Neural Tissue and Neurophysiology

Comprehensive practice flashcards covering neural tissue histology, neuron anatomy, glial cells, resting potentials, graded potentials, action potentials, synaptic transmission, and neurotransmitters.

Central Nervous System (CNS)

The division of the nervous system consisting of the brain and spinal cord, responsible for integrating, processing, and coordinating sensory data and motor commands.

Peripheral Nervous System (PNS)

All neural tissue located outside the central nervous system, including sensory receptors, nerves, and ganglia.

Somatic Nervous System (SNS)

The subdivision of the motor division of the PNS that provides voluntary control over skeletal muscle contractions.

Autonomic Nervous System (ANS)

The subdivision of the motor division of the PNS that provides automatic, involuntary regulation of smooth muscle, cardiac muscle, glands, and adipose tissue.

Receptors

Sensory structures that detect changes in the internal or external environment.

Effectors

Target organs—such as skeletal muscle, smooth muscle, cardiac muscle, glands, and adipose tissue—whose activities change in response to neural commands.

Dendrites

Highly branched neuronal cell processes bearing dendritic spines ($0.5\text{--}1\,\mu\text{m}$ long) where a neuron receives most stimuli from the environment or other neurons.

Perikaryon

The cytoplasm surrounding the nucleus within the neuron cell body, containing organelles, neurofilaments, and neurofibrils.

Axon Hillock

The thickened base or initial segment of an axon connecting to the cell body, where action potentials typically originate.

Axolemma

The specialized plasma membrane that covers the axoplasm of a neuronal axon.

Axoplasmic Transport

The bidirectional transport of materials such as enzymes and lysosomes along neurotubules between the cell body and the synaptic terminals.

Retrograde Flow

The movement of materials along neurotubules backward toward the neuronal cell body.

Telodendria

Fine terminal axonal extensions branching at the end of the axon trunk that terminate at synaptic terminals.

Synapse

A specialized site of intercellular communication between a neuron and another cell, such as another neuron or an effector cell.

Synaptic Knob

An expanded structure at the end of a presynaptic axon terminal containing synaptic vesicles filled with neurotransmitters.

Synaptic Cleft

The narrow extracellular space separating the presynaptic membrane from the postsynaptic membrane.

Neuromuscular Junction

A specialized intercellular synapse formed between a motor neuron and a skeletal muscle fiber.

Neuroglandular Junction

A synapse where a neuron communicates with and regulates the secretory activity of a gland cell.

Anatomical Classes of Neurons

The four structural categories of neurons based on morphology: anaxonic, bipolar, unipolar, and multipolar neurons.

Anaxonic Neurons

Small neurons where all cell processes look alike (axons cannot be distinguished from dendrites); located in the brain and special sense organs.

Bipolar Neurons

Small ($\approx 30\,\mu\text{m}$) neurons with two distinct processes: one dendritic process that branches at its distal tip and one axon; rare and found in special sense organs.

Unipolar Neurons

Neurons in which the dendrites and axon are continuous (fused) and the cell body lies off to one side; make up most sensory neurons in the peripheral nervous system.

Multipolar Neurons

Neurons possessing two or more dendrites and a single axon; the most common structural type in the central nervous system.

Interoceptors

Sensory receptors that monitor internal organ systems and provide sensations of deep pressure, pain, and taste.

Proprioceptors

Sensory receptors that monitor the position and movement of skeletal muscles and joints.

Exteroceptors

Sensory receptors that monitor the external environment, providing information about touch, temperature, pressure, and the special senses.

Interneurons

Association neurons located entirely within the brain and spinal cord ($\approx 20\text{ billion}$ in the body) that distribute sensory information and coordinate motor activity.

Ependymal Cells

Glial cells forming a simple epithelium (ependyma) lining fluid-filled passageways in the brain and spinal cord that assist in producing, circulating, and monitoring cerebrospinal fluid (CSF).

Microglia

Mobile phagocytic glial cells in the CNS, embryologically related to monocytes and macrophages, that remove cellular debris, waste products, and pathogens.

Astrocytes

The most numerous neuroglia in the CNS that maintain the blood–brain barrier, provide structural support, regulate interstitial solute concentrations, and form scar tissue after injury.

Oligodendrocytes

CNS glial cells whose cytoplasmic extensions wrap around axons to form membranous myelin sheaths, stabilizing axonal architecture and increasing conduction speed.

Internodes

The large areas of an axon that are completely wrapped in myelin sheaths.

Nodes of Ranvier

Small gaps situated between adjacent internodes along an axon where the axolemma remains exposed to extracellular fluid.

White Matter

Regions of the central nervous system dominated by myelinated axons, named for their glossy white appearance due to high lipid content.

Gray Matter

Areas of the central nervous system dominated by neuron cell bodies, dendrites, and unmyelinated axons.

Schwann Cells

Glial cells of the peripheral nervous system that form a protective sheath around every peripheral axon, producing myelin around myelinated internodes.

Neurilemma

The outer surface of a Schwann cell wrapping a peripheral axon.

Satellite Cells

PNS neuroglia that surround neuron cell bodies in peripheral ganglia, regulating the surrounding interstitial microenvironment much like astrocytes.

Wallerian Degeneration

The progressive degeneration of an axon and its myelin sheath distal to the site of injury in a peripheral nerve, followed by Schwann cell proliferation to guide regrowth.

Transmembrane Potential

The electrical potential difference resulting from an unequal distribution of positive and negative charges across a cell membrane.

