Neuroscience & Behaviour

Cellular Components of the Nervous System

• Neurons: Specialized communication cells that synthesize neurotransmitters, receive incoming signals, and transmit electrochemical impulses.

  • Soma (Cell Body): Contains the cell nucleus and metabolic machinery; synthesizes proteins and neurotransmitters.

  • Dendrites: Branching extensions from the soma designed to receive messages from adjacent neurons.

  • Axon: Elongated nerve fiber conducting electrical impulses away from the soma toward target cells.

  • Axon Terminal (Synaptic Knob): Terminal bulb of an axon branch housing synaptic vesicles.

• Glial Cells: Provide structural scaffolding to anchor neurons, transport nutrients, remove metabolic waste, mediate immune defense, and generate myelin.

• Myelin Sheath: Fatty insulating layer wrapped around axons that accelerates the propagation velocity of electrical impulses.

  • Multiple Sclerosis (MS): Autoimmune pathology characterized by progressive degradation of the myelin sheath, disrupting signal conduction.

Electrochemical Signal Transmission

• Membrane Potential: Electrical charge differential across the semipermeable neuronal membrane between intracellular fluid (predominantly negative ions) and extracellular fluid (predominantly positive ions).

• Resting Potential: Baseline polarization state when no neurotransmitters are acting on the neuron, maintaining an intracellular charge of $-70\,\text{millivolts}$.

• Action Potential: Rapid electrical impulse triggered when incoming stimulation exceeds the excitation threshold.

  • Membrane ion channels open transiently, allowing positive ions to surge into the interior of the axon.

  • Maximal positive charge accumulation triggers an outflow of positive ions back to the exterior to re-establish resting polarity.

• All-or-None Law: Neurons fire completely or not at all; signal speed remains constant, and message intensity is mediated solely by the firing frequency rather than impulse magnitude.

• Synapse and Chemical Release:

  • Arrival of an action potential at the axon terminal prompts synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.

  • Neurotransmitters diffuse across the fluid-filled cleft to bind specific postsynaptic receptor sites via a lock-and-key mechanism.

  • Reuptake: The process of reabsorbing excess neurotransmitters back into the presynaptic neuron to terminate signaling.

Neurotransmitters and Neuropharmacology

• Functional Classes:

  • Excitatory: Chemical messengers that depolarize the membrane, increasing the probability that the receiving neuron fires an action potential.

  • Inhibitory: Chemical messengers that hyperpolarize the membrane, preventing or reducing the likelihood of firing.

• Key Neurotransmitters:

  • Acetylcholine (ACh): Mediates skeletal muscle contraction and cognitive processes like memory; reduced production is linked to Alzheimer's disease.

  • Glutamate: Primary excitatory neurotransmitter critical for memory encoding via synaptic alterations; dysregulation is associated with schizophrenia, depression, and Parkinson's disease.

  • GABA: Primary inhibitory neurotransmitter moderating behaviors from feeding to aggression; insufficient GABA levels correlate with anxiety disorders.

  • Dopamine: Drives voluntary motor pathways, learning, reward anticipation, and mood; elevated activity correlates with psychosis and mania (dopamine hypothesis of schizophrenia), whereas depletion causes Parkinson's disease.

  • Serotonin: Modulates sleep, hunger, pain perception, and mood; implicated in depression, impulsivity, stress coping, and aggression.

  • Endorphins: Endogenous neuropeptides that blunt pain perception and produce positive affective states.

• Pharmacological Mechanisms:

  • Selective Serotonin Reuptake Inhibitors (SSRIs): Antidepressant agents that inhibit presynaptic reuptake of serotonin, extending receptor activation.

  • Opiates: Exogenous agonist compounds (e.g., morphine, heroin) that bind endorphin receptor sites with high potency, generating analgesia, euphoria, and high addiction risk.

Structural Divisions of the Nervous System

• Central Nervous System (CNS):

  • Brain: Primary processing organ regulating internal physiology, cognition, and behavior.

  • Spinal Cord: Pencil-thick bundle of nerves running down the back that conducts bidirectional communication between the brain and peripheral structures.

• Peripheral Nervous System (PNS): Composed of long axons and dendrites extending outside the CNS.

  • Somatic Division: Controls voluntary musculoskeletal movements and receives sensory input via sensory (afferent) neurons and motor (efferent) neurons.

  • Autonomic Division: Automatically governs involuntary visceral and organ activities.

  • Sympathetic Division: Mobilizes bodily energy reserves for threat responses (fight-or-flight).

  • Parasympathetic Division: Calms physiological systems post-emergency to maintain homeostasis and conserve energy.

Brain Regions and Functional Localization

• Cerebral Cortex Lobes:

  • Frontal Lobe: Houses the motor cortex, Broca's area (speech production), working memory, executive planning, and emotional regulation.

