Patho 2 exam

Chapter 4: Neuronal Physiology

1. Structure of Neurons

  • Neurons: Basic structural and functional units of the nervous system, responsible for transmitting signals.

    • Dendrites:

      • Serve as the input zone of the neuron.

      • Expand surface area for neurotransmitter receptors.

      • Initiate graded potential that carries signals to the cell body.

    • Cell Body:

      • Contains the nucleus and organelles.

    • Axon Hillock:

      • Acts as the trigger zone for action potentials, containing voltage-gated sodium channels (VGSCs).

    • Axon/Nerve Fiber:

      • A long, single tube that may be myelinated.

      • Conducts signals away from the cell body.

    • Axon Terminals:

      • Output zone of the neuron, where neurotransmitters are released.

2. Linkage Between Neurons

  • Neurons receive converging input from thousands of sources and can diverge their output to many other neurons.

3. Electrical Signal Initiation & Characteristics

  • Graded Potentials:

    • Small, localized changes in resting membrane potential (RMP, typically -70 mV).

    • Initiated by a stimulus leading to Na+ influx, causing depolarization or hyperpolarization.

    • Magnitude and duration are proportional to the triggering stimulus applied.

    • Spread passively in both directions but diminish over distance.

    • Summation can lead to an action potential:

      • Excitatory Input: Causes depolarization.

      • Inhibitory Input: Causes hyperpolarization.

      • Possible outcomes of summation:

        • No Summation: If a second impulse arrives after the first has died down.

        • Temporal Summation: Same location stimulated closely in time.

        • Spatial Summation: Nearby inputs occurring at the same time.

        • Cancellation: Excitatory and inhibitory signals offset each other.

4. Action Potentials

  • Defined as a brief, rapid, large change in RMP, resulting in a positive interior of the cell.

    • Triggered when threshold potential (-55 to -50 mV) is reached, leading to:

      • Opening of VGSCs, allowing Na+ in.

      • Two gates present on VGSCs:

        • Activation Gate: Opens rapidly at threshold, closes slowly at RMP.

        • Inactivation Gate: Closes slowly at threshold, opens back at RMP.

    • Peaks at +30 mV, followed by K+ efflux leading to brief hyperpolarization before returning to RMP:

      • VGPCs (Voltage-Gated K+ Channels): Have one activation gate, delayed opening.

    • Action potentials are:

      • Consistent in magnitude and duration (approximately 1 msec).

      • Conducted along the axon via contiguous conduction, traveling in only one direction due to:

        • Absolute Refractory Period: No new AP can occur when VGSCs are open.

        • Relative Refractory Period: After hyperpolarization, when VGSCs are closed but potentially able to open again.

    • The Na+-K+-ATPase restores ionic balance after some action potentials.

    • Frequency of action potentials relates to stimulus strength.

    • Speed is impacted by axon diameter and myelination:

      • Myelin Sheath: Increases conduction speed and reduces energy expenditure (via saltatory conduction).

      • Types of Nerve Fibers:

        • Type A: Large, myelinated fibers (alpha, beta, delta); for touch and fast responses.

        • Type B: Smaller, myelinated fibers.

        • Type C: Small, unmyelinated fibers.

5. Signal Transmission between Neurons

  • Synapse Structure:

    • Connection between two neurons consisting of terminus of presynaptic neuron, synaptic cleft, and postsynaptic neuron.

    • Process of Signal Transmission:

      1. Arrival of AP at presynaptic terminal opens voltage-gated calcium channels (VGCCs).

      2. Ca++ influx into synaptic knob causes synaptic vesicles to release neurotransmitters into synaptic cleft via exocytosis.

      3. Neurotransmitter binding to ligand-gated ion channels (LGICs) results in graded potentials and can generate an action potential if summed correctly.

    • Types of Synapses:

      • Excitatory Synapses: Allow Na+ in, causing depolarization.

      • Inhibitory Synapses: Allow K+ out or Cl- in, leading to hyperpolarization.

