chapter 2

Neurons: Key Players and Basic Roles

• Focus of the lesson is on interneurons (the big three are mentioned, with interneurons as the primary focus). Other two neuron types are acknowledged as more minor and not the main focus here.
• Interneurons: carry messages back and forth within the nervous system.
• Other neuron types exist, but the emphasis is on interneurons for understanding neural communication.

Myelin Sheath and Signal Speed

• Axons can be coated with a fatty substance called a myelin sheath (the transcript says "nylon sheath" due to likely mispronunciation).
• Purpose of myelin: to make messages travel faster and signals move more efficiently.
• Neurons with myelin are typically more efficient at transmitting messages quickly.

Action Potential: The Neuron’s Message

• Action potential = the electrical signal that travels along the neuron when activated.
• For an action potential to occur, the stimulus must reach the threshold.
• If the stimulus is too weak, the neuron does not fire and the message is not transmitted.
• Analogy: when someone speaks softly, you might hear parts of it but not the full word; the threshold was not reached, so the message isn’t fully received.
• Once the threshold is reached, the action potential is fired and the message moves along to the next neuron.

Resting Potential and Refractory Period

• Resting potential: the neuron is at rest until a new stimulus arrives.
• After an action potential, there is a brief refractory period during which the neuron cannot receive or fire another signal.
• Refractory period is a short recharge phase; then the neuron returns to resting potential and is ready for a new stimulus.
• Analogy: sitting in stadium seating during a game—refractory period is like taking a moment to sit back before standing again.

Neural Communication: Presynaptic vs Postsynaptic

• The neuron that currently has the message and is sending it is the presynaptic neuron.
• The neuron receiving the message is the postsynaptic neuron.
• Communication occurs at the synapse between the axon terminal of the presynaptic neuron and the dendrites of the postsynaptic neuron.

Neurotransmitters: Messengers Across the Synapse

• Neurotransmitters are the common messages sent across the synapse.
• At the axon terminal, neurotransmitters are released into the synapse and travel to docking sites on the dendrite of the next neuron.
• Different neurotransmitters have different functions; not all neurotransmitters are identical in their effects.
• Examples mentioned (not exhaustive):
  • Acetylcholine (ACh)
  • Norepinephrine (noradrenaline) (the transcript mentions a form that is likely norepinephrine)
  • Glutamate (excitatory messaging)
• Glutamate specifically is described as sending excitatory messages that increase bodily arousal (e.g., heart rate, respiration).
• The transcript notes that not every neurotransmitter is the same; there are many different types with diverse roles.

Pharmacology: Agonists and Antagonists

• Drugs can alter brain function by interacting with neurotransmitter systems.
• Agonists: substances that act like a neurotransmitter and activate receptor sites on the postsynaptic neuron, enhancing signaling.
• Antagonists: substances that occupy receptor sites but do not activate them, thereby reducing signaling.
• Example rationale:
  • Antidepressants (SSRIs) can be explained as a mechanism to increase serotonin signaling when serotonin is low.
  • Antagonists can be used to reduce excessive dopamine signaling in conditions like schizophrenia.
• Some medications combine agonist and antagonist actions to balance activation levels at receptor sites (to achieve a desired level of stimulation).
• The course includes a separate pharmacology module focused on how medications affect the brain.

The Brain, Spinal Cord, and Injury Contexts

• The brain sits within the CNS (central nervous system) along with the spinal cord; together they control many body operations.
• Traumatic brain injury (TBI) and brain injuries can be severe and disrupt normal functioning; recovery varies and severe injuries may lead to long-term changes.
• Spinal injuries are mentioned as a spectrum, with the potential for lasting impact.
• The CNS communicates with the peripheral nervous system (PNS) to manage bodily functions.
• The brain is protected by cerebrospinal fluid; physical impacts can cause brain injury by the brain hitting the skull.
• Boxers and others who experience repeated head impacts are at risk for TBIs and related disorders.

The Central and Peripheral Nervous System (CNS vs PNS) and Reflexes

• CNS includes the brain and spinal cord; it is the control center.
• Peripheral nervous system (PNS) is everything outside the CNS and is divided into two main branches:
  • Somatic nervous system: communicates sensory information to the CNS and carries out motor commands; controls voluntary movements and sensory processing.
  • Autonomic nervous system: regulates involuntary functions and is further subdivided into:
  • Sympathetic nervous system: prepares the body for fight-or-flight and energy mobilization.
  • Parasympathetic nervous system: supports rest-and-digest functions and helps regulate energy stores.
• Somatic nervous system details: sensory receptors convey information to the CNS; motor commands travel back out to muscles to enable movement.
• Autonomic nervous system functions continuously to regulate internal bodily processes without conscious effort.
• The sympathetic system is associated with threat detection and energy mobilization; the parasympathetic system helps regulate energy reserves and recovery.

