Fundamentals of the Nervous System and Nervous Tissue Lectrure 4

Structure of a Motor Neuron Neuron cell body Dendritic spine (b) Figure 11.5b Structure of a motor neuron. Neuron Processes • 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 • 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 – An axon contains the same cytoplasmic organelles found in the dendrites and cell body with two important exceptions (lacks rough endoplasmic reticulum and a Golgi apparatus, the structures involved with protein synthesis and packaging) Neuron Processes • 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 • Myelination in the PNS – Formed by Schwann cells ▪ Wraps around axon in jelly roll fashion ▪ One cell forms one segment of myelin sheath – Plasma membranes have less protein ▪ No channels or carriers, so good electrical insulators ▪ Interlocking proteins bind adjacent myelin membranes Schwann cell plasma membrane Schwann cell cytoplasm PNS Nerve Fiber Myelination Axon 1 Schwann cell nucleus Myelin sheath Figure 11.6a PNS nerve fiber myelination. A Schwann cell envelops an axon. Schwann cell cytoplasm 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. 3 The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. (a) Myelination of a nerve fiber (axon) Neuron Processes • 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 Dendrites(receptive regions) Cell body (biosynthetic center and receptive region) Nucleus Initial segmen Chromatophilic t substance(rough of axon endoplasmic Axon hillock Nucleolus reticulum) (a) Axon (impulse-generating and conducting region) Impulse direction Figure 11.5a Structure of a motor neuron. Myelin sheath gap Schwann cell Terminal branches Axon Terminals (secretory region) Neuron Processes • 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 Neuron Processes • 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 Axons Myelin sheath Oligodendrocytes Myelin sheath gap (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Figure 11.4d Neuroglia. Classification of Neurons • 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 • 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