Study Guide: Nervous System Structure and Function

Nervous System: Structure and Function

1. Ion Roles in Neuronal Excitation

Neuronal excitation depends on the movement of ions across the cell membrane, particularly sodium (Na+) and calcium (Ca2+).

• Na+ (Sodium): Responsible for depolarizing the neuron during an action potential. When a nerve terminal is stimulated, voltage-gated sodium channels open, allowing Na+ to enter the cell, making the inside more positive.

• Ca2+ (Calcium): Involved in neurotransmitter release. When the action potential reaches the axon terminal, voltage-gated Ca2+ channels open, and Ca2+ influx triggers synaptic vesicles to release neurotransmitters into the synaptic cleft.

• Example: In neuromuscular junctions, Ca2+ entry causes acetylcholine release, stimulating muscle contraction.

2. Neuron Structure and Function

Neurons are specialized cells for communication in the nervous system. They have distinct structural regions:

• Soma (Cell Body): Contains the nucleus and organelles; responsible for metabolic activities and integration of signals.

• Dendrites: Branch-like extensions that receive signals from other neurons and convey them toward the soma.

• Axon: A long projection that transmits electrical impulses (action potentials) away from the soma to other neurons or effectors.

• Example: Motor neurons have long axons to reach muscles, while interneurons have short axons for local communication.

3. Neuronal Replication

Most neurons are unable to replicate due to their highly specialized structure and the absence of centrioles, which are necessary for cell division.

• Key Point: Neurons are generally considered post-mitotic; they do not undergo mitosis after differentiation.

• Additional info: Some neural stem cells exist in certain brain regions, but most mature neurons cannot divide.

4. Electrochemical Synapse Structure and Function

Synaptic transmission involves several key structures and processes:

• Presynaptic Axon Terminal: Contains synaptic vesicles with neurotransmitters.

• Postsynaptic Dendrite: Receives the neurotransmitter signal.

• Synaptic Vesicles: Release neurotransmitters into the synaptic cleft upon Ca2+ influx.

• Ligand-Gated Ion Channels: Open in response to neurotransmitter binding, allowing ions to flow and generate postsynaptic potentials.

• Voltage-Gated Sodium Channels: Initiate action potentials in the postsynaptic neuron.

• Depolarization: Movement of positive charge from dendrite to axon hillock triggers action potential if threshold is reached.

5. Glial Cells in the Nervous System

Glial cells support and protect neurons. Major types include:

• Astrocytes: Maintain blood-brain barrier, regulate ion balance, and provide structural support.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

• Microglia: Act as immune cells, removing debris and pathogens.

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

6. Anatomical and Functional Divisions of the Nervous System

• Anatomical: Central Nervous System (CNS: brain and spinal cord) and Peripheral Nervous System (PNS: nerves and ganglia).

• Functional: Somatic Nervous System (voluntary control of skeletal muscles) and Autonomic Nervous System (involuntary control of smooth muscle, cardiac muscle, and glands).

7. Sensory vs. Motor Divisions

• Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

• Motor (Efferent) Division: Transmits commands from the CNS to effectors (muscles and glands).

8. Ion Channels and Action Potentials

• Chemically Gated Channels: Open in response to neurotransmitter binding; mediate graded potentials.

• Voltage-Gated Channels: Open in response to changes in membrane potential; mediate action potentials.

• Na+/K+ ATPase: Maintains resting membrane potential by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed.

$3 \text{Na}^+_{\text{in}} + 2 \text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3 \text{Na}^+_{\text{out}} + 2 \text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

9. Brain Regions: Cerebrum, Cerebellum, Pons, Medulla

• Cerebrum: Higher cognitive functions; divided into lobes (frontal, parietal, temporal, occipital).

• Cerebellum: Coordination of movement and balance.

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

• Medulla Oblongata: Controls vital autonomic functions (heart rate, respiration).

• Corpus Callosum: Major white matter tract connecting right and left cerebral hemispheres.

10. Patellar Stretch Reflex

• Receptors: Muscle spindles in the quadriceps detect stretch.

• Afferent Pathway: Sensory neuron transmits signal to spinal cord.

• Integration Center: Synapse in spinal cord (may involve interneurons).

• Efferent Pathway: Motor neuron stimulates quadriceps (extensor); interneuron inhibits hamstrings (flexor).

• Effectors: Quadriceps contract, hamstrings relax.

11. Alzheimer's Disease

• Definition: Neurodegenerative disorder causing memory loss and cognitive decline.

• Affected Regions: Hippocampus, cerebral cortex (especially temporal and parietal lobes).

• Functions Impacted: Memory, reasoning, language, and judgment.

12. Hypothalamus Functions

• Regulates homeostasis: temperature, hunger, thirst, circadian rhythms, and endocrine activity via the pituitary gland.

13. Collections of Cells in the Nervous System

• CNS: Nuclei (clusters of neuron cell bodies).

• PNS: Ganglia (clusters of neuron cell bodies).

14. Brain Regions and Mobility/Decision Making

• Mobility: Cerebellum and basal nuclei are critical for motor coordination.

• Decision Making: Prefrontal cortex is essential for judgment and executive functions.

• Alcohol Effects: Impairs both cerebellum (mobility) and prefrontal cortex (decision making).

15. Inferior and Superior Colliculi

• Superior Colliculi: Visual reflexes (tracking moving objects).

• Inferior Colliculi: Auditory reflexes (startle response to sound).

16. Brain Regions for Autonomic Breathing Control

• Medulla Oblongata and Pons contain respiratory centers that regulate breathing rhythm and depth.

17. Dural Meninges and Hemorrhages

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

• Subdural Bleed: Between dura and arachnoid mater.

• Epidural Hemorrhage: Between skull and dura mater.

18. Brain Protection

• Physical: Skull, meninges, cerebrospinal fluid (CSF).

• Biochemical: Blood-brain barrier, antioxidant enzymes, microglia.

19. Ventricular System and CSF

• Ventricular System: Series of interconnected cavities (lateral, third, fourth ventricles) in the brain.

• Fluid: Cerebrospinal fluid (CSF) circulates, providing cushioning, nutrient delivery, and waste removal.

20. Spinal Cord Organization

• Dorsal/Ventral Root: Dorsal carries sensory input; ventral carries motor output.

• Dorsal/Ventral Horn: Dorsal horn processes sensory information; ventral horn contains motor neurons.

• Lateral Columns: Contain ascending and descending tracts.

• Central Canal: Contains CSF.

• Commissures: Connect left and right sides of the spinal cord.

21. Excitatory and Inhibitory Neurotransmitters

• Excitatory: Glutamate (CNS), acetylcholine (neuromuscular junction).

• Inhibitory: GABA (gamma-aminobutyric acid), glycine.