Neurons, Synapses, Muscle, and Nervous System Organization: Study Notes

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Neurons, Synapses, and Signaling

Neuron Structure and Function

Neurons are specialized cells for receiving, processing, and transmitting information. Their structure reflects their role in information transfer within the nervous system.

Cell body (soma): Contains the nucleus and organelles; integrates incoming signals.
Dendrites: Branch-like extensions that receive input from other neurons.
Axon: Long projection that conducts action potentials away from the soma; originates at the axon hillock, the site of action potential initiation.
Synaptic terminals: Axon endings that form synapses with target cells.
Synapse: Junction where a neuron communicates with another cell.
Neurotransmitter: Chemical messenger released at most synapses to transmit signals across the synaptic cleft.

Information Processing Pathway:

Sensory input: Sensory neurons carry signals from receptors toward the CNS.
Integration: Interneurons and neural circuits process and interpret input.
Motor output: Motor neurons send commands to effectors (muscles or glands).

Types of Neurons

Sensory neurons: Transmit information from sensory receptors to the CNS.
Interneurons: Connect neurons within the CNS for processing and integration.
Motor neurons: Carry signals from the CNS to effectors.

Ion Pumps, Ion Channels, and the Resting Potential

Establishing the Resting Membrane Potential

The resting membrane potential (Vm) is the baseline voltage across the neuronal membrane, typically negative inside relative to outside. It is established by:

Na+/K+ pump: Uses ATP to move 3 Na+ out and 2 K+ in, creating ion gradients.
Selective permeability: The membrane is more permeable to K+ (via leak channels) than to Na+ at rest.

Ion Gradients:

K+: High inside, low outside; tends to diffuse out, making the inside more negative.
Na+: Low inside, high outside; tends to diffuse in.

Equilibrium Potential: Each ion has a voltage where its net diffusion stops. The resting Vm is closer to the equilibrium potential for K+ because K+ permeability dominates.

Depolarization: Vm becomes less negative.
Hyperpolarization: Vm becomes more negative.
Graded potential: Variable size, decays over time and distance.
Threshold: Vm level that triggers an action potential (often ~−55 mV).
Action potential: All-or-none spike that regenerates along the axon.
Refractory period: Time when Na+ channels are inactivated and the neuron cannot fire again immediately.

Key Equations:

Nernst Equation (for equilibrium potential of an ion):
Goldman-Hodgkin-Katz Equation (for membrane potential):

Action Potentials

Action Potential Sequence

Action potentials are all-or-none electrical signals that travel along axons. They are generated when the membrane potential reaches threshold and follow a stereotyped sequence:

Threshold reached at axon hillock.
Voltage-gated Na+ channels open → Na+ influx (depolarization).
Na+ channels inactivate.
Voltage-gated K+ channels open → K+ efflux (repolarization/hyperpolarization).
Undershoot (Vm becomes more negative than resting).
Return to resting potential.

Refractory Period: Na+ channel inactivation prevents immediate re-firing, ensuring unidirectional propagation.

Conduction Speed Increases With:

Larger axon diameter
Myelination (saltatory conduction at nodes of Ranvier)

Graded Potentials vs Action Potentials

Graded potentials: Variable size, decay with distance, occur in dendrites/soma, usually ligand-gated channels.
Action potentials: All-or-none, regenerate along axon, triggered at axon hillock by summed graded potentials reaching threshold.

Summation:

Temporal summation: Same synapse fires repeatedly.
Spatial summation: Multiple synapses active at once.

EPSP vs IPSP:

EPSP (Excitatory Postsynaptic Potential): Depolarizes toward threshold (often Na+ in).
IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes away from threshold (often Cl− in or K+ out).

Action Potentials and Calculus

Action potentials can be analyzed as curves of voltage over time, V(t). The peak of the action potential occurs where the slope is zero:

(maximum or minimum)
For a maximum,

This approach is used in neuroscience and physiology to analyze voltage traces and identify key events.

