nervous tissue

Organization & Functions

  • Central Nervous System (CNS): Composed of the brain and spinal cord; primarily responsible for integration of information.
      - Peripheral Nervous System (PNS): Composed of nerves that connect the CNS to the rest of the body, categorized into:
        - Sensory (afferent): Nerves that carry sensory information to the CNS.
        - Motor (efferent): Nerves that carry commands from the CNS to the body.
          - Somatic: Controls skeletal muscle.
          - Autonomic: Controls smooth muscle, cardiac muscle, and glands.
            - Sympathetic: Activates the fight or flight response.
            - Parasympathetic: Activates the rest and digest response.

  • Functions: The nervous system performs sensory input, integration (interneurons), and motor output.

Neurons vs Neuroglia

  • Neurons: Characteristics include:
      - Excitable
      - Conductive
      - Long-lived
      - Amitotic (do not divide)
      - High metabolic demands

  • Neuroglia (Glial cells): Support and protect neurons; perform various functions:
      - Insulate
      - Guide development
      - Capable of division

Types of Neurons

  • Multipolar Neuron: Composed of the following parts:
      - Soma: Contains the nucleus and Nissl bodies (site of protein synthesis).
      - Dendrites: Responsible for graded input (receiving signals).
      - Axon: Transmits the action potential output.
      - Axon Hillock: The trigger zone where action potentials are initiated.
      - Myelin Sheath: Insulating layer around axon, speeds up signal transmission.
      - Nodes of Ranvier: Gaps in myelin sheath; important for saltatory conduction.

Axonal Transport

  • Involve two types of motor proteins:
      - Kinesin: Transports materials anterogradely (from cell body to axon terminals).
      - Dynein: Transports materials retrogradely (from axon terminals back to cell body).

  • Functions: Moves vesicles, proteins, and organelles essential for neuron function.

Neuroglia

  • CNS Neuroglia Types:
      - Astrocytes: Maintain the blood-brain barrier (BBB) and ion balance.
      - Oligodendrocytes: Form myelin around multiple axons.
      - Microglia: Act as phagocytes that remove debris and pathogens.
      - Ependymal Cells: Line ventricles and produce cerebrospinal fluid (CSF).

  • PNS Neuroglia Types:
      - Schwann Cells: Form myelin around a single axon and aid in repair processes (neurilemma).
      - Satellite Cells: Regulate the environment around neuron cell bodies.

Gray vs White Matter

  • Gray Matter: Composed of neuron cell bodies, dendrites, and synapses.

  • White Matter: Composed of myelinated axons; organized into tracts that facilitate communication between different brain regions.

Electrical Signals

Types of Potentials

  • Graded Potentials:
      - Characteristics: Local, decremental (decrease in strength with distance), vary in magnitude with stimulus intensity, can summate to reach a threshold.

  • Action Potentials (AP):
      - Characteristics: All-or-none response, initiated when the threshold (~ -55 mV) is reached, propagate along the axon without decrement.

Ion Channels

  • Types of Ion Channels:
      - Leak Channels: Primarily for potassium ions (K+).
      - Voltage-Gated Channels: Open in response to membrane potential changes; include sodium (Na+) and potassium (K+) channels.
      - Ligand-Gated Channels: Open in response to neurotransmitter (NT) binding.
      - Mechanical Channels: Open in response to physical deformation (e.g., touch/stretch).

  • Channel Function: Determines whether graded or action potentials are generated.

Resting Membrane Potential

  • Typical Value: Approximately -70 mV.

  • Maintained by: Sodium-potassium ATPase pump (3 Na+ out, 2 K+ in) leading to increased K+ permeability and a surplus of large anions inside the cell contributing to negativity.

Graded Potentials

  • Mechanism: When a stimulus opens ligand or mechanical channels, Na+ influx (depolarization) or K+/Cl- efflux (hyperpolarization) occurs leading to a local change in potential. These changes are decremental and can summate. The sum at the axon hillock determines whether an AP will occur.

Action Potentials

  • Phases:
      1. Depolarization: Rapid influx of Na+ ions as voltage-gated Na+ channels open.
      2. Repolarization: K+ channels open and Na+ channels inactivate, leading to efflux of K+ ions.
      3. Hyperpolarization: Slow closing of K+ channels leads to an overshoot of the resting potential.

