The physical sensation of hunger comes from contractions of the stomach muscles. These contractions are believed to be triggered by high concentrations of the hormone ghrelin. Two other hormones, peptide YY and leptin, cause the physical sensations of being full. Ghrelin is released if blood sugar levels get low, a condition that can result from going long periods without eating.
Psychology 120.3 Lecture 2 2025/09/10
Introduction to Human Neuroscience
Topic 1: Components of the Neurons
Cell Body (Soma):
This is the central part of the neuron, acting as the cell's "trunk" or main body. It houses the nucleus, which contains the neuron's genetic material, as well as other vital organelles that maintain the cell and produce proteins.
Dendrites:
These are branched, tree-like extensions that receive signals from other neurons. They serve as the neuron's input, allowing it to communicate with other cells and perceive its environment.
Axon:
A long, cable-like projection that extends from the cell body and carries electrochemical signals, known as nerve impulses, away from the cell. The axon allows the neuron to send messages over long distances to other cells.
Axon Terminal:
At the end of the axon, there are specialized structures called axon terminals. These terminals are the communication points where the signal is transferred from one neuron to the next, a process that often involves chemical signals crossing a synapse.
https://www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-life-and-death-neuron
Topic 2: Types of Neurons
The structural components of a neuron determine whether it is unipolar, bipolar, or multipolar. These components are called axons, which transmit information, and dendrites, which receive information.
A unipolar neuron has one axon which extends into dendrites, though this splits into two parts in pseudounipolar neurons.
A bipolar neuron has two completely independent structures extending from the cell body, one of which is an axon and the other a dendrite.
A multipolar neuron only has one axon extending from the cell body, but multiple dendrites grow out of it, making transmitting information easier.
https://www.medicalnewstoday.com/articles/unipolar-vs-bipolar-vs-multipolar-neurons
An example: what type of neuron is a Purkinje cell? Where is it located, and what does it do?
A Purkinje cell is found in the cerebellar cortex of the brain, characterized by a flask-shaped cell body, a massive and elaborate dendritic tree, and a single long axon. These cells primarily use the neurotransmitter GABA to inhibit other neurons, playing a crucial role in coordinating and controlling voluntary motor movements and balance by providing the sole output from the cerebellar cortex
https://www.sciencedirect.com/topics/neuroscience/purkinje-cell
Drawing Copyright: Patrick Guenette | Dreamstime.com
Topic 3: Neuron Action Potential
https://www.youtube.com/watch?v=A0ucST0jyqw
(essential viewing)
Remember: The Salty Banana meme.
For this class, just memorize the five stages of the action potential:
(1) Stimulus; (2) Depolarization; (3) Repolarization; (4) Refractory Period; (5) Resting Potential.
The meme for the Refractory Period is "flushing a toilet (quickly) twice in a row". The neuron has to recharge.
The final meme for the Action Potential is the "Saskatchewan Rough Riders Wave".
And here is the news! All you need to learn from this is that each of those dendrites in the Purkinje cell is transmitting a unique signal.
https://www.youtube.com/watch?v=KeVOG-X_Fck
(optional viewing)
For the exam: the action potential is an all-or-none response, not like turning up the volume of an old-fashioned radio, but like the ones and zeros of a digital computer. That is one of the reasons that artificial neural networks have been designed for over thirty years; the brain is not a digital computer, but an artificial intelligence can be designed with neural networks to imitate the brain.
Topic 4: Glial Cells: A Neuron's Pit Crew
Glial cells are critically important because they . Their functions include providing structural support, forming the myelin sheath for efficient nerve signal transmission, maintaining homeostasis of the neuronal environment, regulating synaptic activity by controlling neurotransmitters, forming the blood-brain barrier to protect the brain, and playing a key role in the brain's immune response and recovery from injury.
Key Functions and Importance
Structural Support and Homeostasis:
Glial cells provide the physical support for neurons and maintain the optimal ionic and water balance in the extracellular fluid surrounding them.
