Neuroscience: Neuron Functions and Properties

Neurology Microanatomy & Physiology

• Neurons are the functional units of the nervous system
• Neurons have high oxygen requirements
• Neurons cannot reproduce via mitosis
• Neurons can regenerate their processes if the cell body remains intact
• Neurons receive and send impulses

Basic Neuron Structure

• Cell body (soma or perikaryon)
• Dendrites receive signals from other cells
• Axons transmit signals to other cells
• Dendrites are afferent processes; numerous and branching
• Axons are efferent processes; a single extension from each neuron
• A nerve is a bundle of axons

Neurons Continued

• Axons are covered by a fatty sheath called myelin
• Myelin gives white matter its white appearance
• Oligodendrocytes are the glial cells in the brain and spinal cord, producing myelin
• Schwann cells are glial cells in peripheral nerves, producing myelin

Myelin Sheath

• Oligodendrocytes and Schwann cells form a tight wrap around axons
• The myelin sheath lines the axon end to end
• Nodes of Ranvier are small gaps between myelin segments
• Myelin increases the speed of nerve impulse conduction; myelinated is faster than unmyelinated

Afferent Impulses

• Afferent nerve fibers conduct impulses from the periphery to the central nervous system (CNS)
• Typically sensory in nature
• Efferent nerve fibers conduct impulses from the CNS to the periphery
• Typically motor in nature
• Nerves are bundles of afferent and efferent axons
• Sensory nerves contain only sensory fibers
• Motor nerves contain only motor fibers
• Mixed nerves contain both sensory and motor fibers

Autonomic vs. Somatic

• Somatic nervous system is conscious, involving both motor and sensory nerves and impulses to skeletal muscle
• Autonomic nervous system is unconscious, involving both motor and sensory nerves and impulses to smooth, cardiac muscle, and glands

Depolarization & Repolarization

• A nerve "firing" is depolarization
• Repolarization is the return to the resting state

Sodium Potassium ATPase Pump

• At rest, the neuron is polarized
• The sodium-potassium pump maintains this polarization
• 2 potassium ions are pumped inside the cell from outside
• 3 sodium ions are pumped outside the cell from inside
• The membrane is polarized because one side is more positively charged (outside) and the other more negatively charged (inside)

Resting Membrane Potential

• Resting membrane potential is the difference in electrical charge across the neuron.
• The resting membrane potential is -70 mV

Depolarization

• Stimulation of a nerve opens sodium channels
• Sodium flows into the cell by diffusion
• The inside of the cell becomes positively charged (depolarization)

Repolarization

• Sodium channels close, and potassium channels open
• Potassium flows out of the cell by diffusion
• The inside of the cell returns to its negative resting potential

Repolarization vs. Resting State

• Resting membrane potential: K+ inside, Na+ outside
• Repolarization: K+ outside, Na+ inside
• Na+/K+ ATPase pump restores the resting state

Threshold Stimulus

• A threshold of stimulus intensity is required for depolarization.
• Weak stimuli may cause some sodium channels to open, but not enough for depolarization.

Wave of Depolarization

• Adjacent channels open once a stimulus opens some sodium channels
• A wave of depolarization spreads through the neuron
• This process is called conduction of the action potential, which occurs in practical terms in a "nerve impulse."

All-or-Nothing Principle

• If a stimulus reaches threshold, the impulse is generated along the axon with the same strength.
• A strong or weak stimulus that reaches the threshold produces the same impulse.

Refractory Period

• This is the time when a neuron cannot produce another impulse
• Sodium and potassium channels are temporarily closed/open during this period
• Relative refractory period happens when cells may produce a depolarization response but only with a stronger stimulus.

Saltatory Conduction

• Myelin sheaths increase impulse speed, preventing sodium flow along the neuron, except at the Nodes of Ranvier.
• Impulse "jumps" from node to node.

