Neurotransmission and Brain Structures

Neurochemical Messages and Myelin Sheath

  • Interference with Messages: Neurochemical messages can interfere with brain functions, acting as a "juggernaut" or getting in the way of processes.

  • Myelin Sheath and Damage: The myelin sheath, acting as insulation for neural messages, can be damaged.

    • Early Life Development: This damage can start very young. An example is an infant's tight grip suddenly releasing, causing them to hit themselves, indicating immature myelin development.

    • Aging and Degradation: Myelin sheath can degrade as people age. This degradation is a known contributor to memory problems in Alzheimer's disease.

  • Consequences of Damage: Damage to specific brain parts has direct implications or consequences for their function. Understanding these consequences helps us learn about brain functions.

Neurotransmitter Pathway: Seven Steps of Communication

Neurons communicate by passing messages from one neuron to the next through a sequence of steps.

  1. Dendrite Reception: Information arrives at the dendrites (of the postsynaptic neuron - implied from last lecture).

  2. Cell Body Processing: The message moves through the cell body (implied from last lecture).

  3. Axon Transmission and Myelin Sheath: The electrical impulse travels down the axon, which is covered by the myelin sheath for efficient signal transmission.

  4. Terminal Buttons (Axon Terminals) and Synapse:

    • Terminal Buttons: At the end of the axon are small structures called terminal buttons, or axon terminals.

    • Synapse/Synaptic Cleft: When the message reaches the terminal button, it confronts a space, a gap, or an "avenue" between the presynaptic neuron (sending) and the postsynaptic neuron (receiving). This space is called the synapse or synaptic cleft. Crucially, the neurons are very close but do not touch.

  5. Electrical Impulse (Action Potential) and Firing:

    • Resting Potential: When a neuron is not firing, it is in a resting state with an electrical impulse that has not yet fired. The cell is relatively negative inside compared to the outside.

    • Action Potential: For a message to be passed, an electrical impulse must reach a sufficiently high threshold.

    • Firing: When the neuron fires, it's an action potential. This involves a rapid, temporary reversal of the electrical charge from negative to positive, and then back to negative. It's a very quick process.

    • All-or-Nothing Principle: Neurons either fire completely or not at all; there is no "partial firing." This means the strength of a message isn't determined by how much it fires, but by other factors.

    • Strength of Messages: Strong sensations (e.g., seeing, tasting) result from very rapid firing of neurons. Weak sensations result from slower firing of neurons.

  6. Neurotransmitter Release and Binding:

    • Synaptic Vesicles: Neurotransmitters (neurochemical messages) are stored in tiny sacs called synaptic vesicles within the terminal buttons.

    • Release: When an action potential occurs, these synaptic vesicles release neurotransmitters into the synaptic cleft. They are in a state of "purgatory," waiting to be received.

    • Absolute Refractory Period: After release, there's an absolute refractory period where the neuron cannot fire again for a brief time.

    • Binding (Lock and Key): Neurotransmitters must then bind to specific receptor sites on the dendrites of the postsynaptic neuron. This is a "lock and key" mechanism: the shape of the neurotransmitter must perfectly match the shape of the receptor site for binding to occur.

    • Fate if Not Bound: If neurotransmitters don't bind, two things can happen:

      • Degradation: An enzyme breaks them down, and they go away.

      • Reuptake: They are reabsorbed back into the presynaptic neuron (like being "sucked back up into an alien ship"). This is the mechanism by which SSRIs (Selective Serotonin Reuptake Inhibitors) work for depression, by preventing serotonin reuptake, leaving more serotonin in the synaptic cleft.

  7. Postsynaptic Potential (PSP):

    • Electrical Impulse: This is another electrical impulse dgenerated in the postsynaptic neuron after neurotransmitter binding.

    • Excitatory PSPs (EPSPs): These increase the likelihood that the postsynaptic neuron will fire, telling it to "do something."

    • Inhibitory Neurotransmitters: Some neurotransmitters, like GABA, are inhibitory. They decrease the likelihood of the postsynaptic neuron firing (e.g., promoting sleep by acting as an "on/off switch" for neural activity at night).

How We Understand Brain Functions

Our understanding of brain function comes from various methods:

  • Lesioning:

    • Animal Studies: Involves surgically removing parts of the brain in animals (e.g., mice) to observe the resulting behavioral impacts.

    • Human Case Studies: The classic example is Phineas Gage, an $-19^{th}$-century railroad worker who survived a rod going through his brain. His dramatic personality and behavioral changes provided early evidence for the frontal lobe's role in personality.