Myelination:
Certain glial cells, like oligodendrocytes and Schwann cells, form myelin, an insulating sheath that allows for rapid and efficient transmission of electrical signals along neurons.
Synaptic Regulation:
Glia actively participate in synaptic activity by clearing neurotransmitters from the synaptic cleft, releasing neurotrophic factors that support neuronal survival, and influencing synapse formation and elimination.
Metabolic Support:
They transport nutrients to neurons, control energy sources for brain activity, and express receptors for metabolic hormones.
Blood-Brain Barrier Formation:
Astrocytes, a type of glial cell, are crucial in forming the blood-brain barrier, which protects the brain from harmful substances while allowing essential molecules to pass through.
Immune Response and Injury Recovery:
Microglia, another glial cell type, act as the brain's immune cells, clearing debris and pathogens. Glial cells also play a role in brain injury, either by contributing to damage or by promoting recovery.
Neural Development:
During development, glial cells guide neuronal migration and produce molecules that influence the growth of axons and dendrites.
What is the relationship between glial cells and Amytrophic Lateral Sclerosis (ALS)?
Glial cells contribute to motor neuron degeneration. The first evidence of glial dysfunction in ALS, in patients and in animal models, came from studies in the mid-1990's examining astroglial glutamate transporters. Microglia, located in the brain, are the immune cells for neural matter.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4241182/
At issue here is microgliosis, the process that involves changes in the microglia's shape, an increase in their number at the affected site, and alterations in their gene expression and surface markers, which are all part of the brain's protective and inflammatory response. Microgliosis plays a role in various neurological conditions, including neurodegenerative diseases and infections, and can have both beneficial and detrimental effects
Topic 5: Synaptic Gap
A good image for the synaptic gap is that of a symphony orchestra, with the conductor on the sending neuron, and the instruments of the orchestra on the receiving neuron.
The synaptic gap, also known as the synaptic cleft, is the small space located between the two. This tiny gap serves as the site where chemical signals, called neurotransmitters, are released from the presynaptic neuron and then diffuse across to the postsynaptic neuron to transmit information.
A synapse, which includes the synaptic gap, consists of several key parts:
Presynaptic Neuron:
The neuron that sends the signal. Its axon terminal contains vesicles filled with neurotransmitters.
Postsynaptic Neuron:
The neuron that receives the signal. Its dendrite or cell body has receptors for neurotransmitters..
Synaptic Cleft:
The space between the presynaptic and postsynaptic cells where neurotransmitters are released.
Topic 6: Neurotransmitters
With respect to human neurology, the five most important neurotransmitters are: dopamine, serotonin, GABA, glutamate and acetylcholine.
Here are the roles of these key neurotransmitters:
Dopamine: Influences the brain's reward system, contributing to feelings of pleasure and motivation. It also plays a vital role in motor control, with its deficiency linked to Parkinson's disease.
Serotonin: Affects mood, sleep, appetite, and digestion. Imbalances are often associated with depression and other mood disorders.
GABA (Gamma-aminobutyric acid): This is the primary inhibitory neurotransmitter in the brain, helping to reduce neuronal excitability and promoting calmness and relaxation.
Glutamate: The most common excitatory neurotransmitter in the nervous system, vital for learning and memory by stimulating neurons to fire.
Acetylcholine: The first neurotransmitter discovered, it directly affects muscle function and is essential for muscle contractions and other cognitive processes like attention and memory.
Topic 7: Agonists and Antagonists
Agonists act as substitutes for one of the five main neurotransmitters; while antagonists block those transmitters. Here is where we are faced with counter-intuitive evidence, caffeine being a good example.
Caffeine is primarily an antagonist, meaning it blocks the effects of another substance rather than activating a receptor itself. It acts as a non-selective adenosine receptor antagonist, competing with the body's natural adenosine to bind to adenosine receptors. By preventing adenosine from binding, caffeine dampens the sleep-promoting signals that adenosine normally transmits, leading to increased wakefulness and alertness.
