Nervous Tissue and Neuron Structure Review

Nervous Tissue Review

• Central Nervous System (CNS): Composed of the brain and spinal cord.
• Peripheral Nervous System (PNS): Includes nerves branching out from the CNS.

Neuron Structure

• Neuron Parts: Cell body, axon, dendrites.
  • Impulse Direction: Dendrites receive impulses, axons transmit impulses away from the cell body.
  • Myelin: Axons and some parts of the dendrites can be myelinated.
  • Nissl Bodies: Structures within the cell body, involved in protein synthesis.
  • Neurofibrils: Provide internal support for the neuron.
  • Cell Division: Neurons generally do not divide.
  • Axon Hillock: The area where the axon joins the cell body.
  • Synaptic Knobs: Bulbs at the ends of axon branches, filled with neurotransmitter vesicles. Neurotransmitters contained within proteins called vesicles.

Neuron Types

• Unipolar (Pseudounipolar): Single process extending from the cell body, often sensory neurons relaying information from the body to the CNS.
• Bipolar: Two processes extending from the cell body, found in special senses (e.g., eye and ear).
• Multipolar: Multiple processes extending from the cell body, common in neurons of the CNS.
• PNS vs. CNS: Unipolar and bipolar neurons are mainly in the PNS, multipolar primarily in the CNS.
• Sensory/Motor/Association: Neurons can be classified as sensory (relaying information inward to the CNS), motor (transmitting signals outward to muscles or glands), or association (linking other neurons in the CNS).

Neuroglia (Glial Cells)

• Functions: Support, protect, and nourish neurons.
• Location: CNS and PNS.
• Specific Cell Roles: Different types of glial cells have specific roles, e.g., myelin production (oligodendrocytes in CNS and Schwann cells in PNS), axon repair (Schwann cells), phagocytosis of microbes (microglia), blood-brain barrier formation (astrocytes), CSF formation and movement (ependymal cells), and cleaning up debris (astrocytes and microglia).
• Myelin Gaps (Nodes of Ranvier): Gaps between myelin segments, where action potentials jump during saltatory conduction.
• Myelin Function: Increases the speed of nerve impulse transmission.

Axon Fiber Types

• Fiber Classification: Based on the amount of myelin and axon diameter (A, B, C fibers)
• Myelination/Diameter and Speed: A fibers (most myelin, largest diameter) conduct impulses fastest; C fibers (least myelin, smallest diameter) conduct impulses slowest.
• Information Carried: A fibers typically carry motor information; C fibers typically carry sensory information like pain.

Ligands and Receptors (Signaling Molecules)

• Categories: Hormones and neurotransmitters.
• Hormones: Produced by endocrine glands; travel via bloodstream to target cells. Made of peptides or steroids; peptides are water-soluble (bind to cell membrane receptors) and steroids are fat soluble (bind to intracellular receptors).
• Neurotransmitters: Released from neurons; travel across synapses to target cells. Binding to receptors can be ionotropic (opening ion channels, leading to fast responses) or metabotropic (activating second messenger systems, leading to slower, longer-lasting effects).
• Hormone Effects/Types of Receptors: Water-soluble hormones bind to membrane receptors, eliciting rapid responses, while fat-soluble hormones bind to intracellular receptors, triggering slower but longer-lasting effects.

Ion Channels

• Types on Neurons: Leakage (passive), chemically gated, voltage-gated.

Action Potentials and Potentials

• Resting Potential: The difference in electrical charge across the neuronal membrane at rest (approximately -70 mV).
• Action Potential: Rapid reversal of the membrane potential, triggered by depolarization above threshold.
• Ion Movement: Action potential generation involves the opening and closing of voltage-gated ion channels leading to movement of sodium (Na+) and potassium (K+).
• Resting Membrane Potential: Due to a greater permeability of the neuron to potassium.

Refractory Period

• Absolute Refractory Period: The period when the neuron cannot fire another action potential, regardless of the stimulus strength.
• Relative Refractory Period: The period after the absolute refractory period, during which a stronger-than-normal stimulus is needed to trigger an action potential.

Chemical Synapse

• Structure: Presynaptic neuron, synaptic cleft, postsynaptic neuron
• Neurotransmitter Release: Voltage-gated calcium channels open in the presynaptic axon terminal, triggering the release of neurotransmitters into the synaptic cleft.
• Neurotransmitter Removal: Neurotransmitters are removed from the synaptic cleft through degradation, reuptake, or diffusion. Blocked reuptake pumps would cause a continued response.

