Nervous System Organization

Central and Peripheral Nervous System

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The CNS includes the brain and spinal cord, while the PNS consists of nerves and ganglia outside the CNS.

• Sensory (dorsal) and Motor (ventral) roots in the spinal cord

• White matter: Contains axons and is myelinated

• Gray matter: Not myelinated

• Spinal reflexes: Initiate responses without input from the brain

Protecting the CNS

The CNS is protected by several structures and barriers:

• Meninges: Three layers (pia mater, arachnoid, dura mater) protect the brain and spinal cord

• Cerebrospinal Fluid (CSF): Cushions and nourishes the CNS

• Blood-Brain Barrier: Regulates passage of substances from blood to CNS

Meningitis: Inflammation of the meninges, can be life-threatening due to infection.

Major Brain Divisions

• Cerebrum: Controls higher functions, sensory perception, voluntary movement

• Cerebellum: Regulates motor control, coordination

• Medulla oblongata: Controls cardiac and respiratory functions

• Pons: Relays signals between cerebrum and cerebellum

Cranial nerves: 12 pairs, including Olfactory (I), Optic (II), Oculomotor (III), Trochlear (IV), Trigeminal (V), Abdominal (VI)

Brain Structure and Function

Cerebral Cortex and Thalamus

The cerebral cortex is highly folded and contains more white matter. The thalamus relays all sensory information except smell to the cortex.

• Frontal lobe: Coordinates info from other areas, controls behavior

• Parietal lobe: Processes sensory info from skin, musculoskeletal system, viscera, taste

• Occipital lobe: Visual processing

• Temporal lobe: Auditory processing

Corpus callosum: Connects left and right hemispheres

Language areas: Visual, auditory, and motor integration; language tends to be on the left side

Cortical Association Areas

• Wernicke's Area: Language comprehension; damage causes inability to understand language

• Broca's Area: Speech production; damage causes inability to speak

Neural Activity and Blood Flow

Blood Flow and Metabolic Demand

Increased neural activity leads to increased metabolic demand for glucose and oxygen, resulting in increased blood flow to active brain regions.

• PET (Positron Emission Tomography): Measures brain activity by detecting radioactive tracers

• MRI: Measures changes in hemoglobin oxygenation

• EEG: Measures electric potential

• MEG: Measures magnetic field

Neurons and Glial Cells

Neuron Structure

Neurons are excitable cells that transmit electrical signals. They consist of dendrites, a cell body (soma), and an axon.

• Multipolar neurons: Motor neurons, pyramidal cells, Purkinje cells

• Unipolar, bipolar, pseudo-unipolar: Other neuron morphologies

Glial cells outnumber neurons by 10:1 and provide support and insulation.

• Schwann cells: Produce myelin in PNS

• Oligodendrocytes: Produce myelin in CNS

Membrane Potential and Ion Channels

Resting Membrane Potential (RMP)

The resting membrane potential is the voltage difference across the neuronal membrane, typically around -60 mV.

• Na+/K+ pump: Maintains RMP by active transport

• Uses over 50% of the metabolic energy in the mammalian brain

Equilibrium potential: The voltage at which the net flow of a particular ion is zero.

Nernst equation:

Neurons have higher sodium concentrations outside and higher potassium concentrations inside.

Ion Channels and Signal Transmission

Neurons signal by changes in membrane potential (Vm), which arise from opening and closing of ion channels.

• Permeability: How many ions allowed

• Selectivity: Which ions are allowed

• Gating: What opens/closes the channel

Voltage-gated channels change conductance with voltage; channels have open and closed states.

Action Potentials

Generation and Propagation

Action potentials are the basis for neural information transmission. They are rapid, all-or-none electrical signals.

• Depolarization: Triggers Na+ channel activation gates to open, Na+ enters cell, causing further depolarization

• Positive feedback: One of the only positive feedback loops in biology

• Propagation: Action potentials travel along axons; myelin increases speed via saltatory conduction

Schwann cells produce myelin in PNS; Oligodendrocytes in CNS. Demyelinating diseases (e.g., Multiple Sclerosis) impair propagation.

Action Potential Equations

• Ohm's Law: or

• Membrane capacitance:

Synaptic Transmission

Types of Synapses

• Electrical synapses: Passive diffusion via gap junctions

• Chemical synapses: Neurotransmitter release activates postsynaptic cell

SNARE proteins catalyze fusion of vesicles with the presynaptic membrane.

• Synaptotagmin: Binds Ca2+, triggers exocytosis, located close to Ca2+ channels for fast release

Neurotransmitter Clearance

• Return to axon terminal

• Enzymatic inactivation

• Diffusion out of synaptic cleft

Certain drugs inhibit neurotransmitter reuptake (e.g., SSRIs, cocaine).

Neurotransmitter Receptors

• Excitatory transmission: Increases postsynaptic neuron excitability

• Inhibitory transmission: Decreases postsynaptic neuron excitability

Additional info:

• Some context and terminology expanded for clarity (e.g., definitions of SNARE proteins, synaptotagmin, and action potential propagation)

• Equations and formulas provided in LaTeX format for academic completeness