Fundamentals of the Nervous System and Central Nervous System: Study Guide

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Functions of the Nervous System

The nervous system is responsible for controlling and communicating information throughout the body. Its main functions include:

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpreting sensory input to determine an appropriate response.

• Motor Output: Activating effector organs (muscles and glands) to produce a response.

Organization of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

• Peripheral Nervous System (PNS): Consists of nerves outside the CNS.

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

  • Motor (Efferent) Division: Transmits commands from the CNS to effector organs.

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

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

      • Further divided into sympathetic and parasympathetic divisions.

Cell Types in Nervous Tissue

• Neurons: Excitable cells that transmit electrical signals.

• Neuroglia (Glial Cells): Support, protect, and insulate neurons.

  • Neuroglia in CNS: Astrocytes, oligodendrocytes, microglia, ependymal cells.

  • Neuroglia in PNS: Schwann cells, satellite cells.

Neuron Structure and Regions

• Dendrites: Receive incoming signals and convey them toward the cell body.

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

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

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

Myelination

• Myelin Sheath: Insulates axons, increasing the speed of nerve impulse conduction.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated, enabling saltatory conduction.

Membrane Potential

• Resting Membrane Potential: The voltage difference across the membrane of a resting neuron (typically about -70 mV).

• Depolarization: Membrane potential becomes less negative (more positive).

• Hyperpolarization: Membrane potential becomes more negative than the resting potential.

Ion Channels

• Leak Channels: Always open; allow ions to move down their concentration gradients.

• Gated Channels: Open or close in response to specific stimuli.

  • Chemically-gated (Ligand-gated): Open in response to binding of a chemical messenger.

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

  • Mechanically-gated: Open in response to physical deformation of the membrane.

Action Potential Steps

An action potential is a rapid, temporary change in membrane potential that propagates along the axon.

1. Resting State: All voltage-gated channels are closed; neuron is at resting potential.

2. Depolarization: Voltage-gated Na+ channels open; Na+ enters the cell.

3. Repolarization: Na+ channels inactivate; voltage-gated K+ channels open; K+ exits the cell.

4. Hyperpolarization: K+ channels remain open briefly, causing membrane potential to become more negative than resting.

Synapses

• Presynaptic Neuron: Sends (transmits) signals.

• Postsynaptic Neuron: Receives signals.

• Synaptic Cleft: Small gap between neurons where neurotransmitters are released.

• Neurotransmitters (NT): Chemical messengers that transmit signals across the synaptic cleft.

Neuromuscular Junction (NMJ)

• Acetylcholine (ACh): The neurotransmitter at the NMJ; always causes depolarization of the muscle fiber.

• Leads to muscle contraction by triggering an action potential in the muscle cell.

Key Concepts

• Threshold: The membrane potential (typically around -55 mV) that must be reached to trigger an action potential.

• Na+/K+ Pump: Maintains ion gradients by pumping 3 Na+ out and 2 K+ in, using ATP.

  • Equation: $3 \mathrm{Na}^+_{\text{out}} + 2 \mathrm{K}^+_{\text{in}}$

• Myelinated Axons: Conduct signals faster than unmyelinated axons due to saltatory conduction.

Chapter 12: The Central Nervous System

Embryonic Development of the CNS

• Rostral: Toward the head (anterior).

• Caudal: Toward the tail (posterior).

• Neural Tube: Gives rise to the brain and spinal cord.

• Primary Brain Vesicles:

  • Brain Regions

    • Cerebral Hemispheres: Largest part; responsible for higher brain functions.

    • Diencephalon: Contains thalamus, hypothalamus, epithalamus.

    • Brainstem: Includes midbrain, pons, medulla oblongata.

    • Cerebellum: Coordinates movement and balance.

    • Defining Feature: The foramen magnum separates the brainstem from the spinal cord.

    • Pons vs. Midbrain and Medulla: The pons is anterior and bulges outward; the midbrain is superior, and the medulla is inferior and continuous with the spinal cord.

    Prosencephalon: Forebrain

  • Mesencephalon: Midbrain

  • Rhombencephalon: Hindbrain

• Secondary Brain Vesicles:

  • Telencephalon: Becomes the cerebral hemispheres.

  • Diencephalon: Becomes thalamus, hypothalamus, epithalamus.

  • Mesencephalon: Remains as midbrain.