Resting Membrane Potential

The transmembrane potential of an undisturbed cell, which averages approximately $-70\text{ mV}$ (or $0.07\text{ V}$) in a typical neuron.

Graded Potential

A temporary, localized change in the resting potential caused by a typical stimulus, which decreases in magnitude with distance from the stimulus site.

Action Potential

An electrical event involving rapid depolarization and repolarization that propagates along the surface of an excitable axonal membrane toward synaptic terminals.

Types of Ion Channels

The four functional membrane channel categories: leakage (nongated) channels, ligand-gated channels, mechanically gated channels, and voltage-gated channels.

Leakage Channels

Nongated ion channels that remain constantly open, allowing ions such as $\text{K}^+$ and $\text{Na}^+$ to diffuse continuously across the membrane.

Ligand-Gated Channels

Chemically gated ion channels that open or close upon binding specific chemical messengers such as acetylcholine.

Voltage-Gated Channels

Ion channels that open or close in response to direct changes in transmembrane potential; characteristic of excitable membranes.

Mechanically Gated Channels

Ion channels that open or close in response to mechanical vibration, touch, or pressure that physically distorts the membrane surface.

Sodium–Potassium Exchange Pump

An ATP-driven membrane carrier that maintains resting potential by ejecting $3\text{ Na}^+$ from the cytoplasm for every $2\text{ K}^+$ recovered from the extracellular fluid.

Equilibrium Potential

The transmembrane potential at which the electrical gradient and chemical concentration gradient for a specific ion are equal in magnitude and opposite in direction.

Potassium Equilibrium Potential

The transmembrane potential ($-90\text{ mV}$) reached if the plasma membrane were freely permeable only to potassium ions.

Sodium Equilibrium Potential

The transmembrane potential ($+66\text{ mV}$) reached if the plasma membrane were freely permeable only to sodium ions.

Depolarization

Any shift of the transmembrane potential from the resting potential toward $0\text{ mV}$ or positive values, typically mediated by sodium influx.

Repolarization

The restoration of the normal resting potential following membrane depolarization, involving the inactivation of $\text{Na}^+$ channels and opening of $\text{K}^+$ channels.

Hyperpolarization

An increase in the negativity of the resting potential beyond normal resting levels (e.g., shifting from $-70\text{ mV}$ toward $-80\text{ mV}$ or $-90\text{ mV}$), typically caused by potassium outflow.

Threshold

The critical transmembrane potential (approximately $-60\text{ mV}$ to $-55\text{ mV}$) required to activate voltage-gated sodium channels and trigger an action potential.

All-or-None Principle

The physiological concept stating that any stimulus strong enough to bring the membrane to threshold triggers a complete, uniform action potential, whereas subthreshold stimuli trigger none.

Absolute Refractory Period

The timeframe from voltage-gated sodium channel activation until sodium channel inactivation ends, during which the membrane cannot generate another action potential regardless of stimulus size.

Relative Refractory Period

The period following the absolute refractory period during which the membrane can generate another action potential, but only if stimulated by a larger-than-normal depolarizing stimulus.

Continuous Propagation

The step-by-step propagation of an action potential along each adjacent segment of an unmyelinated axon, proceeding at a speed of approximately $1\text{ m/s}$.

Saltatory Propagation

The rapid propagation of an action potential along a myelinated axon, where the impulse jumps from one exposed node of Ranvier to the next.

Cholinergic Synapse

A widespread type of chemical synapse that synthesizes and releases acetylcholine (ACh) into the synaptic cleft.

Acetylcholinesterase (AChE)

An enzyme present in the synaptic cleft and postsynaptic membrane that rapidly breaks down acetylcholine into acetate and choline within $20\text{ msec}$.

Synaptic Delay

The brief time lag of approximately $0.2\text{--}0.5\text{ msec}$ between the arrival of an action potential at the synaptic knob and the generation of an effect on the postsynaptic membrane.

Synaptic Fatigue

The weakening or failure of a synapse occurring under intensive stimulation when the resynthesis and recycling of neurotransmitter cannot keep pace with demand.

Electrical Synapse

A rare type of synapse in which presynaptic and postsynaptic cell membranes are connected directly by gap junctions, allowing ions and local currents to flow instantaneously between cells.

Excitatory Postsynaptic Potential (EPSP)

A graded depolarization caused by neurotransmitter arrival that shifts the postsynaptic transmembrane potential closer to threshold (facilitation).

Inhibitory Postsynaptic Potential (IPSP)

A graded hyperpolarization of the postsynaptic membrane (such as through the opening of chemically gated $\text{K}^+$ channels) that moves the transmembrane potential farther from threshold.

Temporal Summation

The cumulative effect on membrane potential produced by repeated stimuli arriving in rapid succession at a single synapse.

Spatial Summation

The integration of postsynaptic potentials occurring simultaneously at multiple different synapses on a single neuron.

Facilitation

A state in which a neuron is brought closer to threshold by subthreshold excitatory stimuli, requiring smaller subsequent stimuli to reach threshold.

Norepinephrine (NE)

A neurotransmitter operating indirectly via G proteins and second messengers in the brain and sympathetic nervous system, involved in attention, consciousness, and temperature control.

Glutamate

The most important excitatory neurotransmitter in the brain, functioning directly on calcium/sodium channels and indirectly via G proteins, crucial for memory and learning.

Gamma-Aminobutyric Acid (GABA)

An inhibitory neurotransmitter that exerts direct inhibitory effects by opening $\text{Cl}^-$ channels and indirect effects by opening $\text{K}^+$ channels and blocking $\text{Ca}^{2+}$ entry.