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Chapter 3

PSY2301 Chapter 3: What are the Functional Units of the Nervous System?

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Patrick Davidson

Chapter 3: Functional Units of the Nervous System How can we see brain cells? • To highlight individual cells in the nervous system: o to make the cells firm we soak them in formaldehyde (removes water from the tissue) o then the tissues are sliced into small thin slices and looked at in a microscope o visualization of different parts of the cells is done by dyes Cajal's neuron theory: • neurons are the functional units of the nervous system , this includes the idea that it is the interactions between these discrete cells that enables behaviour • 3 divisions of a neuron o cell body/soma : core region o dendrites: neurons branching extensions- collect information from other cells  one axon but many dendrites per neuron o axon: carries messages to other neurons Information processing • neurons acquire information from sensory receptors, pass the information on to other neurons, and make muscles move to produce behaviour o at the same time, they regulate body processes such as breathing, heartbeat, and body temperature Structure and Function: • surface area of a neuron is increased by its extending dendrites and the axon • Dendrite spines: further protrusions of the dendrite that greatly increases the SAof the dendrite and is the usual point of dendritic contact with the axons of other cells. • Aneuron may have 1-20 dendrites each may have many branches and the spines on the branches may be thousands • the dendrites collect information from other cells, and the spines are the points of contact with other neurons • the extent of a cell's branches corresponds to is information processing capacity • each neuron has only one single axon that carries messages to other neurons • axon hillock: the axon begins at one end of the cell body at an expansion known as the axon hillock • axon collateral: the axon may branch out into one or many more axons collaterals that joins at a right angle (collateral's are branches of axons) • the lower tip of the axon can branch into many mini branches called teleodendria o at the end of the teleodendria is a knob called terminal button • terminal button (end foot): knob at the tip of an axon that conveys information to other neurons • the terminal button sits very close to the dendrite spin or some other part of another cell making an 'almost connection' o this includes the surfaces of end foot and the neighbouring dendrite spine as well as the space between them called the synapse Cell body (soma): core region of the cell containing the nucleus and other organelles for making proteins Axon: 'root' or single fibre of a neuron that carries messages to other neurons Synapse: junction between one neuron and another that forms the information transfer site between neurons. Sensory neuron: neuron that carries incoming information from the sensory receptors into the spinal cord and brain. Interneuron: association neuron interposed between a sensory neuron and a motor neuron; thus, in mammals, interneurons constitute most of the neurons of the brain. Motor neuron: neuron that carries information from the brain and spinal cord to make muscles contract. Bipolar neuron: sensory neuron with one axon and one dendrite Somatosensory neuron: brain cell that brings sensory information from the body into the spinal cord Pyramid cell: distinctive interneuron found in the cerebral cortex Purkinje cell: distinctive interneuron found in the cerebellum 3 types of neurons • Sensory neurons are designed to bring information into the brain from sensory receptors, interneurons, to associate sensory and motor activity in the central nervous system, and motor neurons to carry information out of the brain and spinal cord to the body's muscle. Sensory Neurons • they are the simplest neurons structurally • a bipolar neuron found in the retina eye has a single short dendrite on one side of its cell body and a single short axon on the other side o Bipolar neurons transmit afferent (incoming) sensory information from the retina's light receptors to the neurons that carry information into the visual centres of the brain. • Asensory neuron that is a bit more complicated is the somatosensory neuron, which brings sensory information from the body into the spinal cord o the somatosensory dendrite connects directly to its axon, so the cell body sits on one side of this pathway Interneurons • these cells are also called association cells, because they link up sensory and motor neurons, interneurons branch extensively, to better collect information from many sources • animals with small vs large brains is that large brained animals have more interneurons • Specific interneuron called stellate (star-like) cell, is very small, with many dendrites extending around the cell body , its dendrites are hard to see • Apyramidal cell has a long axon, a pyramid shaped cell body, and two sets of dendrites, one set from the apex of the cell body and the other from its sides. o These interneurons carry information from the cortex to the rest of the brain and spinal cord. • Purkinje cell is a distinctive output cell with extremely branched dendrites that form a fan shape. o Carries information from the cerebellum to the rest of the brain and spinal cord Motor Neurons • To collect information from many sources, motor neurons have extensive networks of dendrites, large cell bodies, and long axons that connect to muscles. • Motor neurons are located in the lower brainstem and spinal cord o All efferent (outgoing) neuron information must pass through them to reach the muscles. Neural Networks • sensory neurons collect afferent information from the body and connect to interneurons that process the information and then pass it on to motor neurons whose efferent connections move muscles and so produce behaviour • neurons are 'networkers' and the physical appearance of the neuron reveals its role • neurons that project for long distances, such as somatosensory neurons, pyramidal neurons, and motor neurons are larger than other neurons • in general, neurons with large cell bodies have extensions that are long, whereas neurons with small cell bodies like satellite interneuron have short extensions • long extensions= carries out information to distant parts of the nervous system; short= engaged in local processing o ex: somatosensory neuron dendrites at big toe and axon is at the brain Excitation and Inhibition • neurons excite (turn them on) other neurons or inhibit them (turn them off) • a neuron spurs into action when its excitatory inputs exceed its inhibitory inputs o the same goes for the reverse Glial cell: nervous system cell that provides insulation, nutrients, and support and that aids in repairing neurons and eliminating waste products. Ependymal cell: glial cell that makes and secretes cerebrospinal fluid; found on the walls of the ventricles in the brain. Tumor: mass of new tissue that grows uncontrolled and independent of surrounding structures Hydrocephalus: buildup of pressure in the brain and, in infants, swelling of the head caused if flow of cerebrospinal fluid is blocked; can result in retardation Astrocyte: star shaped glial cell that provides structural support to neurons in the central nervous system and transports substances between neurons and blood vessels Blood-brain barrier: tight junctions between the cells that compose blood vessels in the brain, providing a barrier to the entry of an array of substances, including toxins, into the brain Five Types of Glial Cells: Ependymal: small, ovoid- secretes CSF Astrocyte: star shaped, symmetrical- nutritive and support function Oligodendroglia: asymmetrical- forms myelin around CNS axons in brain and spinal cord Schwann cell: asymmetrical; wraps around peripheral nerves to form myelin Microglial: small, mesodermally derived- defensive function • Glial cells (comes from the word glue) are the support cells of the nervous system • they do not transmit information themselves, they help neurons carry out this task, binding them together and providing support, nutrients, and protection among other functions • glial are different than neurons because they can replace themselves and errors that can come from this results in abnormal growth i.e. tumors Ependymal cells • on the walls of ventricles, the cavities inside the brain are ependymal cells that secretes CSF that fills the ventricles • CSF is constantly being formed and flows through the ventricles toward the base of the brain, where it is absorbed into the blood vessels o CSF function: shock absorber o creates a medium in which waste is removed o helps maintain regular brain temperature o is a source of nutrients for the brain • CSF flows through the ventricles, passes through narrow passages which runs through the brainstem o if the fourth ventricle (very narrow) is fully or partly blocked, the fluid flow is restricted o if its blocked, CSF builds up and the ventricle expands putting pressure on the brain o if this happens in an infant before the skulls brain is solidified, the pressure of the brain is conveyed to the skull and the baby's head swells causing hydrocephalus  doctors at a 'shut' tube inside the ventricle and one inside vein and drain the CSF Astrocyte • provide structural support within the nervous system • their extensions attach to blood vessels and to the brain's lining, creating scaffolding that holds neurons in place • these extensions provide pathways for movement of some nutrients between blood vessels and neurons • astrocytes also excrete chemicals that keep neurons