Fundamentals of the Nervous System and Nervous Tissue

Nervous System Functions

• Sensory Input: The nervous system uses its millions of sensory receptors to monitor changes occurring both inside and outside the body. These changes are called stimuli, and the gathered information is called sensory input.
• Integration: The nervous system processes and interprets the sensory input and decides what should be done at each moment – a process called integration.
• Motor Output: The nervous system activates effector organs, i.e., muscles and glands, to cause a response, called motor output.

Nervous System Organization

• Central Nervous System (CNS): Brain and spinal cord. Integration and command center.
• Peripheral Nervous System (PNS): Paired spinal and cranial nerves. Carries messages to and from the spinal cord and brain.
  • Afferent (Sensory) Division:
    • Somatic Sensory Fibers: Convey impulses from skin, skeletal muscles, and joints to the CNS.
    • Visceral Sensory Fibers: Convey impulses from visceral organs to the CNS.
  • Efferent (Motor) Division:
    • Somatic Nervous System: Voluntary, conscious control of skeletal muscles.
    • Autonomic Nervous System (ANS): Involuntary control of smooth muscles, cardiac muscles, and glands.

Nervous System Cells

• Neuroglia (Glial Cells): Supporting cells.
  • CNS Neuroglia:
    • Astrocytes: Most abundant, versatile, and highly branched glial cells. Support and brace neurons, anchor them to nutrient supply lines, guide migration of young neurons, control the chemical environment, respond to nerve impulses and neurotransmitters, participate in information processing in the brain.
    • Microglia: Small, ovoid cells with thorny processes. Migrate toward injured neurons. Phagocytize microorganisms and neuronal debris.
    • Ependymal Cells: Range in shape from squamous to columnar. They line the central cavities of the brain and spinal column. Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells.
    • Oligodendrocytes: Branched cells. Processes wrap CNS nerve fibers, forming insulating myelin sheaths.
  • PNS Neuroglia:
    • Satellite Cells: Surround neuron cell bodies in the PNS. Similar function to astrocytes of CNS.
    • Schwann Cells (Neurolemmocytes): Surround all nerve fibers in the PNS and form myelin sheaths in thicker nerve fibers. Similar function to oligodendrocytes. Vital to regeneration of damaged peripheral nerve fibers.
• Neurons: Excitable cells that transmit electrical signals.
  • Structural Components:
    • Cell Body (Soma): Contains the nucleus and most normal organelles. Biosynthetic center of neuron.
    • Dendrites: Receptive (input) region of a neuron. Convey incoming messages toward the cell body as graded potentials.
    • Axons: One axon per neuron arising from the axon hillock. Impulse-generating and conducting region. Transmits impulses away from the cell body.
  • Myelin Sheaths: Whitish, fatty (protein-lipoid), segmented sheath around most long or large-diameter axons. Functions to protect the axon, electrically insulate fibers from one another, and increase the speed of nerve impulse transmission.

Neuron Classification

• Structural Classification: Based on the number of processes extending from their cell body.
  • Multipolar: Three or more processes (1 axon, rest dendrites). Most common type; major neuron in CNS.
  • Bipolar: Two processes (1 axon and 1 dendrite). Rare, e.g., retinal neurons.
  • Unipolar: One T-like process (2 axons). Also called pseudounipolar, e.g., sensory neurons.
• Functional Classification: Based on the direction in which the nerve impulse travels relative to the CNS.
  • Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward the CNS.
  • Motor (Efferent) Neurons: Carry impulses from the CNS to effectors.
  • Interneurons (Association Neurons): Lie between motor and sensory neurons. Shuttle signals through CNS pathways. Most are entirely within the CNS.

Membrane Potential

• Voltage: Measure of potential energy generated by separated charge.
• Potential Difference: Voltage measured between two points. In neurons, the potential difference is also called membrane potential.
• Resistance: Hindrance to charge flow (insulator has high resistance, conductor has low resistance).

Resting Membrane Potential (RMP)

• RMP Range: -40mV to -90mV (depending on neuron type). Typically around -70mV.
• Factors Generating RMP:
  • Differential permeability of the membrane to Na+ and K+.
  • Operation of the sodium-potassium pump (Na^+/K^+\text{ ATPase}).
• Electrochemical Gradient: Combination of electrical and concentration gradients.
  • Concentration Gradient: Ions diffuse along chemical concentration gradients from an area of higher concentration to an area of lower concentration.
  • Electrical Gradient: Ions diffuse along electrical gradients toward areas of opposite charge.
  • Membrane Permeability: Affects the electrochemical gradient by determining which ions can cross the membrane and how easily they can do so. Ion channels are crucial for this.
• Depolarization: A decrease in membrane potential (toward zero and above). Inside of the membrane becomes less negative (more positive) than resting potential.
• Hyperpolarization: An increase in membrane potential (away from zero). Inside of the membrane becomes more negative than resting potential.
• Repolarization: The process by which a depolarized membrane returns to its resting potential.

