Nervous System Overview and Functions

Nervous System Objectives

• Students should be able to describe the structure of neurons and the function of their components.
• Students should be able to list the divisions of the nervous system and describe the characteristics of each.
• Students should be able to list the functions of the nervous system.
• Students should be able to differentiate between types of neurotransmitters and their functions.
• Students should be able to explain how an action potential is generated and propagated.
• Students should be able to explain the processes involved in synaptic transmission.
• Students should be able to differentiate voluntary neural pathways versus reflex arcs.
• Students should be able to explain the structure of neurons, including dendrites, cell bodies, and axons, and the function of glial cells.
• Students should be able to categorize neurons as multipolar, bipolar, or unipolar, based on their structure.

Overview of Nervous System

• The nervous system is a network of neurons that generate, modulate, and transmit information throughout the human body.

Cells of the Nervous System (Neuron vs Glial Cells)

• The nervous system comprises neurons and glial cells.
• Neurons generate and transmit nerve impulses.
• Glial cells support, nourish, and protect neurons.
• Neurons are categorized as multipolar, bipolar, or unipolar based on their structure.

Divisions of the Nervous System

• The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS includes cranial nerves, spinal nerves, sensory neurons, and motor neurons.
• Sensory neurons transmit information from sensory organs to the CNS.
• Motor neurons transmit signals from the CNS to effectors (muscles and glands).
• The somatic nervous system controls voluntary movements.
• The autonomic nervous system controls involuntary actions.
• The sympathetic division is associated with "fight-or-flight."
• The parasympathetic division is associated with "rest-and-digest."

CNS vs PNS

• The CNS and PNS work together.
• The CNS processes information and generates responses.
• The PNS relays information between the CNS and the rest of the body.

Somatic vs Autonomic

• The somatic nervous system is involved in voluntary actions.
• The autonomic nervous system controls involuntary actions.
• The sympathetic and parasympathetic divisions have opposing effects.

Neurotransmitters

• Neurotransmitters are chemical messengers that transmit signals across synapses.
• Excitatory neurotransmitters cause the neuron to fire.
• Inhibitory neurotransmitters prevent the neuron from firing.
• Neurotransmitters include adrenaline, noradrenaline, dopamine, serotonin, GABA, acetylcholine, glutamate, and endorphins.

How Are Neurotransmitters Removed from Synapse?

• Neurotransmitters are removed through diffusion, reuptake (by proteins), and enzymatic degradation.

What Can Go Wrong?

• Issues with neurotransmitter production/release, receptor function, and reuptake can disrupt nervous system function.

CNS Stimulants

• Central nervous system stimulants include caffeine, nicotine, cocaine, methylphenidate, amphetamines, methamphetamine, modafinil, and phentermine.
• Psychomotor stimulants cause increased alertness, attention, energy, and activity.
• Hallucinogens distort perception.

Hallucinogens

• Marijuana is extracted from cannabis plants.

Cholinergic System

• The cholinergic system uses acetylcholine (ACh) as a neurotransmitter.
• ACh is crucial in parasympathetic functions and some CNS activities.

Cholinergic Receptors

• Two main types of cholinergic receptors are muscarinic and nicotinic.
• Muscarinic receptors are generally associated with parasympathetic actions.
• Nicotinic receptors are typically found at synapses and neuromuscular junctions.
• Different subtypes of muscarinic (M1-M5) and nicotinic receptors (Nn, Nm) are expressed in various parts of the body.

Cholinergic Agonists

• Direct-acting agonists bind directly to cholinergic receptors.
• Indirect-acting agonists inhibit the enzyme that breaks down ACh.

Cholinergic Antagonists (Antimuscarinics)

• Antagonists block ACh actions at the receptor sites.
• They cause effects that oppose parasympathetic activity (e.g. increased heart rate, decreased GI motility, etc.).
• Examples include atropine, ipratropium, oxybutynin, and tolterodine.

Cholinergic Antagonists (Nicotinic antagonists)

• Antagonists block nicotinic receptors.
• Examples include rocuronium, which is used for muscle relaxation during surgery and mechanical ventilation.

Neuronal Pathways

• Voluntary pathways involve conscious decision-making.
• Reflex arcs are automatic responses to stimuli, bypassing the brain.
• Both involve neurons (sensory, interneurons, motor) and effector organs.

Action Potential

• Action potentials are rapid changes in membrane potentials along neurons.
• The resting potential (-70mV) exists due to differing ion concentrations inside and outside the cell.
• Depolarization involves opening of voltage-gated sodium channels, increasing membrane potential.
• Repolarization reverses the change in voltage due to opening of potassium channels and allows sodium channels to close.
• A stimulus triggers an action potential if it surpasses the threshold.
• Action potentials are propagated along the axon via sequential depolarizations, ensuring their transmission throughout the body.

Synapses

• Synapses are the junctions between neurons or between neurons and effectors.
• Most synapses are chemical, transmitting signals via neurotransmitters.
• Electrical synapses transmit signals directly via ion flow through gap junctions.