Study Guide: Nervous System Structure and Function (Ch. 11, BIOL 145)

Introduction to the Nervous System

Anatomical Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integrating, processing, and coordinating sensory data and motor commands.

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

Functional Divisions of the Nervous System

• Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion). Subdivided into sympathetic and parasympathetic divisions.

• Enteric Nervous System (ENS): Governs the function of the gastrointestinal tract.

Neuronal Structure and Function

Structure of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles; metabolic center of the neuron.

• Dendrites: Receive incoming signals from other neurons.

• Axon: Conducts electrical impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

Axonal Transport

• Definition: Movement of materials between the cell body and axon terminals.

• Types:

  • Anterograde Transport: From soma to axon terminal (e.g., neurotransmitters).

  • Retrograde Transport: From axon terminal to soma (e.g., recycled materials).

Classification of Neurons

• By Structure: Multipolar, bipolar, unipolar, anaxonic.

• By Function: Sensory (afferent), motor (efferent), interneurons.

Groupings in the Nervous System

• Ganglia: Clusters of neuron cell bodies in the PNS.

• Nuclei: Clusters of neuron cell bodies in the CNS.

• Nerves: Bundles of axons in the PNS.

• Tracts: Bundles of axons in the CNS.

Neuroglia (Glial Cells)

CNS Glia

• Astrocytes: Maintain blood-brain barrier, regulate ion/nutrient concentrations, structural support.

• Oligodendrocytes: Form myelin sheaths in CNS.

• Microglia: Act as phagocytes, removing debris and pathogens.

• Ependymal Cells: Line ventricles, produce and circulate cerebrospinal fluid (CSF).

PNS Glia

• Satellite Cells: Surround neuron cell bodies in ganglia; regulate environment.

• Schwann Cells: Form myelin sheaths in PNS; assist in axon regeneration.

Myelin, Blood-Brain Barrier, and CSF

• Myelin: Insulating layer around axons; increases speed of impulse conduction.

• Blood-Brain Barrier: Selective barrier formed by endothelial cells and astrocytes; protects CNS from toxins.

• Cerebrospinal Fluid (CSF): Cushions CNS, circulates nutrients, removes waste.

Demyelination

• Causes: Autoimmune diseases (e.g., multiple sclerosis), infections, toxins.

• Consequences: Slowed or blocked nerve conduction, neurological deficits.

Regeneration in the PNS

• Steps: Wallerian degeneration, macrophage cleanup, Schwann cell proliferation, axon regrowth.

Neuronal Physiology

Resting Membrane Potential

• Definition: The voltage difference across the neuron's membrane at rest (typically -70 mV).

• Maintained by: Sodium-potassium pump, leak channels, impermeability to large anions.

Electrochemical Gradients for Na+ and K+

• Na+: Higher outside; moves inward when channels open.

• K+: Higher inside; moves outward when channels open.

Ion Channels at Rest

• Leak Channels: Always open; allow passive movement of ions.

• Na+/K+ ATPase Pump: Actively transports 3 Na+ out and 2 K+ in per cycle. Equation: $3\ \mathrm{Na}^+_{\text{in}} + 2\ \mathrm{K}^+_{\text{out}} + \mathrm{ATP} \rightarrow 3\ \mathrm{Na}^+_{\text{out}} + 2\ \mathrm{K}^+_{\text{in}} + \mathrm{ADP} + \mathrm{P}_i$

Ion Channels During Stimulation

• Voltage-Gated Channels: Open in response to changes in membrane potential.

• Chemically Gated Channels: Open in response to neurotransmitter binding.

• Mechanically Gated Channels: Open in response to physical deformation.

Membrane Potential Changes

• Depolarization: Membrane potential becomes less negative (Na+ influx).

• Repolarization: Return to resting potential (K+ efflux).

• Hyperpolarization: Membrane potential becomes more negative than resting.

Local (Graded) Potentials

• Definition: Small, localized changes in membrane potential; decrease with distance.

• Role: Can trigger action potentials if threshold is reached.

Action Potential (AP)

• All-or-Nothing: AP occurs fully if threshold is reached; otherwise, not at all.

• Phases: Depolarization (Na+ in), repolarization (K+ out), hyperpolarization.

• Threshold: Minimum depolarization needed to trigger AP (typically -55 mV).

• Refractory Periods:

  • Absolute: No new AP possible.

  • Relative: Stronger stimulus needed for AP.

• Propagation:

  • Continuous: Unmyelinated axons; AP moves along every segment.

