Intro to the Nervous System — Comprehensive Study Notes

Brain

A physical, material organ with mass, occupying space, composed of neurons and glial cells, performing complex electrochemical processes.

Mind

Thoughts, consciousness, decisions, and experiences; an emergent property intimately linked to the complex activity and information processing within the brain.

Foundational Principle of Neuroscience (Reed Montague)

The brain processes information; the mind is the pattern of information processing running on the brain.

Single Neuron as an Information-Processing Unit

A cascade of events from receptors o second messengers o kinases o gene expression changes o protein changes, all contributing to long-term alterations in neuronal function and structure.

Synapses

Specialized junctions where neurons communicate, enabling precise information transfer.

Central Nervous System (CNS)

Comprises the brain and spinal cord, serving as major processing and integration centers for complex thought, motor control, and sensory interpretation.

Peripheral Nervous System (PNS)

Includes everything outside the CNS, consisting of nerves connecting the CNS to the rest of the body (muscles, organs, sensory receptors).

Sensory (Afferent) Pathways

Nerve pathways that bring information into the CNS from the external world and internal states.

Motor/Autonomic (Efferent) Pathways

Nerve pathways that carry commands out of the CNS to muscles and organs.

Somatic Nervous System

A subdivision of the PNS that controls voluntary movement by connecting CNS motor neurons directly to skeletal muscles.

Autonomic Nervous System (ANS)

A subdivision of the PNS that regulates visceral organ function largely subconsciously, further divided into sympathetic and parasympathetic divisions.

Cerebral Cortex

The large, outer, convoluted sheet of gray matter (roughly 2-4 \text{ mm} thick) responsible for high-level cognitive processing, divided into four major lobes and organized into six layers.

Cerebellum

Located at the back of the brain, crucial for fine-tuning smooth, coordinated movements, maintaining posture and balance, and involved in motor learning and timing, acting as an 'error correction' system.

Brainstem

Connects the cerebrum and cerebellum to the spinal cord, containing vital nuclei that release neuromodulators influencing global brain states like arousal, attention, mood, and sleep-wake cycles.

Thalamus

A pair of deep, egg-shaped structures above the brainstem, acting as the 'gateway to the cerebral cortex' by routing almost all sensory information (except olfaction) and regulating sleep, wakefulness, and consciousness.

Neurons

Primary information-processing cells, specialized for electrical signaling and intercellular communication via synaptic inputs and outputs.

Astrocytes

Star-shaped glial support cells that provide metabolic support, regulate the chemical brain environment, and form part of the blood-brain barrier.

Oligodendrocytes (CNS) / Schwann cells (PNS)

Glial cells that elaborate myelin, a fatty sheath insulating axons to increase action potential conduction velocity.

Microglia

Resident immune cells of the CNS, responding to infection/injury/inflammation, acting as phagocytes, clearing debris, and playing roles in synaptic pruning and plasticity.

Soma (Cell Body)

Contains the nucleus and organelles, serving as the main site of protein synthesis and metabolic maintenance for a neuron.

Dendrites

Highly branched, tree-like extensions serving as primary input antennas, receiving synaptic contacts from other neurons.

Axon

A typically single, long process extending from the soma that functions as the output cable, propagating action potentials to downstream neurons.

Action Potentials (Spikes)

Brief, rapid, 'all-or-none' electrical events traveling along axons without decrement, typically lasting \approx 1-2 \times 10^{-3} \text{ s} and peaking around +30 \text{ mV}.

Resting Membrane Potential

The inside of a neuron is typically at about V_{\text{rest}} \approx -70 \text{ mV} relative to the outside, maintained by differential ion distribution and the Na+/K+ pump.

Depolarization

A change in membrane potential where the inside of the cell becomes more positive, often leading to an action potential.

Hyperpolarization

A change in membrane potential where the inside of the cell becomes more negative, making an action potential less likely.

Ion Channels

Integral membrane proteins forming pores selective for specific ions (Na^+ , K^+ , Cl^- , Ca^{2+}) , which are often gated (open/close in response to stimuli).

Myelin

A lipid-rich sheath insulating axons, dramatically increasing action potential conduction velocity via saltatory conduction.

Saltatory Conduction

The process by which action potentials 'jump' from one Node of Ranvier to the next along myelinated axons, increasing conduction speed.

Excitatory Synapses

Synapses primarily using glutamate as a neurotransmitter, causing depolarization (EPSP) in the postsynaptic neuron and making it more likely to fire an action potential.

Glutamate

The most prevalent excitatory neurotransmitter in the CNS, essential for activity-dependent synaptic plasticity but neurotoxic in excess.

Inhibitory Synapses

Synapses primarily using GABA as a neurotransmitter, causing hyperpolarization or stabilization (IPSP) in the postsynaptic neuron, making it less likely to fire an action potential.

GABA (gamma-aminobutyric acid)

The main inhibitory neurotransmitter in the brain, crucial for balancing brain activity.

Neuromodulatory Synapses

Synapses that use monoamines or neuropeptides as neurotransmitters, primarily binding to metabotropic receptors to activate intracellular second messenger cascades, leading to slower, diffuse, and long-lasting alterations in cellular properties and overall brain tone.

Metabotropic Receptors

G-protein coupled receptors that, when activated by neurotransmitters (like neuromodulators), initiate intracellular second messenger cascades, leading to slower and more diffuse cellular changes.

Synaptic Plasticity

The ability of synapses to strengthen (LTP) or weaken (LTD) over time in response to activity, considered the cellular basis for learning and memory.

Excitotoxicity

Neuronal damage or death caused by excessive activation of glutamate receptors, leading to excessive Ca^{2+} influx, implicated in stroke, epilepsy, and neurodegenerative diseases.