Synaptic Transmission

Chemical and Electrical Synapses

Neurons communicate with other cells at synapses, which can be chemical or electrical.

Chemical synapses: Use neurotransmitter release and receptor binding; most common type.
Electrical synapses: Use gap junctions (connexons) for direct cytoplasmic continuity; fast and often bidirectional.

Chemical Synapse Sequence

Action potential arrives at synaptic terminal.
Depolarization opens voltage-gated Ca2+ channels.
Ca2+ influx triggers synaptic vesicle fusion (exocytosis).
Neurotransmitter released into synaptic cleft.
Neurotransmitter binds to postsynaptic receptors (ionotropic or metabotropic).
Postsynaptic response (EPSP or IPSP) generated.
Signal terminated by enzymatic breakdown, reuptake, uptake by glia, or diffusion.

Ionotropic receptors: Ligand-gated ion channels; fast, short-lived responses. Metabotropic receptors: G-protein-coupled receptors; slower, longer-lasting, amplified responses.

EPSP and IPSP Recap

EPSP: Small voltage change that brings the postsynaptic cell closer to firing an action potential.
IPSP: Small voltage change that makes the postsynaptic cell less likely to fire.
Summation of EPSPs and IPSPs at the axon hillock determines whether an action potential is triggered.

Muscle Types and Contraction

Muscle Types

Muscle Type	Structure	Control	Location	Key Features
Smooth	Spindle-shaped, non-striated	Involuntary	Walls of hollow organs	Slow, sustained contractions
Cardiac	Striated, branched, intercalated discs	Involuntary	Heart	Coordinated, rhythmic contractions
Skeletal	Striated, long cylindrical, multinucleate	Voluntary	Attached to bones	Strong, fast contractions

Muscle Cell Structure

Whole muscle → bundles → muscle fibers (cells) → myofibrils → sarcomeres → thin/thick filaments
Sarcomere: Basic contractile unit, bounded by Z lines.
Z lines: Anchor thin filaments.
M line: Center of sarcomere, anchors thick filaments.

Sliding Filament Model

Thin filaments (actin) slide past thick filaments (myosin), shortening the sarcomere.
Actin and myosin do not shorten; overlap increases.
Contraction: Z lines move closer together.

Filament Components

Thin filament: Actin (with myosin-binding sites), tropomyosin (blocks binding sites), troponin (binds Ca2+).
Thick filament: Myosin (motor protein with heads that bind actin and ATP).

Cross-Bridge Cycle (Power Stroke)

Ca2+ binds troponin → tropomyosin shifts, exposing actin binding sites.
Myosin head binds actin (cross-bridge forms).
Power stroke: Myosin pivots, pulling actin filament inward.
ATP binds myosin → cross-bridge breaks.
ATP hydrolysis resets myosin head.
Cycle repeats as long as Ca2+ is present.

Muscle Contraction Control

Motor neuron action potential reaches neuromuscular junction (NMJ).
Acetylcholine (ACh) released, binds muscle membrane receptors.
Muscle action potential spreads along sarcolemma and T tubules.
Sarcoplasmic reticulum releases Ca2+.
Contraction begins; relaxation occurs when Ca2+ is pumped back into SR.

Whole-Muscle Control

Recruitment: Activating more motor units increases force.
Summation: Repeated stimulation before relaxation increases tension.
Tetanus: High-frequency stimulation produces fused contraction.

Clinical Anchors

Myasthenia gravis: Autoimmune attack on ACh receptors at NMJ; causes muscle weakness. Treated with acetylcholinesterase inhibitors or immune suppression.
Rigor (rigor mortis): ATP depletion prevents myosin detachment from actin, causing muscle stiffness after death.

Nervous System Organization and Glia

Evolution and Organization

Nerve net: Diffuse network (e.g., cnidarians).
Cephalization: Clustering of sensory and interneurons at the anterior end.
CNS (Central Nervous System): Brain and spinal cord; integration hub.
PNS (Peripheral Nervous System): Nerves and ganglia; carries information to/from CNS.

