1 Cells of the nervous system Structure and Function

2 Input (sensation) Output (action) In order to understand behavior, we must understand the operating mechanism of the nervous system What is the function of the nervous system? The ultimate goal of the nervous system is to produce ecologically relevant behavior to enhance survival So how do we start probing the nervous system? The reductionist method

3 There are an estimated 10^11 cells in the brain alone! The nervous system is not a homogenous tissue Comprised of cells that can be classified based on many different parameters including: Biochemistry (what proteins does it express?) Anatomy (where are they found?) Function (what do they do?) Morphology (shape) Connectivity (who do they connect to? Local/global?) Known 100 different cell types (probably more) Basket cells, Betz cells, Medium spiny neurons, Purkinje cells, Pyramidal cells, Renshaw cells, Alpha motor neurons, Granule cells, Oligodendrocytes, …

1) Central nervous system (CNS): Brain Spinal Cord 2) Peripheral nervous system (PNS): outside the brain and spinal cord Two major divisions to the nervous system Classification of neurons

Structure and function of two cell types making up the nervous system: 1.Neurons Functional classification Sensory Motor Interneurons Morphological classification Uni – Bi – Multi - polar 2. Glia Astrocytes Microglia Oligodendrocytes There are more glia cells than neurons

6 The Golgi silver staining method (1873): Only 1% of the cells Shows the entire cell Neuron structure Santiago Ramon Y Cajal Nobel prize to both in 1906 Camillo Golgi

The neuron: basic structure Four parts: 1.Dendrites 2.Soma 3.Axon 4.Terminal buttons

Neurons are built to receive, process, and transmit information How does information transfer occur? Within the cell – unidirectional from dendrite through axon to terminal buttons - electrical Between cells – through the synapse chemical

Types of Sensory Receptors: Chemoreceptors Mechanoreceptors Thermoreceptors Photoreceptors Nociceptors

Connection specificity – information transmission among neurons is not random but well planned. Connections are not permanent, and can change with experience.

Neurons are built to receive, process, and transmit information Example 1 : Withdrawal reflex CNSPNS Sensory neuron Motor neuron

Neurons are built to receive, process, and transmit information Example 2 : Voluntary action

Neurons differ in structure unipolar bipolar multipolar

Transfer sensory information from the periphery to the central nervous system bipolar unipolar Sensory neurons

Motor neurons generate a motor response – control muscles multipolar

Types of cells Sensory Neurons: unipolar/bipolar Motor Neurons: multi-polar Neurons: multi-polar – interneurons Differ in function and structure

100 sensory neurons are needed to trigger a motor neuron A single motor neuron contacts several muscle fibers But this is a much more complex system

100 sensory neurons are needed to trigger a motor neuron – convergence A single motor neuron contacts several muscle fibers - divergence Divergence and convergence For example: touch receptors on the skin of the hand will have less convergence (higher resolution) compared with touch receptors on the back

They come in different shapes… Most neurons are multipolar neurons 150,000!! contacts Purkinje cell in cerebellum

They come in different shapes…

21 They come in different shapes And sizes …

Classifications of Neurons 1. Function/direction a. sensory neurons (afferent) b. Motor neurons (efferent) c. interneurons

Classifications of Neurons 1. Function 2. Structure a. unipolar b. bipolar c. multipolar

Neurons need a lot of energy, maintenance, protection Glia Cells (supporting cells): more abundant than neurons. 1.Astrocyte cells 2.Microglia 3.Oligodendrocytes / Schwann cells Types of glial cells

1. Astrocytes (stars) Insulate: Isolate the neuron and synapse Hold in place: Glue Nourish: lactate (from glucose) Clean: Phagocytosis of dead neurons Control the chemical surrounding of the neuron

Types of glial cells 1.Astrocytes 2.Microglia scavengers; remove cellular debris (phygocytes); Involved in immune processes in the brain

Types of glial cells 1.Astrocytes 2.Microglia 3.Oligodendrocytes myelinated axons of central nervous system

Produce myelin sheath: fatty tissue, insulate the axon, increase conduction Nodes of Ranviar – 1-2 micrometer gaps every 1 mm of sheath

Neuron: one axon Nerve – bundle of many axons within a membrane

3. Schwann Cells In the peripheral nervous system One for each myelin segment Allows regeneration of axons

OligodendrocytesSchwann cells PlaceCNSPNS MyelinOne cell up to 50 segments One cell per segment Damageinhibit axon regrowthSupport axon regrowth following damage Oligodendrocytes vs. Schwann cells

Structure of a cell

Membrane Defines the shape of the cell Divides between the intra and extra cellular fluid Made of phospholipid molecules 6-8 nm thick Hydrophylic phosphate Hydrophobic lipid

Membrane 4 ways to cross the membrane:

1. Simple diffusion movement of molecules from a high concentration to a low concentration

2. A specific channel Allows passive transport of hydrophilic molecules

3. Active transport against the concentration gradient

4. Endocytosis / Exocytosis Endocytosis Exocytosis

Membrane 4 ways to cross the membrane: 1.Simple Diffusion 2.A specific channel – facilitated diffusion 3.Active transport - against the concentration gradient 4.Endocytosis/Exocytosis Proteins embedded in the membrane

Summary The ultimate goal of the nervous system is to produce ecologically relevant behavior to enhance survival It achieves this goal by actively sensing the environment (receiving information), processing this information, and finally translating it into behavior It consists of two major divisions: CNS, PNS Highly heterogenous (both at functional and structural levels) Two cell types – Neurons and Glia All cells, and the brain in general, maintain their identity by use of selectively permeable membranes