Tigroid substance (Nissl substance) (TY-groid SUB-stuns): Coarse, basophilic clumps seen in the cytoplasm of a neuron's soma and dendrites; this represents the large amounts of rough endoplasmic reticulum and free ribosomes, indicating the cell's high rate of protein synthesis.
Dendrites (DEN-dryts): Numerous, short, highly branched cytoplasmic extensions that project from the soma of a neuron; they are the primary sites for receiving and integrating incoming synaptic information from other neurons.
Soma/cell body (SOH-muh): The central portion of the neuron, containing the nucleus, major organelles, and the tigroid substance; it is the metabolic and genetic center of the neuron.
Axon (AK-son): A single, long, thin cytoplasmic extension projecting from the soma at the axon hillock; its primary function is to transmit the efferent action potential away from the soma to the target cell (another neuron, muscle, or gland).
Neurolemmocyte (Schwann cell) (noo-roh-LEM-uh-syt): A glial cell found in the peripheral nervous system (PNS); its function is to wrap around an axon to form an insulating myelin sheath, or to simply surround unmyelinated axons.
Myelin sheath (MY-uh-lin sheeth): A thick, lipid-rich insulating layer formed by the wrapping of the neurolemmocyte (PNS) or oligodendrocyte (CNS) plasma membrane around an axon; it dramatically increases the speed of action potential conduction.
Myelin sheath gap (Node of Ranvier) (MY-uh-lin sheeth gap/nohd uh RAHN-vee-ay): A small, non-myelinated gap or interruption that occurs at regular intervals between adjacent segments of the myelin sheath; this is the site where the action potential is regenerated (saltatory conduction).
Axon terminals (Synaptic boutons) (AK-son TUR-mih-nuls/sih-NAP-tik boo-TAHNZ): The distal, specialized swellings or enlargements at the very end of an axon; they contain synaptic vesicles and are responsible for releasing neurotransmitters into the synaptic cleft.
Multipolar neuron (mul-tih-POH-ler NOO-ron): A neuron characterized by having one axon and two or more dendrites extending from the soma; this is the most common neuronal type (e.g., motor neurons, interneurons).
Pseudounipolar neuron (soo-doh-yoo-nih-POH-ler NOO-ron): A neuron characterized by having a single short process extending from the soma that immediately bifurcates into a peripheral process (functioning as a dendrite) and a central process (functioning as an axon); found in most sensory ganglia (e.g., dorsal root ganglia).
Bipolar neuron (by-POH-ler NOO-ron): A neuron characterized by having one axon and one dendrite extending from opposite sides of the soma; a relatively rare type, found in special sensory organs (e.g., retina, olfactory epithelium).
Pia mater (PEE-uh MAH-ter): The innermost, delicate layer of the meninges (membranes covering the brain and spinal cord); it is composed of flattened cells and fine collagen fibers, closely investing the surface of the CNS.
Molecular layer (cerebral cortex) (muh-LEK-yuh-ler LAY-er uv thuh suh-REE-brul KOR-teks): The outermost layer (Layer I) of the six-layered cerebral cortex; it is composed mainly of neuropil (axons, dendrites, glia) and contains relatively few scattered small neurons.
External granular layer (cerebral cortex) (eks-TUR-nul GRAN-yuh-ler LAY-er): Layer II of the cerebral cortex; it is a dense layer composed primarily of small, numerous granular cells and interneurons.
External pyramidal layer (cerebral cortex) (eks-TUR-nul pih-RAM-ih-dul LAY-er): Layer III of the cerebral cortex; it is dominated by small to medium-sized pyramidal cells that project to other cortical areas.
Internal granular layer (cerebral cortex) (in-TUR-nul GRAN-yuh-ler LAY-er): Layer IV of the cerebral cortex; it is the main receptive layer, densely packed with granular cells that receive sensory input from the thalamus.
Internal pyramidal layer (cerebral cortex) (in-TUR-nul pih-RAM-ih-dul LAY-er): Layer V of the cerebral cortex; it contains the largest pyramidal cells whose axons project outside the cortex (e.g., to the brainstem and spinal cord), making it the primary motor layer.
Multiform layer (cerebral cortex) (mul-tih-FORM LAY-er): The innermost layer (Layer VI) of the cerebral cortex, adjacent to the white matter; it contains a diverse population of neurons, including fusiform cells, which project to the thalamus.