Spinal Cord and PNS

Spinal cord: Relays information, generates locomotion patterns, runs reflex circuits.
Afferent neurons: Sensory input to CNS.
Efferent neurons: Motor output from CNS to effectors.
Motor system: Controls skeletal muscle.
Autonomic system: Controls smooth/cardiac muscle and glands (sympathetic, parasympathetic, enteric divisions).

Glia Cells

Schwann cells: Myelinate axons in PNS.
Oligodendrocytes: Myelinate axons in CNS.
Astrocytes: Support blood–brain barrier, regulate ion balance, maintain homeostasis.
Microglia: Immune defense and cleanup in CNS.
Radial glia: Guide neuron migration during development.

Glia support, insulate, and protect neurons, but do not conduct action potentials.

Enteric Nervous System (ENS)

Network of neurons in digestive tract walls; controls GI function locally.
Can operate independently of CNS, but modulated by autonomic system.

Vertebrate Brain Regional Specialization

Major Brain Regions

Region	Main Functions
Forebrain	Olfaction, sleep, learning, complex processing
Midbrain	Sensory routing and coordination
Hindbrain	Involuntary functions (e.g., breathing), movement coordination (cerebellum)

Key Structures and Functions

Thalamus: Sensory relay/sorting station.
Hypothalamus: Homeostasis and body regulation (temperature, hunger, thirst, hormones, autonomic state).
Medulla oblongata: Controls vital reflexes (breathing, heart rate).
Cerebellum: Movement coordination and balance.
Prefrontal cortex: Executive functions and decision making.

Sleep and Biological Clocks

Reticular formation: Regulates arousal, sleep, and sensory filtering.
Suprachiasmatic nucleus (SCN): Circadian pacemaker; synchronizes biological rhythms to light/dark cycles.

Cerebral Cortex: Voluntary Movement and Cognition

Functional Organization

Sensory input → primary sensory areas → association areas → prefrontal planning → motor cortex → brainstem/spinal cord → motor neurons → skeletal muscle
Lobes: Frontal (executive, motor), temporal (auditory, language), occipital (visual), parietal (sensory integration).

Language and Lateralization

Broca’s area: Speech production.
Wernicke’s area: Speech comprehension.
Lateralization: Left hemisphere often dominant for language/math; right for spatial/pattern recognition.

Learning, Memory, and Plasticity

Neuronal Plasticity

Learning and memory involve activity-dependent remodeling of synapses.
Developmental processes include neuron competition, programmed cell death, and synaptic pruning.

Memory Types

Short-term memory: Temporary links, hippocampus involved.
Long-term memory: Stable connections in cerebral cortex; sleep may aid consolidation.

Long-Term Potentiation (LTP)

Lasting increase in synaptic strength; involves glutamate, NMDA/AMPA receptors, and coincidence detection.

Disorders of the Nervous System

Schizophrenia: Dopamine pathway hypothesis; genetic and environmental factors; treated with dopamine receptor blockers.
Depression: Linked to biogenic amines; medications often increase monoamine activity (e.g., SSRIs).
Addiction: Involves reward system (dopamine release from VTA to nucleus accumbens, prefrontal cortex); drugs alter signaling.
Alzheimer’s disease: β-amyloid plaques, tau tangles, neuron loss (especially hippocampus/cortex).
Parkinson’s disease: Loss of dopamine neurons affecting basal nuclei; treated with l-dopa or stimulation.

Key Terms and Definitions

Neuron: Cell specialized for information transfer.
Dendrite: Receives synaptic input.
Axon hillock: Action potential initiation site.
Myelin sheath: Insulating layer that speeds conduction.
Node of Ranvier: Gap in myelin where action potentials are regenerated.
Saltatory conduction: Action potentials jump between nodes.
Ionotropic receptor: Ligand-gated ion channel.
Metabotropic receptor: G-protein-coupled receptor.
Acetylcholinesterase (AChE): Enzyme that breaks down acetylcholine.
GABA: Major inhibitory neurotransmitter in the brain.

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard biology textbooks.