Let's try another one. How does Botox work?
Botox acts as an antagonist to the neurotransmitter acetylcholine, blocking nerve signals that control muscle movement and causing muscle paralysis or weakening. This mechanism effectively blocks the agonist (activator) action, allowing Botox to relax muscles by preventing them from receiving signals from motor nerves.
One more: How does nicotine work?
Nicotine is primarily an agonist, meaning it activates nicotinic acetylcholine receptors (nAChRs), which are involved in neurotransmission in the brain and nervous system. By binding to and activating these receptors, nicotine triggers the release of various neurotransmitters, including dopamine, leading to its stimulating and psychoactive effects. However, nicotine can also act as a net antagonist because it also causes the desensitization of these receptors over time, reducing their response to subsequent stimulation.
We will discuss psychoactive drugs in depth, but that is enough for the first exam.
Topic 8: Divisions of the Nervous System:
The human nervous system is divided into:
1. Central Nervous System (CNS)
Brain:
The control centre for all bodily functions, including thoughts, movements, and memory.
Spinal Cord:
Connects to the brain and transmits signals back and forth between the brain and the rest of the body.
2. Peripheral Nervous System (PNS)
Somatic Nervous System:
Controls voluntary muscle movements and carries sensory information from the body to the CNS.
Autonomic Nervous System (ANS):
Regulates involuntary bodily functions like digestion, heart rate, and breathing.
Sympathetic Nervous System: Activates the "fight-or-flight" response, preparing the body for increased activity.
Parasympathetic Nervous System: Responsible for "rest-and-digest" functions, conserving energy and slowing heart rate.
Enteric Nervous System: An independent network of nerves that regulates the digestive tract.
Let's apply this information by delving deeper into the working of Ozempic
Ozempic influences digestion by activating GLP-1 receptors, which slow stomach emptying and intestinal movement. It directly affects the vagus nerve, a key component of the parasympathetic nervous system that regulates digestion.
Ozempic affects the gut (stomach) by delaying gastric emptying. This keeps food in your stomach longer, slows down digestion and can make you feel bloated. This is a direct action on the enteric nervous system.
To repeat and clarify:
The vagus nerve acts as a direct line of communication between the brain and the enteric nervous system (ENS), the "second brain" of the gut. It relays sensory information about gut conditions to the brain and sends motor commands from the brain to the gut to regulate digestive processes like nutrient absorption, acid secretion, and muscle contractions. This bidirectional connection is crucial for maintaining gut health, regulating mood and energy, and controlling immune responses.
Yes, Ozempic directly affects the digestive system by activating GLP-1 receptors that slow stomach emptying and intestinal movement, influencing the vagus nerve and the parasympathetic nervous system's regulation of digestion. While this is a designed effect to slow digestion, for some people, this can lead to a severe condition known as gastroparesis, or stomach paralysis, where the muscles and nerves in the stomach function improperly, preventing food from moving through the digestive tract.
Topic 9: Components of the Central Nervous System
The two main components of the central nervous system (CNS) are . The brain is responsible for controlling thought, movement, and senses, while the spinal cord acts as a pathway, carrying messages between the brain and the rest of the body. We will deal in greater detail with the brain in Lecture Three, but this is a good place to show how our CNS and glandular systems interact.
Topic 10: The HPA Axis
The HPA (Hypothalamic-Pituitary-Adrenal) axis is . When a stressor is detected, the hypothalamus releases CRH (corticotropin-releasing hormone), which signals the pituitary gland to release ACTH (adrenocorticotropic hormone), prompting the adrenal glands to produce cortisol. Cortisol then helps the body cope with stress by mobilizing energy, but high levels are also detected by the brain to shut off the stress response through negative feedback.