Neurotransmitters

• Excitatory/Inhibitory: Neurotransmitters can be either excitatory increase, (e.g., glutamate) or inhibitory (e.g., GABA or glycine). By altering the postsynaptic membrane potential.
• Examples: Acetylcholine (can be ionotropic or metabotropic), glutamate, GABA, glycine. These affect postsynaptic membrane potential by influencing the movement of specific ions (e.g., Na+, K+, Cl-) into or out of the neuron.

Neural Pathways

• Convergence: Multiple presynaptic neurons synapse with a single postsynaptic neuron.
• Divergence: A single presynaptic neuron synapses with multiple postsynaptic neurons.

Brain Structures

• Cerebrum, Hemispheres, Cortex, Medulla: Cortex and hemispheres are parts of the cerebrum. The cortex has different layers of gray and white matter; the underlying medulla are white matter.
• Hemisphere Separation: Longitudinal fissure.
• Lobes and Sulci/Fissures: Lobes are separated by sulci (shallow grooves) and fissures (deep grooves). The ridges on the cortex are called gyri (or convolutions).
• Functional Areas (Brodman Areas): Specialized regions with specific functions (speech, movement, sensation).

Brain Medulla

• Tracts/Fiber Types: Commissural fibers connect hemispheres; projection fibers connect the brain to the spinal cord; association fibers connect different areas within a hemisphere.

Basal Ganglia

• Composition: Gray matter.
• Movement Regulation: Involved in planning and coordinating movement.
• Substantia Nigra: Influences basal ganglia function; degeneration of this area is involved in Parkinson's disease.

Hemispheric Dominance

• Language/Logic: Left hemisphere typically dominant for language and logical reasoning.
• Spatial/Creativity: Right hemisphere typically dominant for spatial reasoning and creativity.

Ventricles and CSF

• CSF Formation and Function: Formed by choroid plexuses in the ventricles. Cushions and protects the brain.
• Ventricular Location and Connection: Chambers within the brain; interconnected.
• CSF Path: Ventricles, then spinal cord.

Diencephalon

• Thalamus: Important relay center for sensory input to the cortex.
• Hypothalamus: Regulates homeostasis and basic drives (Sweet Hat Mnemonic).
• Pineal Gland: Secretes melatonin, regulated by light levels; influential in circadian rhythms.
• Pituitary Gland: Anterior (glandular) and posterior (neural) lobes; releases many hormones. Linked to/controlled by the hypothalamus.

Midbrain

• Cerebral Peduncles: Major tracts carrying nerve impulses.
• Corpora Quadrigemina: Four bumps on the midbrain; superior colliculi regulate visual reflexes; inferior colliculi regulate auditory reflexes.

Hindbrain

• Pons: Involved in respiration and relaying nerve signals.
• Cerebellum: Important for motor control and coordination.
• Medulla Oblongata: Controls vital functions (heartbeat, breathing).

Reticular Activating System (RAS)

• Location: Network of neurons throughout the brainstem.
• Alertness and Arousal: Keeps the brain alert and awake by filtering sensory information, controlling alertness and sleep.

Limbic System

• Emotions and Memory: Involved in forming emotions and memories.
• Hippocampus (Memory): Crucial for forming short-term memories and spatial navigation.
• Amygdala (Fear): Processes fear and prepares the body for a threatening situation.
• Hypothalamus: Integrates emotions and stimuli directly linked to long-term memories

Meninges

• Layers: Dura mater, arachnoid mater, pia mater.
• Spaces: Epidural space (between dura and skull); Subdural space (between dura and arachnoid); Subarachnoid space (between arachnoid and pia). Contains CSF.
• Dura Mater: Two layers; a dural sinus between them.

Cerebrospinal Fluid/Blood Circulation

• Choroid Plexus (CSF formation)
• Arachnoid Granulations/Villi (CSF absorption)
• Dural Sinuses (Blood drainage)
• CSF Circulation/Drainage: Movement through ventricles to the subarachnoid space and drainage into the dural sinuses.

This quiz covers key concepts related to nervous tissue, focusing on the central and peripheral nervous systems. It delves into neuron structure, including parts like cell body, axon, and dendrites, as well as various neuron types. Test your understanding of these essential components of the nervous system.