11.3 Neurons

11.3 Neurons • Neurons (nerve cells) are structural units of nervous system • Large, highly specialized cells that conduct impulses • Special characteristics – Extreme longevity (lasts a person’s lifetime) – Amitotic, with few exceptions – High metabolic rate: requires continuous supply of oxygen and glucose • All have cell body and one or more processes Neuron Cell Body (1 of 2) • Also called the perikaryon or soma • Biosynthetic center of neuron – Synthesizes proteins, membranes, chemicals – Rough ER (chromatophilic substance, or Nissl bodies) • Contains spherical nucleus with nucleolus • Some contain pigments • In most, plasma membrane is part of receptive region that receives input info from other neurons Neuron Cell Body (2 of 2) • Most neuron cell bodies are located in CNS – Nuclei: clusters of neuron cell bodies in CNS – Ganglia: clusters of neuron cell bodies in PNS Neuron Processes (1 of 10) • Armlike processes that extend from cell body – CNS contains both neuron cell bodies and their processes – PNS contains chiefly neuron processes • Tracts – Bundles of neuron processes in CNS • Nerves – Bundles of neuron processes in PNS • Two types of processes – Dendrites – Axon IP Anatomy Review Animation: Nervous I Click here to view ADA compliant Animation: IP Anatomy Review Animation: Nervous I Structure of a Motor Neuron (1 of 3) Figure 11.5a Structure of a motor neuron. Neuron Processes (2 of 10) • Dendrites – Motor neurons can contain 100s of these short, tapering, diffusely branched processes  Contain same organelles as in cell body – Receptive (input) region of neuron – Convey incoming messages toward cell body as graded potentials (short distance signals) – In many brain areas, finer dendrites are highly specialized to collect information  Contain dendritic spines, appendages with bulbous or spiky ends Structure of a Motor Neuron (2 of 3) Figure 11.5b Structure of a motor neuron. Neuron Processes (3 of 10) • The axon: structure – Each neuron has one axon that starts at cone-shaped area called axon hillock – In some neurons, axons are short or absent; in others, axon comprises almost entire length of cell  Some axons can be over 1 meter long – Long axons are called nerve fibers – Axons have occasional branches called axon collaterals – Axons branch profusely at their end (terminus)  Can number as many as 10,000 terminal branches – Distal endings are called axon terminals or terminal boutons Neuron Processes (4 of 10) • The axon: functional characteristics – Axon is the conducting region of neuron – Generates nerve impulses and transmits them along axolemma (neuron cell membrane) to axon terminal  Terminal: region that secretes neurotransmitters, which are released into extracellular space  Can excite or inhibit neurons it contacts – Carries on many conversations with different neurons at same time – Axons rely on cell bodies to renew proteins and membranes – Quickly decay if cut or damaged Neuron Processes (5 of 10) • The axon: functional characteristics (cont.) – Axons have efficient internal transport mechanisms  Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements – Movement occurs in both directions  Anterograde: away from cell body – Examples: mitochondria, cytoskeletal elements, membrane components, enzymes  Retrograde: toward cell body – Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins Clinical – Homeostatic Imbalance 11.1 • Certain viruses and bacterial toxins damage neural tissues by using retrograde axonal transport – Example: polio, rabies, and herpes simplex viruses, and tetanus toxin • Research is under way to investigate using retrograde transport to treat genetic diseases – Viruses containing “corrected” genes or microRNA to suppress defective genes can enter cell through retrograde transport Neuron Processes (6 of 10) • Myelin sheath – Composed of myelin, a whitish, protein-lipid substance – Function of myelin  Protect and electrically insulate axon  Increase speed of nerve impulse transmission – Myelinated fibers: segmented sheath surrounds most long or large-diameter axons – Nonmyelinated fibers: do not contain sheath  Conduct impulses more slowly Neuron Processes (7 of 10) • Myelination in the PNS – Formed by Schwann cells  Wraps around axon in jelly roll fashion  One cell forms one segment of myelin sheath – Outer collar of perinuclear cytoplasm (formerly called neurilemma): peripheral bulge containing nucleus and most of cytoplasm – Plasma membranes have less protein  No channels or carriers, so good electrical insulators  Interlocking proteins bind adjacent myelin membranes PNS Nerve Fiber Myelination (1 of 2) Figure 11.6a PNS nerve fiber myelination. PNS Nerve Fiber Myelination (2 of 2) Figure 11.6b PNS nerve fiber myelination. Neuron Processes (8 of 10) • Myelination in the PNS (cont.) – Myelin sheath gaps  Gaps between adjacent Schwann cells  Sites where axon collaterals can emerge  Formerly called nodes of Ranvier – Nonmyelinated fibers  Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling; one cell may surround 15 different fibers Structure of a Motor Neuron (3 of 3) Figure 11.5a Structure of a motor neuron. Neuron Processes (9 of 10) • Myelin sheaths in the CNS – Formed by processes of oligodendrocytes, not whole cells – Each cell can wrap up to 60 axons at once – Myelin sheath gap is present – No outer collar of perinuclear cytoplasm – Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia Neuron Processes (10 of 10) • Myelin sheaths in the CNS (cont.) – White matter: regions of brain and spinal cord with dense collections of myelinated fibers  Usually fiber tracts – Gray matter: mostly neuron cell bodies and nonmyelinated fibers Neuroglia Figure 11.4d Neuroglia. Classification of Neurons (1 of 3) • Structural classification – Three types grouped by number of processes 1. Multipolar: three or more processes (1 axon, others dendrites) – Most common and major neuron type in CNS 2. Bipolar: two processes (one axon, 1one dendrite) – Rare (ex: retina and olfactory mucosa) 3. Unipolar: one T-like process (two axons) – Also called pseudounipolar – Peripheral (distal) process: associated with sensory receptor – Proximal (central) process: enters CNS Table 11.2-1 Comparison of Structural Classes of Neurons Table 11.2-1 Comparison of Structural Classes of Neurons Table 11.2-2 Comparison of Structural Classes of Neurons Table 11.2-2 Comparison of Structural Classes of Neurons Classification of Neurons (2 of 3) • Functional classification of neurons – Three types of neurons grouped by direction in which nerve impulse travels relative to CNS 1. Sensory – Transmit impulses from sensory receptors toward CNS – Almost all are unipolar – Cell bodies are located in ganglia in PNS 2. Motor – Carry impulses from CNS to effectors – Multipolar – Most cell bodies are located in CNS (except some autonomic neurons) Classification of Neurons (3 of 3) • Functional classification of neurons (cont.) – Three types (cont.) 3. Interneurons – Also called association neurons – Lie between motor and sensory neurons – Shuttle signals through CNS pathways – Most are entirely within CNS – 99% of body’s neurons are interneurons Table 11.2-3 Comparison of Structural Classes of Neurons Table 11.2-3 Comparison of Structural Classes of Neurons