Cellular Neuroanatomy II The Prototypical Neuron: Neurites Reading: BCP Chapter 2
Major Internal Features of a Neuron The neuron is the functional unit of the nervous system. A typical neuron has a soma (which contains a nucleus that holds genetic information and organelles that support the life of the cell) and neurites, including dendrites and an axon, that are specialized structures designed to receive and transmit information.
Neurites Neurites are protoplasmic protrusions that extrude from the cell body of a neuron; there are two types: dendrites and axons. Axons can be distinguished from dendrites by several features including shape, length, myelination and function. Dendrites often taper off in shape, are shorter (usually <200 mm) and branch profusely at all angles. They do not have myelin sheaths, and receive electrochemical signals. In contrast, axons tend to maintain a constant radius, be long (up to 2 m) and branch relatively sparingly at right angles. They have myelin sheaths, and transmit electro-chemical signals.
The Cytoskeleton Cytoskeleton is the scaffolding that give a neuron its shape. The bones of the cytoskeleton are microtubules, microfilaments and neuro-filaments. Largest of the three types, microtubules run longitudinally down neurites. Elements of the cytoskeleton are dynamically regulated (assembled and disassembled) thus neuron shape changes continually. In addition to giving shape to neurons, the cytoskeleton also provides mechanical support, aids in transport of substances, allows cells to migrate, and plays a role in segregating chromosomes during cell division.
The Axon 1 The axon, found only in neurons, is a highly specialized structure for the transmission of information (in the form of electrical activity) over long distances. The axon begins at a region called the axon hillock, which tapers away from the soma to form the initial segment of the axon. Two features distinguish the axon from the soma: there are few, if any, ribosomes in the axon (bound or free) thus there is no protein synthesis; the protein composition of the axonal membrane is fundamentally different than that of the soma. The diameter of an axon proper (constant radius) is variable, ranging from 1 to 25 mm; the thicker the axon, the faster the transfer of information. Axons may give off branches (called axon collaterals) (some of which may return to contact the cell itself, called recurrent) allowing neurons to communicate with many parts of the nervous system.
The Axon 2 The axon ends at the axon terminal or terminal button. The terminal is a site where the axon comes in close proximity to other neurons (usually their dendrites or cell body) and passes information on to them (i.e., provides innervation to them). The point of near-contact is called the synapse. The cytoplasm of the axon terminal differs from that of the axon proper in several respects: microtubules do not extend into the terminal; the terminal contains numerous small bubbles (50 nm in diameter) of membrane called synaptic vesicles; the terminal contains many mitochondria indicating high energy needs. Information (in the form of chemicals) flows in a direction from pre- to post-synaptic sides of the synapse (i.e., from axon terminal of one neuron to dendrite or soma of the next cell).
Axoplasmic Transport Axons lack ribosomes, thus proteins must be synthesized in the soma and then shipped down the axon. Axoplasmic transport can be slow (up to 10 mm/day; diffusion) or fast (up to 1000 mm/day). Fast axoplasmic transport is via the microtubules. Material is enclosed in vesicles, which is then walked along the microtubules at the expense of ATP. The legs differ depending on direction of movement: anterograde ( terminal): kinesin retrograde ( soma): dynein
Tract tracing techniques: used to trace the paths of axons Anterograde: to trace axons projecting away from cell bodies Tract Tracing Retrograde: to trace axons projecting into an area of cell bodies www.seriousmadscience.com cmbn-approd01.uio.no
Dendrites 1 The term dendrite is derived from the Greek word for tree reflecting the fact that these neurites resemble the branches of a tree (called the dendritic tree). Dendrites function as the antennae of the neuron, thus are covered with thousands of synapses (stained red at right). Dendritic trees have a large variety of shapes and sizes to enhance this functionality. In addition, the dendritic membrane has many specialized protein molecules called receptors that detect the chemicals released at the synapse. cell bodies: blue microtubules: green axon terminals: red
Dendrites 2 The dendrites of some neurons are covered with specialized structures called dendritic spines. A lack (or abnormal shape) of these structures can lead to cognitive disabilities. Dendritic spines are thought to isolate various chemical reactions that are triggered by some types of synaptic activity. For the most part, the cytoplasm of dendrites resembles that of axons. However, free ribosomes have been observed at the base of dendritic spines suggesting that protein synthesis occurs here (memory storage). free ribosomes
Classification of Neurons Neurons are classified based on the morphology of dendrites, axons and the structures they innervate. There are four common schemes. Unipolar Number of neurites 1 (unipolar), 2 (bipolar), and 3 or more (multipolar) Dendritic tree shape and/or presence of spines (spiny vs aspinous) Bipolar Multipolar Stellate cell Connections sensory, motor, interneurons Axon length Golgi type I: projection neurons, long Golgi type II: local circuit, short Pyramidal cell
Helper cells (glia = glue ) Outnumber neurons up to 5:1 Provide structural/metabolic support to neurons Non-Neuronal Cells: The Glia Recent evidence for glial communication and modulatory effects of glia on neuronal communication www.jaynejubb.com
Classes of Glial Cells There are four types of glial cells: Oligodendrocytes: extensions rich in myelin; create myelin sheaths around axons in CNS Schwann cells: similar to function of oligodendrocytes, but in PNS; can guide axonal regeneration Microglia: involved in response to injury or disease Astrocytes: largest glia; star-shaped; many functions blogs.scientificamerican.com
Myelination of Neurons
Microglia Microglia exist in a ramified state at rest. When activated, these cells retract their processes, then move towards and engulf injured or diseased tissue. ramified motile ucsf.edu
Astrocytes Astrocytes control and communicate widely with many neurons: form a barrier to unwanted substances entering the brain control blood flow to neurons maintain the proper chemical state outside of neurons/remove waste surround synapses and can modify neuronal signals send nutrients (glucose) to neurons digest old neuronal parts secrete neurotransmitters and glialtransmitters www.nature.com