Introduction to Biological Psychology Part II Chapter 3 Under the Microscope Cells of the Nervous System

CELLS OF THE NERVOUS SYSTEM I 141 . UNDER THE MICROSCOPE CELLS OF THE NERVOUS SYSTEM Catherine Hall Learning Objectives By the end of this chapter , you will be aware of the main cells in the nervous system what these cells look like what their functions are . In each region of the central and peripheral nervous systems

142 CELLS OF THE NERVOUS SYSTEM are specialised cells that perform or support the fundamental function of the nervous system the detection of information about the world , integration with information about the internal body state and past experience , and the generation of an appropriate behaviour . These cells can broadly be as either neurons or glia . Neurons are the cells that perform the signalling and information processing . They detect inputs , integrate information , and send signals to other cells , be they other neurons ( forming neuronal circuits ) or cells ( such as muscles or endocrine cells ) to produce a behavioural or physiological effect . Glia or glial cells play numerous supporting roles for the neurons . This support was originally thought to be structural the term glia is derived from the Greek for glue but is now appreciated to be highly complex , involving dynamic communication between glia , neurons and other cell types , and is able to modulate neuronal communication . In addition to neurons and glia , neural tissue contains a large number of vascular cells . As we saw in Chapter , Exploring the brain , brain tissue is densely in order to provide sufficient metabolites for COl '

CELLS OF THE NERVOUS SYSTEM I 143 Neurons terminal Fig . A typical neuron . Dendrites branch out from the cell body , where the nucleus is located . The axon is located where the cell body transitions into the axon . The axon begins at the axon and ends at the presynaptic terminal , which can branch into multiple terminals . The basic form of a neuron is shown in Fig . It has a cell body , or soma , with branching processes called dendrites and a thin process called an axon . The axon can also be branched , forming axon . We talked in the last chapter ( Exploring the brain ) about the general function of the brain being to take in information , perform a computation on that information to work out what to do next , then to produce an output . As mentioned above , this same function is also performed by individual neurons . The dendrites , or dendritic tree , are where most inputs to

144 I CELLS OF THE NERVOUS SYSTEM the cell are received . These inputs are integrated across the dendritic tree and soma before the cell decides whether the inputs are strong enough to trigger an electrical output signal down the axon ( the action potential , see Chapter Neuronal transmission ) The site of this decision is the top of the axon furthest from the terminals , termed the axon initial segment . The action potential travels along the axon to the axon terminal . The axon terminal is very close to , but not touching , a dendrite of another cell . This tiny gap is specialised for passing messages between two cells and is called a synapse . At the synapse , action potentials cause release of a chemical messenger a neurotransmitter which transmits the signal across the gap to the next cell in the circuit . Neuron morphology affects computation All neurons have this basic morphology , but nevertheless come in a multitude of shapes and sizes .

CELLS OF THE NERVOUS SYSTEM 145 Dendrites Axon Dendrites . Axon Fig . Neuron structure is variable , but the main components of cell body ( shown in black ) dendrites ( shown in brown ) and axon ( shown in blue ) are common among all neurons . Most neurons are multipolar neurons , with a branched dendritic tree and a single axon . Some neurons , particularly sensory neurons ( eg . in the retina ) are bipolar having a single dendrite coming out from one end of the soma , and a single axon from the other ( though these may be branched near their ends ) neurons have a single process , as an axon , which receives inputs at one end and releases transmitter at the other end . These different shapes and sizes alter how neurons perform computations .

146 CELLS OF THE NERVOUS SYSTEM Fig . Different categories Unipolar neuron Bipolar neuron Multipolar neuron neuron Because the job of a neuron is to add up all its inputs and decide whether to an output action potential , the number of these inputs and where they are located affects how this summation happens . For example , in the cerebellum ( Figure ) the

CELLS OF THE NERVOUS SYSTEM 147 cells receive inputs from granule cell axons and axons from deep cerebellar nuclei in the pons , called climbing . The climbing are very branched and make lots of connections ( synapses ) to each cell , while the granule cells axons , called parallel , are simple and form only a single synapse to each cell . This means that the connection between a single granule cell and a cell is weaker than the connection between the climbing neuron and the cell . Another way that neuron morphology can affect the computations it undertakes can be seen if we zoom in on the dendrites ( Figure ) Fig . Dendrites branch out from the soma . is to receive information from other neurons . Some dendrites have small protrusions called spines that are important for communicating with other neurons .