Pyramidal cell (cerebral cortex) (pih-RAM-ih-dul sell): The most characteristic neuron of the cerebral cortex, possessing a triangular soma and a large apical dendrite; these are the principal efferent neurons (output).
Fusiform cell (cerebral cortex) (FYOO-zuh-form sell): Spindle-shaped neurons found primarily in the multiform layer (Layer VI) of the cerebral cortex; they are involved in cortical output to the thalamus.
Granular cell (cerebral cortex) (GRAN-yuh-ler sell): A small, round neuron with a limited branching pattern, found primarily in the external and internal granular layers (Layers II and IV) of the cerebral cortex; they function as interneurons.
White matter (wyt MAT-er): The deep regions of the central nervous system (CNS); it consists primarily of myelinated axons (giving it the pale color) and glial cells (oligodendrocytes and astrocytes), functioning as communication tracts.
Purkinje cell (cerebellum) (per-KIN-jee sell): Very large, highly distinctive neurons located in the single-cell Purkinje layer of the cerebellum; they possess an elaborate, extensive dendritic tree that spreads into the molecular layer and are the sole efferent neurons (output) of the cerebellar cortex.
Granule cell (cerebellum) (GRAN-yool sell): The smallest and most numerous neuron in the CNS, located densely in the granular layer of the cerebellum; they are excitatory neurons whose axons ascend into the molecular layer to synapse with Purkinje cell dendrites.
Stellate cell (cerebellum) (STEL-ayt sell): An inhibitory interneuron found in the outer part of the molecular layer of the cerebellum; its axons inhibit the dendrites of the Purkinje cells.
Molecular layer (cerebellum) (muh-LEK-yuh-ler LAY-er): The outermost, cell-sparse layer of the cerebellar cortex; it is dominated by the vast, branching dendritic trees of the underlying Purkinje cells and the axons of the granular cells.
Granular layer (cerebellum) (GRAN-yuh-ler LAY-er): The innermost, cell-dense layer of the cerebellar cortex; it is densely packed with the small granule cell soma.
Neuropil (NOOR-uh-pil): The complex, felt-like meshwork of unmyelinated axons, dendrites, and glial cell processes that fills the space between somas in the gray matter of the central nervous system (CNS); this is the primary site of synaptic communication.
Glial cells (Neuroglia) (GLEE-ul sells): The non-neuronal cells of the nervous system (e.g., astrocytes, oligodendrocytes, microglia, ependymal cells, neurolemmocytes, ganglion satellite cells); they provide metabolic, structural, and immune support to neurons.
Gray matter (gray MAT-er): Regions of the central nervous system (CNS) characterized by a high concentration of neuron cell bodies (somas), dendrites, and neuropil; it is the primary site of synaptic processing.
Ependymal cells (uh-PEN-duh-mul sells): Cuboidal or columnar epithelial cells that line the ventricles of the brain and the central canal of the spinal cord; they are often ciliated and help circulate cerebrospinal fluid (CSF).
Central canal (spinal cord) (SEN-trul kuh-NAL): A small, fluid-filled channel that runs through the center of the spinal cord's gray matter; it is lined by ependymal cells and is continuous with the brain's ventricular system.
Myelinated axon (MY-uh-luh-nay-ted AK-son): An axon covered by a thick, insulating myelin sheath (formed by a Neurolemmocyte in the PNS or an oligodendrocyte in the CNS); this allows for rapid, saltatory conduction of the action potential.
Unmyelinated axon (un-MY-uh-luh-nay-ted AK-son): An axon that lacks the thick, multiple-layered myelin sheath; it is only partially wrapped or surrounded by a single fold of the Neurolemmocyte plasma membrane, resulting in slower conduction speeds.
Epineurium (ep-ih-NOOR-ee-um): The outermost, dense, irregular connective tissue layer that surrounds and cushions an entire peripheral nerve, binding together multiple nerve fascicles.
Perineurium (per-ih-NOOR-ee-um): A layered sheath of specialized flattened epithelial cells (perineurial cells) and collagen that encircles and isolates a bundle of axons (nerve fascicle); it forms the blood-nerve barrier.
Endoneurium (en-doh-NOOR-ee-um): The delicate layer of loose connective tissue and fine reticular fibers that immediately surrounds the Schwann cells and individual axons within a nerve fascicle.