healthy and help them heal if injured o Contribute to protective partition between blood vessels and the brain blood-brain barrier. Types of Brain Tumors: • Gliomas: arise from glial cells, slow growing and usually not malignant and easy to treat if they arise from astrocytes. If they arise from precursor blast or germinal cells, then they are more malignant, grow quicker and usually return. • Meningiomas: tumor attached to the meninges and grows outside the brain. The tumors are usually well encapsulated and if they are located in non-dangerous areas, they can be removed • Metastatic tumors become established by a transfer of tumor cells from one region of the body to the next. These tumors are usually in multiple areas in the body making it hard to treat, symptoms of condition only usually appear when it has reached the brain. o Treatment consists of radiation and surgery, only given chemo when there are tumors in other parts of the body and not the brain, brain tumors cannot be treated by chemo because the chemicals are hard to pass the blood brain barrier Microglia: • originate in the blood as an offshoot of the immune system and migrate through the nervous system, where they make up of 20 % glial cells • the brain is largely 'immune privileged” immune cells do not reach the brain because of the blood-brain-barrier • microglia monitor the health of brain tissue and play a role of its immune system o they identify and attack foreign tissue o when brain cells are damaged, microglia invade the area to provide growth factors that aid in repair • there are different types of microglia and different shapes o microglial engulf any foreign tissue and dead brain cells, an immune process called phagocytosis o when they are full, they take on a distinctive appearance  the stuffed no longer functioning microglia are dark bodies in and near regions of that brain that have been damaged  because microglia protect the nervous system and remove waste from the brain, it is thought that microglia slow downAlzheimer's disease ( this disease is caused by plaques on regions of the brain)  microglial can be harmful: ingesting inflamed regions instead of protecting it Oligodendroglia and Schwann cells • 2 kinds of glial cells insulate axons of neurons o oligodendroglia and Schwann cells • myelin prevents adjacent neurons from short circuiting • oligodendroglia : myelinate axons in the brain and spinal cord by sending out large, flat branches that enclose and separate adjacent axons • Schwann cells: myelinate axon in the peripheral nervous system o each Schwann cells wraps repeatedly around a part of axon , forming a structure somewhat like bead on a string  Schwann cells and oligodendroglia contribute to neuron's nutrition and function by absorbing chemicals that are released by neurons and releasing chemicals that are absorbed by neurons • the more an axon is myelinated the faster the flows along the neuron • MS: the myelin formed by the oligodendroglia is damaged and the function of the neurons whose axons it encases is disrupted Glial Cells and Neuron Repair: • a deep cut on the body or finger may cut the axons connecting your spinal cord to muscles and to sensory receptors • severed motor neurons axons will render you unable to move affected part of your body; whereas severed sensory fibres will result in loss of sensation from the body part o cessation of both movement and sensation is paralysis o in a period of weeks to months after motor and sensory axons are severed, movement and sensation will return • both microglia and Schwann cells play a part in repairing damage to the peripheral nervous system • when a PNS axon is cut, it dies back to the cell body o microglia remove all debris left by the dying axon o the Schwann cells that provided the axon's myelin shrink and then divide, forming numerous smaller glial cells log the path the axon form took o the neuron then sends out axon sprouts that search for a path made by Schwann cells and follow it o eventually one sprout reaches the intended target and this sprout becomes the new axon  all other sprouts retract • the Schwann cells envelope the new axon, forming new myelin and restoring normal function o Schwann cells serve as signposts to guide axons to their appropriate end points (axons can get lost) • When the CNS is damaged (spinal cord is cut), regrowth and repair do not occur, even though the distance that damaged fibres must bridge is short. o In nonmammalian vertebrates CNS repair is possible o oligodendrocytes play a role in inhibiting neuron regrowth in the CNS Neuron Repair steps 1. When peripheral axon is cut, the axon dies. Schwann cells first shrink and then divide, forming glial cells along the axon's former path 2. the neuron sends out axon sprouts, one of which finds the Schwann-c
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