148 CELLS OF THE NERVOUS SYSTEM Some dendrites are covered with small protrusions , called dendritic spines , whereas others are smooth . Synapses can form on to the spine , or onto the neck of the spine , and this means that some inputs can gate the effect of other inputs , altering their impact on the neuron . Different classes of neurons There are many different ways in which neurons can be and subdivided , depending on what aspect of neuronal function is being focussed on . As we have seen above , we can neurons by their morphology , and morphology can also be used to Further classify the multitude neurons For example , pyramidal cells have a characteristic pyramidal shaped soma , long dendrite pointing upwards ( an apical dendrite ) tufty basal dendrites , and an axon that often forms several . cells of the cerebellum have a round soma , a Hat highly branched dendritic tree at the top of the soma and a single long axon . Granule cells have a small cell body , a simple dendritic tree and an axon that splits in two . Chandelier cells have a highly branched axon that forms distinctive connections with the initial segments of lots of axons .

CELLS OF THE NERVOUS SYSTEM 149 Hippocampal pyramidal cell Cerebellar , Cerebellar granule cells chandelier cell ( dendrites are blue . axon is red ) Fig . Examples of different types of multipolar neuron We can also neurons by their effect on other neurons , being excitatory or inhibitory , depending on whether they make the neurons they connect to more or less likely to an action potential ( more of this in Chapter Neuronal transmission ) Of the examples given above , pyramidal cells and granule cells are excitatory and cells and chandelier cells are inhibitory . Neurons can also be by the type of neurotransmitter they release neurons release glutamate , neurons release ( gamma

150 I CELLS OF THE NERVOUS SYSTEM acid ) neurons release dopamine , and so on . As we will see later , these categories broadly overlap neurons are excitatory , because glutamate excites cells and neurons are inhibitory , because inhibits cells . However other neurotransmitters such as dopamine can have different downstream effects depending on what proteins that cell expresses at the synapse . Neurons can also be based on their connectivity and role in a circuit , but this can get complicated ! Neurons that project a long way to a different brain region are termed principal neurons , while those that project locally are termed . Principal neurons are often excitatory , but not always ( for example cells output information from the cerebellum and are inhibitory ) However , in some brain areas it is hard to decide whether a cell should be termed an or not . Is it helpful to call pyramidal cells that project to far cortical areas principal cells but very similar cells that project to the next cortical column ?

Are cerebellar granule cells because they project within the cerebellum , though they project to a distinct cell layer ?

Instead , the term is only commonly used for inhibitory cells , referred to as inhibitory . A chandelier cell is an example of an inhibitory . Excitatory cells in local circuits are instead usually referred to by other features , location and morphology ( a Layer pyramidal cell as

CELLS OF THE NERVOUS SYSTEM 151 distinct from a Layer pyramidal cell in the example given above ) Glia There are main types cells .

152 CELLS OF THE NERVOUS SYSTEM Zig . cells green ) blue ) microglia ( maroon ) and ependyma cells ( Blood vessels are shown in , thus termed of their morphology , have many , many that encircle synapses each astrocyte can contact up to mil ion synapses . These processes not only a physical support to neuronal connections but also lots active roles to support neuronal function and communication . For example , are an important for removing neurotransmitter

CELLS OF THE NERVOUS SYSTEM 153 from synapses , taking it up across their membranes to reset synapses after synaptic transmission , and can also regulate the levels of ions in the extracellular space . can also release many substances onto neurons and other cells ( lactate , glucose ) modulating their activity and providing metabolic support . Specialised astrocyte processes , termed end feet communicate with local blood vessels , altering Fig . green ) wrap around blood vessels ( magenta ) as well as up glucose from the blood . local blood flow and taking These end feet surround blood vessels , forming part of the , and others extend to the surface of the brain , forming a thin layer just under the pia . This barrier of astrocyte end feet is termed the glia , and stops ( or regulates ) molecules and cells from entering or leaving the nervous tissue . also react to damage to brain tissue , becoming activated and expressing different molecules when they are exposed to infection . They can form a scar around sites of damage . While this can be helpful , it also causes problems if cells remain activated for a long time , and

154 CELLS OT THE NERVOUS SYSTEM that forms Ol Ol ! making connections through . and Cells perform similar roles in the and respectively . Both cells wrap layers of a fatty substance called myelin around neuronal axons . do so by sending multiple processes to nearby axons , whereas cells in the each have only one process . These layers insulate axons allowing action potentials to be conducted more quickly and robustly ( see Chapter Neuronal transmission ) Being so closely associated with axons , and cells also provide support to axons by releasing some molecules and taking up others , regulating the extracellular ! around axons Fig . green ) with cells nuclei labelled in blue . role of at in a similar manner to the synapses . In multiple sclerosis , the body immune cells seem to attack , leading to of axons . This impairs signalling in the axons that were by the