  • Electrical Stimulation:

    • William Penfield's Work: A pioneering Canadian neurosurgeon who stimulated awake patients' brains during seizure surgeries. He discovered specific brain areas corresponded to sensations (e.g., a patient smelling "burnt toast" when a particular region was stimulated, which helped identify the seizure's origin).

  • Modern Technology: Advanced technologies now provide more sophisticated ways to study the brain without invasive procedures.

Brain Structure: An Overview

The Hand Analogy for Brain Structures

  • Cerebral Cortex (Skin of hands): The outer, wrinkled layer covering the cerebrum, which consists of two hemispheres (left and right). It's like a "cap" on top.

    • Gray Matter: The cell bodies of neurons, located on the surface (like the top skin of the hand).

    • White Matter: Axons and their myelin sheaths, extending deeper into the brain (like muscles deep in the hand).

    • Corpus Callosum: A large band of white matter that connects the two hemispheres, allowing them to communicate. This is formed by axons.

  • Convolutions (Hills and Valleys): The "c-like structures" or the folds (gyri and sulci) on the cerebral cortex. Squeezing your hand to create wrinkles demonstrates how these folds allow more information to fit into a tiny space than a flat surface would.

  • Hemispheric Control (Crossed Wrists): Crossing your wrists, with your right hand on the left side and left hand on the right side, illustrates that the right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body.

    • Interdependence: Most functions and skills require both hemispheres, although there can be dominance (e.g., the left hemisphere is dominant for language, but spatial reasoning for directions involves the right hemisphere, requiring both for communication).

Brain Lobes (Using Hand Analogy)

  • Frontal Lobe (Fingers/"F" for Frontal):

    • Location: The foremost part of the brain.

    • Functions: Complex decision-making, planning, attention.

    • Prefrontal Cortex: Involved in impulse control, not fully developed until around age 25. (Example: impulsive actions in youth).

    • Motor Cortex (Knuckle movement): Located within the frontal lobe, responsible for voluntary movements. Moving your knuckle is a voluntary action.

  • Parietal Lobe (Just above wrist/"P" for Pain, Pressure, Touch):

    • Location: Behind the frontal lobe.

    • Functions: Processes sensations of pain, pressure, temperature, and other forms of touch.

    • Somatosensory Cortex: Located within the parietal lobe, this area receives and processes sensory information from the body.

      • Sensory Homunculus: A distorted representation mapping the amount of cortical real estate dedicated to different body parts. Areas like the lips, tongue, and hands have a very high density of neurons, making them highly sensitive. This is a "map" of sensitivity rather than size.

      • Link to Motor Cortex: The somatosensory cortex and motor cortex are adjacent because they are closely linked to voluntary movements and the sensations experienced during those movements.

  • Occipital Lobe (Back of hand/wrist - "eyes in the back of my head"):

    • Location: The rear-most lobe.

    • Functions: Contains the visual cortex, responsible for processing visual information and sight.

  • Temporal Lobe (Thumbs, spanning across multiple lobes):

    • Location: Situated on the side of the head.

    • Functions: The left temporal lobe is especially important for speech production. It connects with areas involved in touch (parietal lobe), motor control, and prefrontal functions, highlighting its role in complex functions like language.

Major Brain Structures
Hindbrain

  • Cerebellum (1a): The "cauliflower-like" structure at the back of the brain.

    • Functions: Fine muscle movement, balance.

    • Clinical Relevance: Alcohol affects the cerebellum, impairing balance, which is tested in sobriety checks.

  • Medulla (1b): Located at the base of the brainstem.

    • Functions: Controls vital involuntary functions such as breathing and circulation.

    • Implication of Damage: Damage to the medulla is typically fatal.

  • Pons (1c): Situated above the medulla.

    • Functions: Involved in sleep, arousal, and movement. It connects the cerebellum to the brainstem.

    • Brain Stem: Connects the brain to the spinal cord. Damage can lead to paralysis.

Midbrain

  • Reticular Formation (2): A network of neurons extending through the midbrain.

    • Functions: Crucial for regulating sleep, arousal, and overall consciousness.

    • Implication of Damage: Damage can result in a loss of consciousness (e.g., coma).

Forebrain

  • Cerebrum: The largest part of the brain in humans, responsible for higher-level functions.

    • Evolutionary Development: The cerebrum's significant development in humans compared to other species (e.g., rats, cats, chimps) has provided an evolutionary advantage. (Discussion ended here).