  • Metencephalon: Becomes pons and cerebellum.

  • Myelencephalon: Becomes medulla oblongata.

Distribution Patterns: Grey vs. White Matter

• In the Brain: Grey matter is superficial (cortex); white matter is deep.

• In the Spinal Cord: White matter is superficial; grey matter is deep (forms an H-shaped core).

• White Matter: Composed of myelinated axons.

• Grey Matter: Composed of neuron cell bodies, dendrites, and unmyelinated axons.

• Grey Part of the Brain: Called the cerebral cortex.

Features and Lobes of the Brain

• Lobes: Frontal, parietal, temporal, occipital, insula.

• Fissures:

  • Longitudinal Fissure: Separates right and left hemispheres.

  • Central Sulcus: Separates frontal and parietal lobes.

  • Lateral Sulcus (Fissure): Separates temporal from frontal and parietal lobes.

  • Parieto-occipital Sulcus: Separates parietal and occipital lobes.

• Gyrus/Gyri: Elevated ridges of the brain.

• Sulcus/Sulci: Shallow grooves between gyri.

• Cerebrospinal Fluid (CSF): Cushions and nourishes the brain and spinal cord.

• Ventricles:

  • Lateral Ventricles: In each cerebral hemisphere.

  • Third Ventricle: In the diencephalon.

  • Fourth Ventricle: Between pons and cerebellum.

  • Ventricles are connected by the interventricular foramen and cerebral aqueduct.

• Corpus Callosum: Connects the two hemispheres.

Organization of the Cerebral Cortex

• Motor Cortex: Controls voluntary movements.

• Primary Motor Cortex: Located in precentral gyrus; initiates voluntary movement.

• Premotor Cortex: Plans movements.

• Broca’s Area: Controls speech production.

• Frontal Eye Field: Controls voluntary eye movements.

• Sensory Cortex: Processes sensory information.

• Primary Somatosensory Cortex: Located in postcentral gyrus; receives sensory input from skin, muscles, joints.

• Somatosensory Association Cortex: Integrates sensory input.

• Primary Visual Cortex: Receives visual information.

• Visual Association Cortex: Interprets visual stimuli.

• Primary Auditory Cortex: Receives auditory information.

• Auditory Association Cortex: Interprets sounds.

• Precentral Gyrus: Contains primary motor cortex.

• Postcentral Gyrus: Contains primary somatosensory cortex.

• Association Areas: Integrate diverse information for purposeful action.

• Lateralization: Specialization of function in each hemisphere.

• Fiber Tracts:

  • 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 cortex with lower brain or spinal cord centers.

• Somatotopy: Point-for-point correspondence between an area of the body and a specific point on the CNS.

• Homunculus: Visual representation of the body within the brain, showing the relative space body parts occupy on the cortex.

Spinal Tracts

• Decussation: Crossing over of nerve fibers from one side of the CNS to the other.

• Symmetry: Pathways are paired symmetrically on each side of the CNS.

Meninges

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

• Spaces: Epidural space (outside dura), subdural space (between dura and arachnoid), subarachnoid space (between arachnoid and pia; contains CSF).

• Spinal Tap (Lumbar Puncture): Removal of CSF from subarachnoid space for diagnostic purposes.

• Epidural: Injection of anesthetic into epidural space; does not enter CSF.

• Difference: Spinal tap accesses CSF; epidural does not.

Spinal Cord

• Origin: Begins at the foramen magnum of the skull.

• Dorsal Horn/Root: Contains sensory (afferent) neurons; ascending information.

• Ventral Horn/Root: Contains motor (efferent) neurons; descending information.

• Ascending/Descending: Also called sensory/motor pathways, respectively.

• Dorsal Column-Medial Lemniscal Pathway: Carries fine touch, vibration, and proprioception; information ascends to the brain.

• Pyramidal Pathway: Carries voluntary motor commands from the cortex to the spinal cord; information descends.

Region	Grey Matter	White Matter
Brain	Superficial (cortex)	Deep
Spinal Cord	Deep (H-shaped core)	Superficial

Example: The dorsal column-medial lemniscal pathway allows you to feel the texture of an object with your fingertips, while the pyramidal pathway enables you to move your fingers to grasp it.

Additional info: Some details, such as the exact threshold value for action potential and the names of all neuroglia, were inferred from standard academic sources.