CELLS OF THE NERVOUS SYSTEM 155 damaged cells , causing a variety of neurological problems , depending on which axons are affected and what signals they were carrying . Microglia are small cells which play an important role in repairing damaged brain tissue . Unlike the rest of the body , immune cells can not readily enter the brain from the blood because they are prevented by the . Instead , microglia act as the brain Fig . Microglia resident immune cell . Their processes constantly extend and retract , surveying the brain for signs of damage or infection . When they find a site of damage , they activate and migrate to this region , forming a barrier between healthy and damaged tissue and removing debris of dying cells . Microglia are also often associated with synapses and blood vessels , and increasingly appreciated to have important roles not just in controlling damage , but in shaping normal brain function as well , regulating synaptic transmission and signalling to blood vessels . Like , while responses to damage are usually thought to be , when they are activated for a long period of time they can themselves be harmful . This may happen in disease such as , and after a stroke .

156 CELLS OF THE NERVOUS SYSTEM The last type of glial cell is the ependymal cell , which we discussed in the last chapter . These cells line the ventricles and produce . Vascular cells We heard in the previous chapter that the brain contains a dense vascular network to provide a constant , tightly regulated supply of energy ( mainly oxygen and glucose ) to the brain . The main cells that make up the blood vessels are endothelial cells , which form the vessel wall next to the blood , and vascular mural cells smooth muscle cells on larger vessels ( arteries and ) and on smaller vessels ( capillaries ) which wrap around endothelial cells ( Figure )

CELLS OF THE NERVOUS SYSTEM 157 muscle cell Cell Fig . Cell types of the A ) and smooth muscle cells ) The neurovascular unit comprises the cells of the blood vessel and and neurons in the brain that wrap around the vessels . As we heard earlier , endothelial cells form tight junctions with adjacent endothelial cells and with , forming the . A major function of endothelial cells is to regulate entry of cells and molecules across the , as well as to clear waste molecules from the brain into the blood . They regulate the entry of small molecules by expressing different transporter proteins which allow certain molecules to cross the into the brain . To allow immune cells into the brain , endothelial cells express proteins that stick to immune cells in the blood which then crawl between or through the endothelial cells . Another function of endothelial cells is control of blood

158 CELLS OF THE NERVOUS SYSTEM . Endothelial cells can respond to signals in the blood or the brain to produce molecules that contract or dilate smooth muscle cells or , altering the diameter of blood vessels and changing blood flow . Smooth muscle cells are cells that wrap around the vessel , while have a distinct cell body and processes that extend along and around the blood vessel . In addition to responding to signals from endothelial cells , these smooth muscle cells and can also constrict and dilate in response to signals from neurons and , changing blood flow to match alterations in neuronal activity . are also important for stabilising newly vessels and work together with endothelial cells to control the . The brain is full ! All these cells with their complex structures are often shown in cartoons with lots of space between them . In reality , however , the different cells processes are closely intertwined and crammed together , taking up almost all the available space . We can see this experimentally by looking at reconstructions from electron images of sequential slivers of a very small bit of tissue taken with an electron microscope .

CELLS OF THE NERVOUS SYSTEM 159 Fig . Electron micrograph of rat brain , segmented to show different structures ( A ) and reconstructed in a volume ( The different structures the space . Spiny dendrites ( yellow ) excitatory axons ( green ) an inhibitory axon ( pink ) synapses ( red ) and astroglia ( light blue ) In these image sequences , structures can be labelled and traced in each image , and then assembled to get a reconstruction of the different cells in a small volume of tissue . From such images we can see in superb detail how different structures connect , which dendrites an axon contacts . However , tracking and reconstructing every process in even a small volume is very expensive , and it not yet possible to do this for even a whole cortical column , never mind a whole brain or brain region .

160 CELLS OF THE NERVOUS SYSTEM Key Takeaways Cells of the nervous system Neurons all have a soma , axon and dendrites but come in lots of shapes and sizes , and produce different neurotransmitter molecules , meaning they can perform lots of different computations . There are different types of glia in the brain ( cells , microglia and ependymal cells ) which have many roles , including controlling the extracellular environment for neurons ( cells , microglia ) providing physical and metabolic support ( cells ) insulating axons to allow fast neuronal transmission ( cells ) detecting and infection and tissue damage ( microglia ,

CELLS OF THE NERVOUS SYSTEM 161 contacting and communicating with blood vessels ( microglia ) producing ( ependymal cells ) Endothelial cells , smooth muscle cells and form a dense network of blood vessels and control the brains energy supply , as well as what cells and molecules can go between the blood and the brain tissue . About the Author Catherine Hall UNIVERSITY OF Catherine Hall is a member of the Neuroscience Steering Committee , the University Senate , convenes the core year module Pry , and lectures on topics relating to basic neuroscience , neurovascular function and dementia .