CNS and Neuro Topics Overview

Introduction and Course Overview

• Final section of the course focused on Neuro topics.
• Upcoming pharmacodynamics course in June.
• Encouragement to ask questions if confused.

CNS Overview

• Body's computer: Takes inputs and generates outputs to control the body.
• Over 100 Billion neurons in CNS.
• Neurons connect with few hundred to 200,000 other neurons.
• Forward transmission: Single axon sends a signal out, generally not receiving signals back.

Sensory Inputs and Signal Transmission

• Afferent signals: From peripheral tissues to brain via spinal cord, brain stem, cerebellum, and thalamus to cerebral cortex.
• Motor (Efferent) Signals: From CNS to muscles or glands, affecting movement and secretion.
• Lower brain (Lizard Brain): Automatic functions e.g., breathing.
• Higher brain (Cortex): Deliberate actions e.g., lecture delivery.

Information Integration

• Mind sifts through massive sensory data, dismisses irrelevant info.
• Stores crucial data as memories.
• Repetitive synapse firing solidifies memories.

Levels of CNS Function

• Spinal Cord: Signal transmission, reflex arcs (motor responses independent of brain signals).
• Lower Brain: Automatic activities (arterial pressure, breathing), some emotional patterns.
• Higher Brain: Memory storage, conscious control, critical thinking.
• Peripheral vs Central Nervous System: Different cells such as Schwann cells and satellite cells (PNS); astrocytes, oligodendrocytes, microglia (CNS).

Myelin Sheath and Blood-Brain Barrier

• Myelin Sheath: Fast transmission of action potentials; diseases like Multiple Sclerosis damage myelin affecting neural transmission.
• Blood-Brain Barrier: Selective permeability to protect the brain, factors affecting permeability (disease states like meningitis).

Neurons and Synapses

• Action Potentials: Fundamental to neuron communication.
• Chemical Synapses: Use neurotransmitters like acetylcholine, serotonin, glutamate.
• Electrical Synapses: Allow bidirectional transmission through gap junctions.

Neurotransmitters

• Acetylcholine (ACh): Involved in both sympathetic and parasympathetic systems; nicotinic and muscarinic receptors.
• Norepinephrine (NE): Key in the sympathetic nervous system; forms from tyrosine; metabolized by MAO and COMT; arousal and wakefulness.
• Dopamine: Reward pathway; synthesized from tyrosine, metabolized by MAO and COMT; receptors D1 (stimulatory) and D2 (inhibitory).
• Serotonin (5HT): Mood, anxiety; synthesized from tryptophan; metabolites through MAO; several receptor types.
• GABA: Major inhibitory neurotransmitter; receptors GABA A (chloride channels) and GABA B.
• Glutamate: Major excitatory neurotransmitter; receptors include NMDA, AMPA; high stimulation can lead to excitotoxicity.
• Others: Glycine (inhibitory in spinal cord), Histamine (H1 receptor affecting arousal), nitric oxide, cannabinoids affecting appetite and nausea.

Action Potentials

• Resting Membrane Potential: Typically around -70mV.
• Depolarization & Repolarization: Involves sodium (Na+) and potassium (K+) channels.
• Hyperpolarization: Important for the refractory period.

Refractory Periods

• Absolute: No action potential possible.
• Relative: Strong stimulus can elicit action potential.

Sensory Functions

• Receptors: For different stimuli (chemo, photo, thermo, mechanoreceptors).
• Proprioception: Body position and movement sensing.
• Cutaneous Receptors: For touch, pressure, heat, pain (Free nerve endings, Merkel's discs, Meissner's corpuscles).

Transmission of Sensory Pathways

• Adaptation: Phasic responses (initial spike in action potentials) vs. Tonic responses (continuous signal until stimulus removal).