Nervous System Organization

Divisions of the Nervous System

The nervous system is a complex network responsible for communication and control throughout the body. It is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and coordinates activity.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. It connects the CNS to limbs and organs.

The PNS is further divided into:

• Somatic Nervous System: Controls voluntary movements.

• Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion).

Electrophysiology of Neurons

Principles of Electrophysiology

Electrophysiology studies the electrical properties of biological cells and tissues, focusing on how neurons generate and transmit electrical signals.

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

• Resting State: The neuron is not transmitting a signal; ion channels maintain the resting potential.

• Activated State: The neuron is depolarized, and an action potential may be generated.

• Inactivated State: Certain ion channels are temporarily unable to open, preventing immediate reactivation.

Key Equation:

• Resting membrane potential is determined by the Nernst equation:

Action Potentials

An action potential is a rapid, temporary change in membrane potential that travels along the neuron.

• Depolarization: Sodium channels open, Na+ enters the cell, making the inside more positive.

• Repolarization: Potassium channels open, K+ leaves the cell, restoring negative charge.

• Hyperpolarization: Membrane potential becomes more negative than resting potential before stabilizing.

Difference between Local and Action Potentials:

• Local Potentials: Small, graded changes in membrane potential that decay with distance.

• Action Potentials: All-or-none, self-propagating signals that do not decrease in strength.

Neuronal Communication

Synapses and Terminology

• Axodendritic: Synapse between an axon and a dendrite.

• Axoaxonic: Synapse between two axons.

• Axosomatic: Synapse between an axon and a cell body (soma).

Chemical vs. Electrical Synapses:

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; most common type.

• Electrical Synapses: Direct electrical connection via gap junctions; allow rapid, synchronized activity.

Neural Integration and Summation

• Summation: The process by which multiple synaptic potentials combine within one postsynaptic neuron.

• Types:

  • Spatial Summation: Multiple inputs from different locations at the same time.

  • Temporal Summation: Multiple inputs from the same location in rapid succession.

Neurotransmitters

• Acetylcholine (ACh): Excitatory at neuromuscular junctions; involved in muscle activation.

• Dopamine: Involved in reward, motivation, and motor control.

• Serotonin: Regulates mood, appetite, and sleep.

• GABA (gamma-aminobutyric acid): Main inhibitory neurotransmitter in the CNS.

• Glutamate: Main excitatory neurotransmitter in the CNS.

Neuronal Circuits

• Diverging Circuit: One neuron stimulates many others.

• Converging Circuit: Many neurons stimulate a single neuron.

• Reverberating Circuit: Neurons stimulate each other in a loop.

• Parallel After-Discharge Circuit: Input diverges to several pathways, which reconverge.

Brain Structure and Function

Basic Structures of the Brain

• Cerebrum: Largest part; responsible for higher brain functions (thought, memory, voluntary movement).

• Cerebellum: Coordinates movement and balance.

• Diencephalon: Contains thalamus and hypothalamus; relays sensory information and controls autonomic functions.

• Brainstem: Includes midbrain, pons, and medulla oblongata; controls vital functions (breathing, heart rate).

White Matter and Gray Matter

• White Matter: Composed of myelinated axons; found deep in the brain and superficial in the spinal cord.

• Gray Matter: Contains neuron cell bodies; found superficially in the brain (cortex) and deep in the spinal cord.

Surface Features of the Brain

• Fissure: Deep groove (e.g., longitudinal fissure separates cerebral hemispheres).

• Sulcus: Shallow groove on the brain surface.

• Gyrus: Raised ridge between sulci.

Lobes of the Brain

• Frontal Lobe: Voluntary movement, planning, reasoning.

• Parietal Lobe: Sensory perception, spatial awareness.

• Temporal Lobe: Hearing, memory, language.

• Occipital Lobe: Vision.

• Insula: Deep within the lateral sulcus; involved in taste, visceral sensation.

Functional Areas

• Primary Motor Cortex: Controls voluntary movements.

• Primary Somatosensory Cortex: Processes sensory input.

• Auditory Cortex: Hearing.

• Visual Cortex: Vision.

• Broca's Area: Speech production.

• Wernicke's Area: Language comprehension.

Basal Nuclei

• Clusters of gray matter deep within the cerebrum.

• Regulate voluntary motor activities and inhibit unnecessary movements.

Fiber Types in the Brain

• Association Fibers: Connect areas within the same hemisphere.

• Commissural Fibers: Connect corresponding areas of the two hemispheres (e.g., corpus callosum).

• Projection Fibers: Connect the cerebrum with lower brain or spinal cord centers.

Limbic System

• Group of structures involved in emotion, motivation, and memory.

• Includes hippocampus, amygdala, and parts of the thalamus.

Brainstem Components and Functions

• Midbrain: Visual and auditory reflexes, eye movement.

• Pons: Relays information between cerebrum and cerebellum; regulates breathing.

• Medulla Oblongata: Controls heart rate, blood pressure, and respiration.

• Reticular Formation: Network of neurons involved in arousal and consciousness.

Cerebrospinal Fluid (CSF)

• Formed by the choroid plexus in the ventricles of the brain.

• Cushions the brain, removes waste, and circulates nutrients.

Hypothalamus

• Part of the diencephalon; regulates homeostasis, endocrine activity, and autonomic functions.

Protection of the Brain and Spinal Cord

• Meninges: Three protective membranes (dura mater, arachnoid mater, pia mater).

• CSF: Provides cushioning.

• Blood-Brain Barrier: Protects brain from harmful substances.

Spinal Cord Structure and Function

Spinal Cord Terminology

• Cauda Equina: Bundle of spinal nerves at the lower end of the spinal cord.

• Posterior (Dorsal) Horn: Contains sensory neurons.

• Lateral Horn: Contains autonomic motor neurons (present in thoracic and upper lumbar regions).

• Anterior (Ventral) Horn: Contains somatic motor neurons.

Ascending and Descending Tracts

• Ascending Tracts: Carry sensory information to the brain (e.g., spinothalamic tract).

• Descending Tracts: Carry motor commands from the brain to the body (e.g., corticospinal tract).

Order of Neurons in Sensory Pathways

• First-Order Neuron: Sensory neuron that detects stimulus and transmits signal to spinal cord or brainstem.

• Second-Order Neuron: Relays signal from spinal cord/brainstem to thalamus.

• Third-Order Neuron: Relays signal from thalamus to cerebral cortex.

Decussation

• Process by which nerve fibers cross from one side of the CNS to the other.

• Occurs in various locations, such as the medulla oblongata (pyramidal decussation).

Somatotopy

• Spatial organization of sensory and motor pathways in the CNS, reflecting the arrangement of the body.

• Example: The homunculus map in the primary motor and sensory cortices.

Additional info: Some details, such as specific tracts or nuclei, may require reference to lecture notes or textbooks for comprehensive coverage.