Week 2, Monday

Histology of the Nervous System pdf

Cell types:

  • subdivided into 2 basic types:

    • Neurons

      • The primary functional cell of the nervous system

      • Function to transmit electrical and chemical signals

    • Neuroglia (glia

      • Supportive cells of the nervous system

      • Come in multiple subtypes, each of which have their own functions

Neuron parts:

  • Cell body: central, contains most organelles including nucleus, carries out metabolism and information processing

    • Aka soma/perikaryon

    • Round/polygon

    • Size= 5-100um

    • The rest of the cell is dependent on the cell body for survival

  • Axon: long & cylindrical, usually only 1, largely functions to send info to other cells

    • Axon hillock: site where the axon attaches to the cell body

    • Initial Segment: just distal to hillock, most electrically excitable area of the axon

    • Collateral branch: side branch of axon

    • Terminal branch: branches at the end of the axon, leading into a synapse

    • Bouton: expanded distal end of terminal branch, specialized for forming synapse, releasing neurotransmitters

  • Dendrite: thin & tampering processes, may be many, functions to receive info from other neurons and relay it to the cell body

    • Most organelles (except ulcers) are found within the dendrite, especially proximally

    • Dendritic spines:

      • Extensive branching

      • Small projections from end of dendritic branches

      • Increases surface area for synapse

      • The formation (or loss) of dendritic spines is thought to play a role in memory and intellect

Neuronal Organelles:

  • Most are found in the cell body

  • The pattern of organelles seen in the neuron are consistent with the neuron’s role as the secretory center

  • Nissl bodies:

    • Makes ribosomes stain dark blue or purple

    • Large amount of rER=high concentration of Nissl dye- neuron looks blue

    • The stained rER is called a Nissl Body

    • Pattern of Nissl bodies present varies in different types of neurons= way to identify neurons

  • Cytoskeleton:

    • 3 types of proteins:

      • microtubules

        • Hollow cylinders

        • Tubulin: main structural component of cylinders

        • Microtubules associated proteins (MAPs): connect tubulin proteins to each other and to outside structures

        • Function: railroad tracks for axonal transportation

      • neurofilaments

        • Intermediate filaments of neuron

        • Structure: cytokeratin proteins

        • Function: structural support

      • microfilaments

        • Consists of Actin protein

        • Functions: move substances to cell membrane, attach molecules to cell membrane, axonal growth

    • Found in all parts of neuron

    • Functions- structural support, shape, transportation

Axonal transport System:

  • axonal transport is necessary for axonal survival

  • Classified by speed- fast or slow

    • Fast

      • subdivided by direction- anterograde (towards synapse) or retrograde (towards cell body)

      • 400mm/day

      • Motor proteins

        • Kinesin= fast anterograde

        • Dynein= retrograde

      • Transports organelles, vesicles, growth factors, endocytic products, and viruses

      • Head: Attach to cargo at one end and microtubules at other

      • Tail: Proteins hydrolyzed ATP to power movement- allows them to “walk” along the microtubules “track” to carry the cargo to its destination

    • Slow

      • Anterograde only

      • Few mm/day

      • Poorly understood mechanism

      • Transports soluble proteins

Synapse:

  • connection between a neuron and another structure (neuron, muscle, gland, etc.)

  • Most occur between 2nerves

  • Can form between any combination of neuronal parts, most often between axon and dendrite

  • Named for the parts involved:

    • AD= axodendritic synapse

    • AA= axonaxionic synapse

    • AS= axosomatic synapse

    • DD= dendrodendritic synapse

  • Each synapse has 3 components:

    • Presynaptic element: structure entering synapse, typically axon, releases neurotransmitters

    • Postsynaptic element: Ede Ives neurotransmitters, responds in various ways depending on the receptor

    • Synaptic cleft: small gap in-between presynaptic and postsynaptic structures. Neurotransmitters must cross he cleft to reach the receptor

  • Active zone= the dark colored region of membrane between axon and dendrite, contains most synaptic structures

Neuronal Classification:

  • the spinal cord is divided into gray and white matter

    • The gray matter is central

      • Divided into horns

      • All levels have anterior (motor) and posterior (somatosensory) horns

      • Some levels have lateral (sympathetic) horns

    • White mater is peripheral

      • Divided into funiculi

      • All levels have anterior (mixed motor and somatosensory), posterior (somatosensory) and lateral (motor)

  • A nerve is a bundle of PNS axons traveling along the same general route

  • Classified by Structure innervated: general vs special

  • Classified by control: somatic vs autonomic (visceral)

  • Classified by function: motor (efferent), sensory (afferent) interneurons

  • All classifications can be combined

  • Most sensory/motor Neurons are partly in CNS and partly in PNS

  • Interneurons are entirely in CNS

  • Individual axons may exit the nerve at various points along its course

  • It is common to refer to nerves as motor, sensory, etc. (this is an oversimplification) it is really the neurons within that have these functions

  • Typically, spinal nerves consist of the following components:

    • Rootlets

    • Roots

    • Mixed nerve

    • Rami

    • Anterior roots/rootlets= motor

    • Posterior roots/rootlets= somatosensory

    • Mixed nerve= anterior and posterior rami are mixed