  • Parietal Lobe: Contains the somatosensory cortex, processing sensations of touch, pain, and temperature, and integrating visual-spatial inputs.

  • Temporal Lobe: Encompasses the auditory cortex, Wernicke's area (speech comprehension), object recognition, and memory storage.

  • Occipital Lobe: Contains the primary visual cortex; cortical damage can produce visual field deficits or hallucinations.

• Subcortical Forebrain Structures:

  • Thalamus: Central sensory switchboard routing non-olfactory sensory inputs to designated cortical regions.

  • Hypothalamus: Master regulator of homeostatic functions, including body temperature, fluid balance, sleep, eating, and sexual behavior.

  • Limbic System: Coordinates emotion, basic drives, and memory; includes the amygdala (fear, aggression, emotion) and the hippocampus (memory acquisition and consolidation).

• Midbrain Structures:

  • Reticular Formation: Regulates alertness, sleep-wake cycles, and motor arousal.

  • Substantia Nigra & Ventral Tegmental Area: Primary sites of dopamine synthesis regulating mood, reward processing, and addiction.

• Hindbrain Structures:

  • Medulla: Controls vegetative autonomic functions (respiration, cardiac rhythm, blood pressure).

  • Pons: Bridges cerebellum and higher brain structures; regulates sleep mechanisms.

  • Cerebellum: Coordinates muscular balance, fine motor execution, and procedural memory.

Hemispheric Lateralization and Split-Brain Effects

• Contralateral Processing: Each cerebral hemisphere governs sensory inputs and motor operations on the opposite side of the body.

• Lateralization of Function:

  • Left Hemisphere: Specializes in verbal capabilities (speaking, reading, writing), numerical processing, and logical reasoning.

  • Right Hemisphere: Dominates nonverbal processing, spatial construction, pattern and face recognition, music perception, and emotional expression.

• Split-Brain Surgery: Surgical severance of the corpus callosum to treat refractory epilepsy, isolating the two hemispheres.

  • Visual Field Mechanics: Stimuli appearing in the right visual field project exclusively to the left (verbal) hemisphere; patients can verbally name the target.

  • Right Hemisphere Isolation: Stimuli appearing in the left visual field project to the right hemisphere; patients cannot verbally name the item, but can correctly identify or draw it using the left hand.

Brain Development, Plasticity, and Imaging Modalities

• Developmental Dynamics:

  • Myelination: Occurs across defined critical periods, such as the surge between $6\,\text{months}$ and $1\,\text{year}$ corresponding with rapid language emergence.

  • Synaptic Pruning: Elimination of unused or redundant synaptic connections to optimize neural efficiency (e.g., pruning of language areas around age $3$).

• Neural Adaptability:

  • Neuroplasticity: Structural and functional reorganization of the brain driven by learning, sensory input, or recovery from damage.

  • Neurogenesis: Synthesis of new functional neurons in select brain regions.

  • Synaptogenesis: Formation of novel synaptic networks and microcircuits.

• Brain Imaging Modalities:

  • Structural Techniques:

  • Computed Tomography (CT): Uses serial rotating X-rays to construct cross-sectional, two-dimensional anatomical images.

  • Magnetic Resonance Imaging (MRI): Employs strong magnetic fields and radio waves to generate high-resolution three-dimensional tissue maps.

  • Functional Techniques:

  • Functional MRI (fMRI): Tracks localized neural activity by measuring blood oxygenation level changes.

  • Positron Emission Tomography (PET): Injects radioactive tracers to monitor metabolic glucose consumption and neurotransmitter receptor occupancy.

The Endocrine System and the Stress Response

• Endocrine Signaling: Ductless glands secrete hormones directly into the vascular system in a pulsatile manner controlled by negative feedback systems, generating slower but longer-lasting physiological modifications compared to nervous signaling.

• Major Endocrine Glands:

  • Pituitary Gland: Known as the master gland; releases trophic hormones regulating peripheral endocrine activity under direct hypothalamic control.

  • Thyroid Gland: Regulates basal metabolic rate.

  • Adrenal Glands: Modulate blood pressure and discharge catecholamines during acute threat.

• Sympathetic-Adrenal Medullary Response (Adrenaline Actions):

  • Increases heart rate and ventricular contraction strength.

  • Constricts peripheral vessels while increasing blood flow directly to skeletal muscle.

  • Dilates bronchial airways to augment oxygen intake.

  • Stimulates glycogenolysis (glycogen converted to glucose) and lipolysis (fat breakdown into fatty acids) for rapid metabolic fuel.

  • Induces pupillary dilation to widen the visual field.

  • Shuts down gastrointestinal secretions and reduces digestive activity.