Ganglion (GANG-glee-un): A cluster of neuron cell bodies (somas) located outside the central nervous system (CNS); these are surrounded by satellite cells and a connective tissue capsule.
Satellite cells (ganglion) (SAT-uh-lyt sells): Small, cuboidal glial cells that form a protective, supportive layer immediately surrounding the neuron cell bodies (somas) within a ganglion.
Autonomic ganglion (aw-tuh-NOM-ik GANG-glee-un): A motor ganglion of the autonomic nervous system, containing multipolar neuron cell bodies; they relay motor signals to smooth muscle, cardiac muscle, or glands.
Sensory/dorsal/spinal ganglion (SEN-suh-ree DOR-sul SPY-nul GANG-glee-un): A sensory ganglion containing the cell bodies of pseudounipolar neurons; these cell bodies are clustered along the dorsal root of the spinal cord and relay sensory information toward the CNS.
Muscle spindle (MUS-ul SPIN-dul): A stretch receptor found embedded within most skeletal muscles; it is a specialized sensory structure composed of a fibrous capsule surrounding small, modified muscle fibers (intrafusal fibers) innervated by Group Ia and Group II sensory neurons.
Group Ia sensory neuron (groop wun ay SEN-suh-ree NOO-ron): A large, rapidly conducting afferent neuron that innervates the central region of the intrafusal fibers in a muscle spindle; it detects the rate of change in muscle length.
Group II sensory neuron (groop too SEN-suh-ree NOO-ron): A large afferent neuron that innervates the peripheral region of the intrafusal fibers in a muscle spindle; it detects the static (absolute) length of the muscle.
Gamma motor neuron (γ motor neuron) (GAM-uh MOH-ter NOO-ron): A smaller efferent neuron that innervates and contracts the specialized poles (ends) of the intrafusal fibers within the muscle spindle; its function is to adjust the sensitivity of the spindle.
Alpha motor neuron (α motor neuron) (AL-fuh MOH-ter NOO-ron): A large efferent neuron that innervates and contracts the main bulk of the muscle (extrafusal fibers); its cell body is located in the ventral horn of the spinal cord.
Intrafusal fiber (in-truh-FYOO-zul FY-ber): Small, modified skeletal muscle fibers located inside the capsule of the muscle spindle; they serve as the sensory receptor for stretch.
Extrafusal fiber (eks-truh-FYOO-zul FY-ber): The large, regular skeletal muscle fibers that make up the main contractile mass of a muscle; they are innervated by alpha motor neurons and generate the force for movement.
Motor end plate (MOH-ter end playt): The highly specialized region of the skeletal muscle fiber membrane that lies directly beneath the axon terminals of the alpha motor neuron; it contains a high concentration of ACh receptors and forms the postsynaptic component of the neuromuscular junction.
Synapse (SIN-aps): A specialized junction between two neurons (or a neuron and an effector cell) where an action potential is transmitted chemically via neurotransmitters, across a synaptic cleft.
Neuromuscular junction (NMJ) (noo-roh-MUS-kyoo-ler JUNK-shun): A specialized type of synapse between a motor axon terminal and an extrafusal muscle fiber; it transmits the signal from the nervous system to the skeletal muscle to initiate contraction.
Calcium channel (KAL-see-um CHAN-ul): A voltage-gated protein in the plasma membrane of the axon terminal; the arrival of an action potential opens this channel, allowing calcium ions to flow into the axon terminal and trigger the release of neurotransmitters.
Synaptic cleft (sih-NAP-tik kleft): The narrow, fluid-filled space that separates the presynaptic membrane of the axon terminal from the postsynaptic membrane (motor end plate or dendrite) at a synapse; neurotransmitters diffuse across this gap.
Synaptic vesicle (sih-NAP-tik VES-ih-kul): Small, membrane-bound sacs found within the axon terminal; they contain neurotransmitters (e.g., Acetylcholine) that are released into the synaptic cleft upon stimulation by calcium influx.
Acetylcholine (ACh) (uh-SEE-tul-koh-leen): A common neurotransmitter found at the neuromuscular junction and in the autonomic nervous system; it binds to ACh receptors on the postsynaptic membrane to generate an excitatory postsynaptic potential.
ACh receptors (ay see aych ri-SEP-ters): Ligand-gated ion channels or G-protein coupled receptors located on the postsynaptic membrane (e.g., motor end plate); they bind Acetylcholine to convert the chemical signal back into an electrical signal.