process of moving materials along the axon
• Anterograde transport moves materials from the cell body to the axon terminal, while retrograde transport moves materials in the opposite direction
• Neural fatigue can occur when axonal transport is disrupted, preventing the transmission of signals

Electrical Signals in Neurons

• Neurons are excitable tissue, able to generate electrical signals
• Membrane potential is influenced by the concentration gradient of ions and membrane permeability
• The Goldman-Hodgkin-Katz equation calculates the membrane potential based on the contribution of all ions that can cross the membrane

Membrane Potential

• Resting membrane potential is the membrane potential of a resting cell
• Graded potentials are temporary, localized changes in resting potential, caused by a stimulus
• Action potentials are electrical impulses that propagate along the axon to the synapse
• Ion movement creates electrical signals, with very few ions moving to create large changes in membrane potential

Ion Channels and Gated Ion Channels

• Ion channels are named for the primary ion that passes through them
• Gated channels control ion permeability, and can be mechanically gated, chemically gated, or voltage-gated
• Threshold voltage varies from one channel type to another, and activation and inactivation rates also vary

Action Potentials

• Action potentials are brief, large depolarizations that allow for rapid signaling over long distances
• Conduction is the high-speed movement of an action potential along an axon
• Action potentials are all-or-none, with a wave of electrical signal at constant amplitude

Generation of an Action Potential

• Na+ and K+ move across the membrane during action potentials
• Action potentials begin when a graded potential reaches the trigger zone and depolarizes to threshold
• Voltage-gated Na+ channels open, allowing Na+ to flow into the cell, and then close, allowing K+ to flow out of the cell

Axonal Na+ Channels

• Axonal Na+ channels have two gates: activation and inactivation gates
• Action potentials will not fire during the absolute refractory period, due to the resetting of voltage-gated Na+ channels

Speed of Conduction

• Axon diameter affects the propagation speed of action potentials
• Larger diameter axons have faster propagation speeds
• Myelination also increases the speed of conduction

Demyelination Disorders

• Demyelination can occur due to various factors, such as heavy metal poisoning, bacterial toxins, and autoimmune disorders

• Multiple sclerosis is a demyelinating disorder affecting the CNS

• Guillain-Barré syndrome is a demyelinating polyneuropathy affecting the PNS### Ion Channels and Action Potentials

• Ion channels for graded potentials allow for varying degrees of opening, whereas those for action potentials are all-or-none.

• Action potentials occur in axons and are essential for long-distance communication.

• They are all-or-none because a single action potential always results in the same amount of neurotransmitter release.

Myelin and Motor Neurons

• Myelin insulates axons and increases the speed of action potential propagation.
• If all motor neurons lost their myelin, muscle activity would be severely impaired.

Cell-to-Cell Communication in the Nervous System

• Neurons communicate at synapses, which can be electrical or chemical.
• Electrical synapses use gap junctions to pass electrical signals.
• Chemical synapses use neurotransmitters that cross synaptic clefts and bind to specific receptors.

Neurotransmitters and Receptors

• Acetylcholine (ACh) binds to cholinergic receptors, including nicotinic and muscarinic receptors.
• Nicotinic receptors are ionotropic and found on skeletal muscles and in the autonomic division of the PNS and CNS.
• Muscarinic receptors are metabotropic and found in the CNS and on target cells for the autonomic parasympathetic division of the PNS.

Amines and Adrenergic Receptors

• Adrenergic neurons secrete norepinephrine, while the adrenal medulla secretes epinephrine.
• Adrenergic receptors bind norepinephrine and epinephrine and are G protein-coupled receptors.

Synaptic Communication and Neurotransmitter Release

• Stronger stimuli result in more neurotransmitter release, but a single action potential always releases a constant amount of neurotransmitter.
• Action potentials code duration and magnitude in the frequency of action potentials produced.

Integration of Neural Information

• Divergence occurs when one neuron sends signals to multiple neurons, while convergence occurs when multiple neurons send signals to one neuron.
• Synaptic plasticity refers to changes in activity at synapses and can be short-term or long-term, enhancing or reducing synaptic activity.

Postsynaptic Responses

• Slow synaptic potentials involve G-protein coupled receptors and second messengers.
• Fast synaptic potentials involve opening of ion channels.
• Excitatory postsynaptic potentials (EPSPs) are depolarizing, while inhibitory postsynaptic potentials (IPSPs) are hyperpolarizing.

Pathways and Integration of Information

• Spatial summation occurs when two or more neurons simultaneously fire and have an additive effect.
• Temporal summation occurs when graded potentials overlap in time and have an additive effect.
• Synaptic activity can be modified by modulatory neurons, presynaptic facilitation, or presynaptic inhibition.

Long-Term Potentiation (LTP) and Synaptic Transmission

• LTP alters synapses and is responsible for acquired behaviors, and is likely involved in learning and memory.
• Glutamate is a key element in potentiation, and disorders of synaptic transmission are responsible for many diseases, including Parkinson's disease, schizophrenia, and depression.

This quiz covers the properties of neurons, including their cellular structure, electrical signals, and cell-to-cell communication in the nervous system. It also explores the integration of neural information transfer and disequilibrium's role in it.