What is the primary purpose of studying the nervous system?

To establish a basic understanding of the brain as an information-processing entity, foundational for molecular, synaptic, and pharmacology details.

Distinguish between the Brain and the Mind.

The Brain is a physical organ with mass and space, composed of neurons and glial cells performing electrochemical processes. The Mind is an emergent property – thoughts, consciousness, decisions, and experiences – intimately linked to the complex activity and information processing within the brain.

According to Reed Montague, how is the mind related to the brain?

The brain processes information, and the mind is the pattern of information processing running on the brain.

What are the four main levels of organization in the nervous system as an information-processing system?

1. Single Neuron as an information-processing unit. 2. Local Circuits of hundreds to thousands of neurons. 3. Regional Systems across multiple areas. 4. Whole-Brain and Nervous-System Level for global emergent properties.

What are the key events in information processing within a single neuron?

Receptors (ionotropic or metabotropic) o Second messengers o Kinases o Gene expression changes o Protein changes.

What is the key distinction between the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)?

The CNS consists of the brain and spinal cord, serving as major processing and integration centers. The PNS includes everything outside the CNS, connecting the CNS to the rest of the body.

What are the two subdivisions of the PNS?

1. Sensory (Afferent) Pathways: Bring information into the CNS. 2. Motor/Autonomic (Efferent) Pathways: Carry commands out of the CNS.

What organ system does the Visceral Sensory System continuously monitor?

Internal states like heart rate, blood pressure, body temperature, blood chemistry, and organ fullness for homeostatic regulation.

What is the role of the Somatic Nervous System?

It controls voluntary movement via motor neurons connecting the CNS directly to skeletal muscles.

What are the two divisions of the Autonomic Nervous System (ANS) and their primary functions?

1. Sympathetic Division: 'Fight or flight' responses. 2. Parasympathetic Division: 'Rest and digest' functions.

Describe the Cerebral Cortex.

It is the large, outer, convoluted sheet of gray matter (roughly 2-4 ext{ mm} thick) in humans, divided into four major lobes. It is the site of high-level cognitive processing, organized into six laminae.

What is the primary function of the Cerebellum?

It is crucial for fine-tuning smooth, coordinated movements, maintaining posture and balance, and plays a significant role in motor learning and timing, acting as an 'error correction' system.

What is the role of the Brainstem?

It connects the cerebrum and cerebellum to the spinal cord. It contains vital nuclei that project broadly, releasing neuromodulators (e.g., norepinephrine, dopamine, serotonin) influencing global brain states like arousal, attention, mood, and sleep-wake cycles.

Why is the Thalamus called the 'gateway to the cerebral cortex'?

It routes almost all sensory information (visual, auditory, somatosensory—except olfaction) to appropriate cortical areas and regulates sleep, wakefulness, and consciousness.

Name the four major cell types in the brain.

1. Neurons 2. Astrocytes 3. Oligodendrocytes (CNS) / Schwann cells (PNS) 4. Microglia

What is the main function of Oligodendrocytes/Schwann cells?

They elaborate myelin, a fatty sheath insulating axons, which dramatically increases action potential conduction velocity through saltatory conduction.

Which part of a neuron typically serves as the primary input antenna, receiving synaptic contacts?

Dendrites.

What is an Action Potential (Spike)?

A brief, rapid, 'all-or-none' electrical event travelling along axons without decrement, typically lasting hickapprox 1-2 imes 10^{-3} ext{ s} and peaking around +30 ext{ mV} from a resting potential of about -70 ext{ mV}. It is driven primarily by Na^+ influx.

What is the typical resting membrane potential of a neuron?

V_{ ext{rest}} hickapprox -70 ext{ mV} relative to the outside.

How does myelin affect axon spike propagation?

Myelin, a lipid-rich sheath, insulates axons and dramatically increases action potential conduction velocity via saltatory conduction (jumping between Nodes of Ranvier).

What are synapses?

Specialized junctions where axon terminals of a presynaptic neuron contact dendrites or soma of a postsynaptic neuron, serving as fundamental sites of information transfer via synaptic transmission.

What neurotransmitter is primarily associated with excitatory synapses in the CNS?

Glutamate.

What neurotransmitter is primarily associated with inhibitory synapses?

GABA (gamma-aminobutyric acid).

What is the main result of neurotransmitter binding at an excitatory synapse?

Influx of positive ions (e.g., Na^+) into the postsynaptic cell, causing depolarization (Excitatory Postsynaptic Potential, EPSP), making the neuron more likely to fire an action potential.

What is the main result of neurotransmitter binding at an inhibitory synapse?

Influx of negative ions (Cl^-) or efflux of positive ions (K^+), causing hyperpolarization or stabilization (Inhibitory Postsynaptic Potential, IPSP), making the neuron less likely to fire an action potential.

How do neuromodulatory synapses differ from excitatory and inhibitory synapses?

They primarily use metabotropic receptors (G-protein coupled receptors) to activate intracellular second messenger cascades, leading to slower, diffuse, and long-lasting alterations in cellular properties, modulating overall brain tone (mood, arousal, attention) rather than rapid excitation/inhibition.

What is the cellular basis for learning and memory?

Synaptic plasticity (e.g., Long-Term Potentiation, LTP; Long-Term Depression, LTD), particularly at glutamatergic excitatory synapses.

What is excitotoxicity, and what neurotransmitter is involved?

Excitotoxicity occurs when excessive glutamate overactivates its receptors, leading to excessive Ca^{2+} influx, resulting in neuronal damage and death. It's implicated in conditions like stroke and epilepsy.

What is the typical range for axonal conduction velocity?

Speeds range from 1 ext{ m/s} (unmyelinated) to over 100 ext{ m/s} (heavily myelinated).