Module 6: The Nervous System & Central Nervous System – Study Notes

Introduction to the Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and processing sensory information. It is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

• CNS vs PNS: The CNS consists of the brain and spinal cord; the PNS includes all neural tissue outside the CNS.

• Motor vs Sensory Divisions: Sensory (afferent) division transmits impulses from receptors to the CNS; motor (efferent) division transmits impulses from the CNS to effectors.

• Key Terms: Afferent refers to incoming signals; efferent refers to outgoing signals.

Nervous Tissue

Structure and Function of Neurons

Neurons are the functional units of the nervous system, specialized for communication.

• Neuron Anatomy: Includes dendrites (receive signals), soma (cell body), axon (transmits signals), and axon terminals.

• Functional Regions: Receptive (dendrites/soma), conducting (axon), and secretory (axon terminals).

• Pseudounipolar vs Multipolar Neurons: Pseudounipolar neurons have a single process; multipolar neurons have multiple dendrites and one axon.

• Neural Structures: Nucleus (CNS cluster of neuron cell bodies), tract (CNS bundle of axons), ganglion (PNS cluster of cell bodies), nerve (PNS bundle of axons).

Neuroglia (Glial Cells)

Glial cells support and protect neurons. There are six main types, each with distinct functions.

• Astrocytes: Maintain blood-brain barrier, regulate nutrients.

• Oligodendrocytes: Form myelin in CNS.

• Schwann Cells: Form myelin in PNS.

• Microglia: Act as immune cells in CNS.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

• Satellite Cells: Support neurons in PNS ganglia.

Myelin is a fatty substance that insulates axons, increasing the speed of nerve impulse transmission.

• White Matter: Regions rich in myelinated axons.

• Grey Matter: Regions rich in neuron cell bodies and unmyelinated fibers.

Electrophysiology of Neurons

Membrane Potential and Action Potentials

Neurons communicate via electrical signals generated by changes in membrane potential.

• Resting Membrane Potential: The voltage difference across the membrane at rest, typically -70 mV.

• Ion Gradients: Established by Na+ and K+ concentration differences.

• Graded Potentials: Local changes in membrane potential; can be depolarizing or hyperpolarizing.

• Action Potentials: Rapid, all-or-none electrical impulses that travel along axons.

• Voltage-Gated Channels: Na+ channels initiate action potentials; K+ channels restore resting potential.

• Absolute and Relative Refractory Periods: Times during which a neuron cannot or is less likely to fire another action potential.

• Saltatory Conduction: Action potentials jump between nodes of Ranvier in myelinated axons, increasing speed.

Formula:

$I = g(V_m - E)$

Where $I$ is ionic current, $g$ is conductance, $V_m$ is membrane potential, and $E$ is equilibrium potential.

Neuronal Synapses

Chemical Synapses and Synaptic Transmission

Synapses are junctions where neurons communicate with other cells.

• Components: Presynaptic terminal, synaptic cleft, postsynaptic membrane.

• Steps of Transmission: Neurotransmitter release, binding to receptors, postsynaptic response.

• Postsynaptic Potentials: EPSP (excitatory) depolarizes; IPSP (inhibitory) hyperpolarizes the postsynaptic cell.

• Temporal Summation: Multiple signals in quick succession.

• Spatial Summation: Multiple signals from different locations.

• Neurotransmitter Receptors: Ionotropic (direct ion flow), Metabotropic (G-protein coupled, indirect effects).

Major Neurotransmitters

• Glutamate: Main excitatory neurotransmitter.

• GABA: Main inhibitory neurotransmitter.

• Dopamine, Serotonin, Norepinephrine: Modulate mood, attention, and arousal.

• Acetylcholine: Involved in muscle activation and memory.

• Peptides: Substance P, opioids, neuropeptide Y.

Receptor Effects: Glutamate receptors typically produce EPSPs; GABA receptors produce IPSPs.

Central Nervous System (CNS)

Brain Structure and Function

The CNS includes the brain and spinal cord, responsible for processing and integrating information.

• Major Brain Divisions: Cerebrum, diencephalon, midbrain, pons, cerebellum, medulla.

• Grey vs White Matter: Grey matter contains neuron cell bodies; white matter contains myelinated axons.

• Development: Primary brain vesicles (forebrain, midbrain, hindbrain) give rise to mature brain structures.

Major Anatomical Landmarks

• Precentral Gyrus: Primary motor cortex.

• Postcentral Gyrus: Primary somatosensory cortex.

• Central Sulcus, Longitudinal Fissure, Lateral Fissure: Major grooves dividing brain regions.

• Basal Nuclei: Involved in movement regulation.

• Limbic System: Emotion and memory.

• Thalamus, Hypothalamus, Pineal Gland: Sensory relay, homeostasis, circadian rhythms.

• Cerebellum: Coordination and balance.

Protection of the Brain

• Blood-Brain Barrier (BBB): Protects brain from toxins; formed by astrocytes and endothelial cells.

• Meninges: Dura mater, arachnoid mater, pia mater.

• Cerebrospinal Fluid (CSF): Cushions and nourishes the brain.

Role of CNS in Homeostasis

• Hypothalamus: Regulates temperature, hunger, thirst, and endocrine functions.

• Pineal Gland: Secretes melatonin, regulates sleep-wake cycles.

• Reticular Formation: Controls arousal and consciousness.

Higher Mental Functions

• Prefrontal Cortex: Decision making, personality, social behavior.

• Memory Consolidation: Transfer of short-term to long-term memory.

• Long-Term Potentiation (LTP): Strengthening of synapses, basis for learning and memory.

Spinal Cord and Pathways

Spinal Cord Structure

• Grey Matter: Contains neuron cell bodies; organized into horns.

• White Matter: Contains myelinated axons; organized into columns.

• Dorsal and Ventral Roots: Dorsal roots carry sensory information; ventral roots carry motor information.

Sensory and Motor Pathways

• Dorsal Column-Medial Lemniscus: Transmits fine touch and proprioception.

• Anterolateral System: Transmits pain and temperature.

• Corticospinal Tract: Controls voluntary movement.

• Basal Ganglia and Cerebellum: Modulate movement and coordination.

Summary Table: Major Neuroglia and Their Functions

Glial Cell Type	Location	Main Function
Astrocyte	CNS	Support neurons, maintain BBB, regulate nutrients
Oligodendrocyte	CNS	Form myelin sheaths
Schwann Cell	PNS	Form myelin sheaths
Microglia	CNS	Immune defense
Ependymal Cell	CNS	Produce cerebrospinal fluid
Satellite Cell	PNS	Support neuron cell bodies in ganglia

Example: Action Potential Generation

• When a neuron is stimulated, voltage-gated Na+ channels open, causing depolarization.

• If the threshold is reached, an action potential is generated and propagated along the axon.

• Voltage-gated K+ channels then open, repolarizing the membrane.

Additional info: Some details, such as the specific names and functions of glial cells, and the formula for ionic current, were inferred from standard academic sources to provide a complete study guide.