Multiple Sclerosis

• An autoimmune disease that damages Schwann cells, destroying myelin sheaths.
• Reduces ability to quickly conduct nerve impulses
• Sensory and motor deficits

How Neurons Communicate: The Synapse

• The synapse is the junction between two neurons or a neuron and target cell; a gap (synaptic cleft)
• There is no physical contact between the cells
• Telodendron is a branched structure at the end of the pre-synaptic neuron; synaptic end bulbs or knobs
• Mitochondria supports the activity.
• Synaptic vesicles hold neurotransmitters

Synaptic Transmission

• Depolarization reaches the synaptic knob
• Calcium channels open, allowing extracellular calcium to enter the synaptic knob.
• Vesicles fuse with the membrane; releasing neurotransmitters into the synaptic cleft
• Neurotransmitters diffuse across the cleft

Postsynaptic Membrane

• Neurotransmitters bind to receptors on the postsynaptic membrane
• This binding results in a target cell response, such as the generation of a new action potential

Excitatory & Inhibitory Neurotransmitters

• Two types of neurotransmitters: Excitatory neurotransmitters typically increase sodium influx to move the postsynaptic membrane closer to threshold.
• Inhibitory neurotransmitters usually hyperpolarize, changing the cell to a more negatively charged state, making it harder to produce another action potential.

Neurotransmitters

• Some neurotransmitters are both excitatory and inhibitory, depending upon the cell they act on.
• Acetylcholine (ACh) is either excitatory or inhibitory.
• Catecholamines (e.g., norepinephrine, dopamine, epinephrine) are often involved in the "fight-or-flight" response.

Catecholamines

• Norepinephrine is involved in the flight-or-flight response.
• Epinephrine is also associated with flight-or-flight.
• Dopamine is involved in brain function, autonomic functions, and muscle control—it’s associated with Parkinson's.

GABA & Glycine

• GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter found in the brain.
• Some drugs increase GABA activity or inhibit brain activity.
• Glycine is primarily found in the spinal cord and is an inhibitory neurotransmitter.

Clinical Case: Metaldehyde Toxicity

• Slug bait toxicity resulting in a "shake-and-bake" effect.
• Pathophysiology: reduces inhibitory neurotransmitter (GABA) and lowers seizure threshold.
• Clinical signs: Severe muscle tremors, hyperesthesia, tachycardia, and hyperthermia.

Clinical Case: Other Neurotoxins

• Pesticides (e.g., strychnine, permethrin, bromethalin) can affect neurons.
• Strychnine inhibits glycine.
• Permethrin affects sodium channels (particularly in cats).
• Bromethalin causes demyelination and cerebral edema (often in rats)

Clinical Case: Gabapentin

• Extremely fractious or nervous cats can be calmer, with sedation, with Gabapentin.
• A drug that increases GABA activity.

Stopping and Recycling Neurotransmitters

• Neurotransmitters are removed from the synaptic cleft by enzymes.
• Acetylcholine is broken down by acetylcholinesterase.
• Organophosphates are common agricultural pesticides that inhibit the activity of acetylcholinesterase, causing an excess of acetylcholine in the synaptic cleft.

Recycling Neurotransmitters

• Some neurotransmitters, like norepinephrine, are broken down using enzymes such as MAO (monoamine oxidase) in the synaptic knob and COMT (catechol-O-methyltransferase) in the synaptic cleft.
• Some antidepressants work by inhibiting these enzymes.

Clinical Case: Myasthenia Gravis

• This is an autoimmune disease resulting in muscle weakness.
• Destruction of acetylcholine receptors, leading to decreased muscle strength.
• Diagnosed through testing (ACh antibody test and Tensilon test)
• Treated with drugs that prolong acetylcholine action in the synaptic cleft

Test your knowledge on the structure and function of neurons with this quiz. Explore essential concepts like dendrites, myelination, and the roles of various ions in neuronal activity. This quiz is perfect for students studying neuroscience and physiology.