  • Saltatory: Myelinated axons; AP jumps between nodes of Ranvier.

Synaptic Transmission

• Synapse: Junction between two neurons or a neuron and effector cell.

• Types:

  • Electrical: Direct ion flow via gap junctions; fast, rare.

  • Chemical: Neurotransmitter release; slower, common.

• Neurotransmission Events: AP arrives, Ca2+ influx, neurotransmitter release, postsynaptic response.

• Neurotransmitters: Examples include acetylcholine, dopamine, serotonin.

• Receptors:

  • Ionotropic: Ligand-gated ion channels; fast response.

  • Metabotropic: G-protein coupled; slower, modulatory effects.

• Termination: Enzymatic degradation, reuptake, or diffusion away from synapse.

Neural Integration

• Processing: Summation of inputs determines neuron response.

• EPSPs (Excitatory Postsynaptic Potentials): Depolarize membrane; increase chance of AP.

• IPSPs (Inhibitory Postsynaptic Potentials): Hyperpolarize membrane; decrease chance of AP.

• Summation:

  • Temporal: Multiple signals in quick succession.

  • Spatial: Multiple signals from different locations.

Neuronal Pools and Circuits

• Neuronal Pools: Groups of interconnected neurons with specific functions.

• Divergence: One neuron sends signals to many.

• Convergence: Many neurons send signals to one.

Central and Peripheral Nervous System Components

CNS and PNS Components

• CNS: Brain (cerebrum, cerebellum, diencephalon, brainstem), spinal cord.

• PNS: Cranial nerves, spinal nerves, ganglia.

Cerebrum

• Lobes: Frontal (motor, planning), parietal (sensory), temporal (hearing, memory), occipital (vision).

• Cerebral Cortex: Outer gray matter; divided into primary motor, primary sensory, and association areas.

• White Matter: Deep to cortex; transmits signals between regions.

• Basal Nuclei: Deep gray matter; regulate movement.

Cerebellum

• Structure: Two hemispheres, cortex, arbor vitae (white matter).

• Function: Coordinates movement, balance, posture.

Diencephalon

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis, endocrine functions.

• Limbic System: Emotion, memory, motivation.

Brainstem

• Midbrain: Visual/auditory reflexes, motor control.

• Pons: Relays signals, regulates breathing.

• Medulla Oblongata: Autonomic functions (heart rate, respiration).

• Reticular Formation: Arousal, consciousness.

Meninges

• Layers: Dura mater (outer), arachnoid mater (middle), pia mater (inner).

• Function: Protect CNS, contain CSF.

• Meningitis: Inflammation of the meninges, often due to infection.

Ventricles and CSF

• Ventricles: Four interconnected cavities in the brain; contain CSF.

• CSF: Produced by choroid plexus; circulates in ventricles, subarachnoid space; drained via arachnoid granulations.

Stroke

• Definition: Disruption of blood flow to the brain; causes tissue damage.

• Effects: Left side—language, right side—spatial, lower brain—vital functions.

Spinal Cord Structure

• Regions: Cervical (C), thoracic (T), lumbar (L), sacral (S), coccygeal (Co).

• Gray Matter: Cell bodies, dendrites; central "H" shape.

• White Matter: Myelinated axons; surrounds gray matter.

• Dorsal/Ventral Roots: Sensory/motor axons entering/leaving cord.

• Dorsal/Ventral Rami: Branches of spinal nerves to back/front of body.

• Connective Tissue Layers: Epineurium (outer), perineurium (fascicles), endoneurium (individual axons).

Reflexes

Types of Reflexes

• Monosynaptic: Single synapse (e.g., patellar reflex).

• Polysynaptic: Multiple synapses (e.g., withdrawal reflex).

Reflex Arc Steps

1. Receptor detects stimulus.

2. Sensory neuron transmits signal to CNS.

3. Integration center processes information.

4. Motor neuron transmits response.

5. Effector carries out response.

Examples of Reflexes

• Corneal Reflex: Blinking in response to corneal stimulation.

• Auditory Reflex: Turning head toward sound.

• Pupillary Reflex: Pupil constriction in response to light.

• Patellar Reflex: Knee-jerk response to tendon tap.

Withdrawal Reflex and Reciprocal Inhibition

• Withdrawal Reflex: Pulling away from painful stimulus.

• Reciprocal Inhibition: Inhibition of antagonist muscles during reflex.

• Crossed Extensor Reflex: Opposite limb extends to support body during withdrawal.