  • Refractory Periods:
      - Absolute Refractory Period: No new AP can be generated regardless of stimulus intensity.
      - Relative Refractory Period: A stronger-than-normal stimulus is needed to elicit an AP.

  • Propagation Mechanisms:
      - Continuous Conduction: Occurs in unmyelinated axons.
      - Saltatory Conduction: Occurs in myelinated axons, jumping from node to node, which increases speed of conduction.

  • Factors Influencing Speed: Myelin presence and larger axon diameter improve conduction velocity.

Synapses

Types of Synapses

  • Electrical Synapses: Utilize gap junctions, allowing fast, bidirectional signal transmission.

  • Chemical Synapses: Unidirectional signal transfer.

Steps of Synaptic Transmission**:

  1. Action potential (AP) arrives at the axon terminal.

  2. Voltage-gated Ca2+ channels open, leading to Ca2+ influx.

  3. Calcium influx triggers vesicles containing neurotransmitters (NT) to undergo exocytosis.

  4. NT bind to receptors on the postsynaptic membrane, opening ion channels.

  5. NT removal occurs via reuptake, enzymatic breakdown, or diffusion.

EPSP/IPSP & Summation

  • Excitatory Postsynaptic Potential (EPSP): Occurs when Na+ ions enter, causing depolarization of the postsynaptic membrane.

  • Inhibitory Postsynaptic Potential (IPSP): Results from K+ efflux or Cl- influx, leading to hyperpolarization of the postsynaptic membrane.

  • Summation Types:
      - Spatial Summation: Involves multiple synaptic inputs occurring simultaneously.
      - Temporal Summation: Involves rapid firing of action potentials from a single synapse over time. This summation at the axon hillock determines if the threshold for an AP is reached.

Neurotransmitters

  • Acetylcholine (ACh): Vital for muscle contraction and functions in the parasympathetic nervous system.

  • Glutamate: The primary excitatory neurotransmitter in the CNS.

  • Gamma-Aminobutyric Acid (GABA): Functions as the primary inhibitory neurotransmitter in the CNS.

  • Dopamine: Involved in movement and reward processing; decreased levels are associated with Parkinson's disease.

  • Serotonin: Influences mood and sleep regulations.

  • Neuropeptides: A category of neurotransmitters that act more slowly and serve modulatory roles in the nervous system.

Neural Circuits

  • Diverging Circuit: One neuron communicates with many other neurons, amplifying signals.

  • Converging Circuit: Multiple neurons send inputs to a single neuron, allowing for signal integration.

  • Reverberating Circuit: A circuit that continues firing in a loop, playing a key role in behaviors such as rhythmic functions like breathing.

  • Parallel After-Discharge Circuit: Involves a single neuron stimulating multiple neurons that converge to one output, facilitating complex processing.

Repair & Plasticity

CNS Repair

  • Limitations: Central nervous system repair is limited due to the presence of inhibitory factors and the lack of a neurilemma for regeneration.

PNS Repair

  • Process: If the cell body remains intact, regeneration is possible through the following steps:
      1. Wallerian degeneration: The axon distal to the injury degenerates.
      2. Macrophages: Clean up the debris.
      3. Schwann Cells: Form a regeneration tube to guide axon regrowth.
      4. Axonal regrowth occurs at an estimated rate of ~1-2 mm/day.

  • Plasticity: Refers to the nervous system's ability to strengthen synapses or reorganize connections.

  • Neurogenesis: Limited ability to generate new neurons, predominantly occurring in areas such as the hippocampus.

Disorders/Imbalances

  • Multiple Sclerosis (MS): Autoimmune disorder leading to demyelination of axons in the CNS, resulting in slowed action potentials.

  • Alzheimer's Disease: Characterized by the accumulation of beta-amyloid plaques, leading to memory impairment.

  • Parkinson's Disease: Associated with dopamine neuron loss, impacting movement control.

  • Toxins/Drugs: Certain substances can affect ion channels or neurotransmitter function, such as botulinum toxin which blocks ACh release.

HIGH-YIELD (EXAM FOCUS)

  • Remember the phases of action potentials and the ion movements involved.

  • Understand the concept of threshold for action potentials.

  • Be able to list the steps occurring at synapses in order.

  • Distinguish between EPSP and IPSP.

  • Recognize the types of summation (spatial and temporal).

  • Understand the role of myelin in conduction of action potentials.

  • Differentiate CNS and PNS glial cells.

  • Be familiar with the steps involved in PNS regeneration.