Graded Potentials

• Short-lived, localized changes in membrane potential. The stronger the stimulus, the more the voltage changes and the farther the current flows. Triggered by stimulus that opens gated ion channels. Results in depolarization or hyperpolarization. Named according to location and function (e.g., receptor potential, postsynaptic potential).

Action Potential (AP)

• A brief, large depolarization of the neuron's plasma membrane. Principle means of long-distance neural communication. Only axons are capable of generating action potentials.
  • Generation of an Action Potential:
    • Threshold: The critical level of depolarization (-55 to -50 mV) that must be reached to trigger an action potential.
    • Propagation: The action potential is self-propagating and travels down the axon. The influx of Na^+ establishes a local current that depolarizes adjacent membrane areas.
  • Refractory Period: The period after initiation of an action potential when the neuron is insensitive to another stimulus.

Conduction Velocity

• Factors affecting conduction velocity include:
  • Axon Diameter: Larger diameter = faster conduction velocity.
  • Degree of Myelination: Myelination dramatically increases conduction velocity.
  • Nerve Fiber Classification: Nerve fibers are classified based on their diameter, degree of myelination, and conduction velocity.

Homeostatic Imbalances

• Multiple Sclerosis (MS): An autoimmune disease that primarily affects the myelin sheaths of the CNS. Results in impaired sensory and motor function.
• Numbness: Loss of sensation due to damage or dysfunction of sensory nerves.

Synapses

• A junction that mediates information transfer from one neuron to the next or from a neuron to an effector cell.
  • Chemical Synapse: Specialized for the release and reception of neurotransmitters. Composed of two parts: axon terminal of the presynaptic neuron (contains synaptic vesicles) and receptor region on the postsynaptic neuron.
  • Electrical Synapse: Less common than chemical synapses. Neurons are electrically coupled (joined by gap junctions). Communication is very rapid and may be unidirectional or bidirectional. Found in some brain regions responsible for eye movements or hippocampus in areas involved in emotions and memory.

Excitatory Synapse and EPSP

• Excitatory Synapse: Neurotransmitter binding depolarizes the postsynaptic membrane.
• Excitatory Postsynaptic Potential (EPSP): A local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing simultaneous flow of Na^+ and K^+. If the EPSP is of sufficient strength, it can trigger an action potential at the axon hillock.
• Excitatory Neurotransmitters: Glutamate, Acetylcholine.

Inhibitory Synapse and IPSP

• Inhibitory Synapse: Neurotransmitter binding hyperpolarizes the postsynaptic membrane.
• Inhibitory Postsynaptic Potential (IPSP): A local hyperpolarization of the postsynaptic membrane that drives the neuron away from AP threshold. Neurotransmitter binding opens chemically gated K^+ or Cl^- channels.
• Inhibitory Neurotransmitters: GABA, Glycine.

Neurotransmitters

• Chemical messengers that transmit signals across a chemical synapse.
  • Classification by Chemical Structure:
    • Acetylcholine (ACh): Released at neuromuscular junctions. Degraded by the enzyme acetylcholinesterase.
    • Biogenic Amines: Include catecholamines (dopamine, norepinephrine, epinephrine) and indolamines (serotonin, histamine).
    • Amino Acids: Include GABA, glutamate, glycine, aspartate.
    • Peptides (Neuropeptides): Include substance P, endorphins, gut-brain peptides.
    • Purines: ATP, adenosine.
    • Gases: Nitric oxide (NO), carbon monoxide (CO).
    • Lipids: Endocannabinoids.
  • Classification by Function:
    • Excitatory: Cause depolarization of the postsynaptic membrane.
    • Inhibitory: Cause hyperpolarization of the postsynaptic membrane.
    • Direct: Neurotransmitters that bind to and open ion channels, producing rapid, direct responses.
    • Indirect: Neurotransmitters that act through intracellular second messengers (e.g., G proteins).
  • Neurotransmitter Receptors:
    • Channel-Linked Receptors: Ligand-gated ion channels. Direct action.
    • G-Protein-Linked Receptors: Indirect action via second messengers. Slower, longer-lasting effects.