    • Neurotransmitters and Neuromodulators:

      • Both are released at synapses and can modulate synaptic activity.

      • Neuromodulators influence synthesis, transport, storage, release of neurotransmitters, and their interactions with receptors.

    • Effect of Drugs/Diseases:

      • Various drugs can inhibit VGSCs, impacting action potential generation (e.g., local anesthetics).

Chapter 6: Afferent Nervous System

1. Nervous System Organization

  • Peripheral (Afferent): Carries stimulus signals to the CNS.

    • Sensory: Includes senses such as sight, smell, taste, pain, sound.

    • Visceral: Relates to internal sensations like hunger and thirst.

  • Central Nervous System (CNS): Composed of the brain and spinal cord.

  • Peripheral (Efferent): Executes outputs from the CNS.

    • Somatic: Involves motor neurons to skeletal muscles.

    • Autonomic: Controls involuntary responses:

      • Sympathetic/Parasympathetic: Managing smooth/cardiac muscle and glands.

      • Enteric: Involves the digestive system.

2. Types of Neurons

  • Afferent Neurons:

    • Respond to various stimuli via sensory receptors generating action potentials:

      • Different receptors for energy forms (photo, mechanosensitive, thermosensitive, nociceptive, osmosensitive, and chemosensitive).

      • Adaptation: Varied receptor potentials over time:

        • Tonic Receptors: Slow or persistently active (e.g., muscle posture).

        • Phasic Receptors: Rapidly adapt; crucial for detecting changes in stimulus intensity (e.g., touch).

  • Interneurons: Entirely within the CNS, connecting afferent and efferent pathways.

  • Efferent Neurons: Carry signals from the CNS to effector organs, traveling along long axons in the PNS.

Chapter 5: Central Nervous System

1. Types of Cells in CNS

  • Neuronal Cells (~10%):

    • Afferent: Containing axon terminals.

    • Interneurons: Forming connections within the CNS.

    • Efferent: Comprising cell bodies and dendrites.

  • Glial Cells (~90%): Supporting cells (do not transmit signals).

    • Astrocytes: Provide structural support, blood-brain barrier establishment, repair, neurotransmitter uptake, and K+ buffering.

    • Microglia: Immune defense cells that remove debris and promote growth.

    • Ependymal Cells: Facilitate cerebrospinal fluid (CSF) flow.

    • Oligodendrocytes: Form myelin sheaths around axons.

    • Glial cells can lead to tumors due to uncontrolled division, while neurons cannot.

2. Protection of the CNS

  • Bone: Provides a hard protective shell, via the vertebral column.

  • Cerebrospinal Fluid (CSF): Acts as a shock absorber, produced in the choroid plexus:

    • Lacks plasma proteins except during infection, absorbs waste, filters nutrients (higher Na+, lower K+).

  • Meninges: Protective membranes envelopes:

    • Dura Mater: Tough outermost layer.

    • Arachnoid Mater: Middle vascularized layer.

    • Pia Mater: Innermost vascularized layer adhering to the brain.

3. Brain Structure

  • Forebrain:

    • Cerebrum:

      • Divided into left/right hemispheres, connected by the corpus callosum.

      • Comprises thin gray matter (cerebral cortex) and thick white matter (myelinated axons).

      • Four lobes with specific functions:

        • Frontal Lobe: Involved in speech, planning, and higher-order functions.

        • Parietal Lobe: Processes sensory input and language comprehension.

        • Occipital Lobe: Responsible for visual input processing.

        • Temporal Lobe: Integrates sensory information related to emotion and memory.

    • Diencephalon:

      • Thalamus: Relay center for sensory information.

      • Hypothalamus: Regulates homeostasis and links nervous and endocrine systems.

    • Cerebellum: Coordinates balance and skilled muscle activity.

    • Brain Stem: Controls involuntary functions and acts as a conduit for signals between the spinal cord and brain.