Cells of the Nervous System
Layout of the Nervous System Central Nervous System (CNS) Brain (in the skull) Spinal Cord (in the spine) Interprets sensory input, initiates movement, and mediates complex cognitive processes Peripheral Nervous System (PNS) Located outside of the skull and spine Serves to bring sensory information into the CNS and carry motor signals out of the CNS
Functional Unit of the Nervous System The functional unit of the nervous system is the neuron. Neurons are specialized cells for the reception, conduction, and transmission of electrochemical signals Many sizes and shapes ~100 billion neurons ~100 trillion connections
Major External Features of a Neuron A typical neuron has a cell body, dendrites and an axon. Flow of information: dendrites receive inputs from other neurons the cell body (or soma) integrates (processes) information the axon transmits the output of processing to other neurons
Major Internal Features of a Neuron Similarities with other cells: Contains a nucleus that holds genetic information Contains organelles that support the life of the cell, including mitochondria and ribosomes Surrounded by a membrane that protects the cell Differences with other cells: Stop dividing (reproducing) after birth Have dendrites and axons, specialized structures designed to receive and transmit information
The Cell Membrane Lipid bilayer with embedded proteins separates inside and outside of cell selective permeability controls flow of substances into and out of the cell The particular composition of embedded proteins in the cell membrane is critical for proper neuron function
Visualizing Neurons 1 The major problem in visualizing neurons is not their minuteness; rather, it is the fact that neurons are so tightly packed and their dendrites and axons intricately intertwined. Golgi stain: for as-yet unknown reasons, silver stains some neurons black in their entirety but stains others not at all. Allows for visualization (under light microscopy) of the shape and connectivity of neurons.
Nissl stain: selectively stains cell bodies (nuclei and ribosomes that contain nucleic acids). Used to indicate the number of neurons in an area. Neurons tend to be organized in clusters rather than spread homogenously throughout a structure. Visualizing Neurons 2
Tract tracing techniques: used to trace the paths of axons Anterograde: to trace axons projecting away from cell bodies Visualizing Neurons 3 Retrograde: to trace axons projecting into an area of cell bodies www.seriousmadscience.com cmbn-approd01.uio.no
Electron microscopy: provides information about the details of neuronal structure With its development (1930s), it provided definitive evidence for the neuron doctrine: the concept that the nervous system is made up of discrete individual cells interconnected through synapses. 3-D reconstructions reveal that axons tend to synapse on dendritic spines Visualizing Neurons 4 dendrite axon synapse spines dendrite arrows: synapses colors: axons
Two photon fluorescence microscopy (1990) uses focused laser beams to scan living tissue. Studies suggest that dendritic spines are remodeled over time, even on the order of minutes. Visualizing Neurons 5 Spines blue: removed white: added green: static
Neuroanatomical Structures Two kinds of gross neural structures; those composed primarily of cell bodies and those composed primarily of axons. Clusters of cell bodies Bundles of axons CNS Nuclei (singular nucleus) Grey matter Tracts White matter PNS Ganglia (singular ganglion) Nerves
Anatomical Classes of Neurons Neurons do not all look alike. One way to classify them is based on the number of processes emanating from their cell bodies.
Functional Classes of Neurons information from tissue/organs integrate intermediate information to effector cells www.studyblue.com
CNS Neurons Are Usually Multipolar
PNS Neurons are Unipolar or Multipolar
Connections Between Neurons Direction: afferent (approaching) efferent (exiting) Type: axon-dendrite but also axon-soma, axon-axon, dendrite-dendrite Neuroscience: Exploring the Brain, 3 rd ed
Glia: The Forgotten Cells of the NS Helper cells (glia = glue ) Outnumber neurons 5:1 Provide structural/metabolic support to neurons Recent evidence for glial communication and modulatory effects of glia on neuronal communication www.jaynejubb.com
Classes of Glial Cells There are four classes of glial cells: Oligodendrocytes: extensions rich in myelin; create myelin sheaths in CNS Schwann cells: similar to function of oligodendrocytes, but in PNS; can guide axonal regeneration Microglia: involved in response to injury or disease; in CNS Astrocytes: largest glia; starshaped; many functions; in CNS blogs.scientificamerican.com
Myelination of neurons
Microglia respond to injury or disease ramified motile ucsf.edu
Astrocytes perform a variety of duties Control and communicate widely with many neurons: form a barrier to unwanted substances entering the brain control blood flow to neurons (activity allows functional imaging) 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