The Endocrine System and Hormones: Brain–Body Communication

• Hormones regulate many body systems: hunger, growth, puberty, sexual development, reproduction, blood pressure, metabolism, etc.
• Hypothalamus: a brain structure that links the nervous system and the endocrine system; coordinates growth and development through hormonal signals.
• Pituitary gland: a pea-sized gland underneath the brain; releases hormones that influence growth and other processes.
• Adrenal glands: involved in the stress response; release adrenaline to meet stress requirements.
• Hormones act as regulators that influence various bodily functions and long-term development.
• The nervous and endocrine systems must work in sync for growth and development to occur.

Brain Anatomy: Structures, Matter, and Basic Functions

• Gray matter vs white matter: gray matter (outer regions) vs white matter (myelinated tracts inside); the presence of myelin relates to fast signaling and efficient communication.
• The brain weighs about three pounds and consumes a lot of oxygen.
• The brain consists of four lobes (mentioned contextually) and a large, complex cerebral cortex that differentiates humans from other species.
• The hypothalamus (discussed earlier) helps coordinate nervous and endocrine functions.
• The brain uses neurotransmitters to communicate; hormones also influence brain function and behavior.
• The brain’s basic functioning includes essential life-supporting processes (breathing, digestion, etc.) managed by various brain regions.
• The cortex is the outer layer of the brain; the cortex is large and highly complex in humans.
• White matter is typically seen inside slices of the brain, whereas gray matter is more on the outer surfaces.

Hindbrain, Midbrain, and Forebrain: A Broad Map

• Hindbrain (lower brain): includes the brainstem components responsible for basic life-sustaining functions.
  • Medulla: autonomic functions like breathing and heart rate regulation.
  • Pons: relay station for signals and involvement in sleep and arousal.
  • Cerebellum: coordinates movement, balance, and fine motor control; supports coordinated actions like raising a hand.
• Midbrain: acts as a relay station for sensory information, including vision and hearing; processes these signals and routes them to appropriate brain areas.
• Forebrain: contains the cerebral cortex, which is large and complex in humans and differentiates our cognitive abilities.
• The brain’s organization across hindbrain, midbrain, and forebrain underpins different levels of processing, from basic autonomic functions to higher-order thinking.

Neuroplasticity and Neurogenesis: Brain Change Across Life

• Neuroplasticity refers to the brain’s ability to change in response to experience.
• Functional plasticity: the brain can reroute functions to different areas if one area is damaged (e.g., language function shifting after injury with therapy).
  • Example: language production might shift to a different brain area after injury if therapy helps regain speech.
• Structural plasticity: physical changes in brain structure (e.g., formation of new connections); popular idea that new learning creates new neural wrinkling, though not literally accurate, aligns with the concept of synaptic changes.
• Neurogenesis (growth of new neurons) in adults is limited but occurs in at least two regions:
  • Olfactory bulb (involved in smell) is a site of adult neurogenesis.
  • Other areas may have neurogenesis, but the olfactory bulb is the explicit example given.
• The brain is still not fully understood; neuroplasticity demonstrates that the brain can adapt and reorganize in response to injury and learning.

Practical Takeaways and Study Suggestions

• When studying neurons:
  • Draw a neuron with a labeled soma, axon, and dendrites to visualize signal flow.
  • Illustrate the presynaptic and postsynaptic sides of a synapse, including the axon terminal, neurotransmitter release, diffusion across the synaptic gap, docking on dendritic receptors, and the role of receptor sites.
• Remember the sequence: resting potential → stimulus threshold → action potential → neurotransmitter release at the synapse → postsynaptic response; followed by a refractory period and return to resting potential.
• Distinguish between: CNS vs PNS; somatic vs autonomic (and within autonomic, sympathetic vs parasympathetic).
• Understand the relationship between the nervous system and the endocrine system (hypothalamus-pituitary-adrenal axis) and how hormones like insulin, cortisol, and others can influence brain function and behavior.
• Be aware of the real-world implications of brain injury and neuroplasticity for rehabilitation and recovery processes.
• If you’re curious to explore deeper: consider the two-way communication between hormones and neural activity, and how drugs (agonists/antagonists) can modulate signaling to treat various conditions.

Quick recap: Key terms to know

• Interneurons, presynaptic vs postsynaptic, neurotransmitters, synapse, dendrite, axon terminal, myelin sheath, action potential, threshold, resting potential, refractory period, glutamate, acetylcholine (ACh), norepinephrine, agonist, antagonist, CNS, PNS, somatic, autonomic, sympathetic, parasympathetic, hypothalamus, pituitary, adrenal glands, neuroplasticity, functional plasticity, structural plasticity, neurogenesis, olfactory bulb, hindbrain, medulla, pons, cerebellum, midbrain, forebrain, cerebral cortex, gray matter, white matter

Note on visual aids used in the lecture

• Encouraged to draw a neuron diagram in your notes to visualize the resting state, threshold crossing, and refractory period.
• Visualize the progression: resting potential → threshold → action potential → synaptic transmission → postsynaptic response → refractory period → back to resting potential.