Brain anatomy

An educational presentation on basics of neuroanatomy.
it define the scientific terminologies and various cells of nervous tissue. structure and function of all nervous tissue is explained. action potential generation is graphically represented.

1. INTRODUCTION TO NEUROANATOMY DR. AYSHAH HASHIMI

2. Nervous System Large Complex organ Serve as center for nervous system Located in head Two parts CNS Brain Spinal cord PNS Sensory Motor

3. Brain Main part of nervous system It integrates the received information and coordinates and influences the activity of all parts of the bodies Brain 3 parts Forebrain/ Prosencephalon Midbrain/ Mesencephalon Hindbrain/ Rhombencephalon Or Cerebrum Cerebellum Brainstem (Midbrain + Pons + Medulla) Cerebrum Cerebellum Brainstem Forebrain Hindbrain Midbrain

4. Forebrain/ Prosencephalon 2 parts Telencephalon (cerebral cortex) consist of 4 lobes Frontal Parietal Occipital Temporal Diencephalon consists of Thalamus Hypothalamus Pineal gland

5. Midbrain consist of Tectum Cerebral peduncle Substantia nigra Hindbrain/ Rhombencephalon Metencephalon (Pons and Cerebellum) Myelencephalon (medulla oblongata)

6. Nervous system Neurons concern with communication signal Glial cells/ Supporting cells What are Gyri and Sulci? Gyrus are ridges on the cerebral cortex surrounded by sulci. They create folded appearance of a brain Now anatomically CNS has 2 parts Grey matter White matter Grey matter is collection of cell bodies of neurons within the CNS White matter is collection of axons within the CNS

7. Division of grey matter Cortex (grey matter at periphery) Nucleus (pieces of grey matter that are embedded or surrounded by white matter ) Division of White matter Tracts : they take information within the CNS upward or downward Ascending tracts Descending tracts Commissural fibres: they connect or transmit information from right to left within the CNS (corpus callosum) Association fibres: they connect anterior to posterior

8. Reticular formation In brain grey matter is at periphery mainly while in spinal cord they are in centre so in brain stem grey matter lies in centre. Motor sense/tract from cortex and sensory sense/tract from spinal cord criss-cross at brainstem fragmenting its grey matter into small pieces. This structure is called as reticular formation. Many nucleus arises from brainstem.

9. Peripheral Nervous System It is one of the two components that makeup the nervous system. Consists of nerves and ganglia outside the brain and spinal cord It serves as a relay between the CNS and the body Sensory PNS is classified into Special senses ( vision, olfaction, taste, hearing, balance) General senses are of two types A. Somatic sensation (skin, locomotor system(pain, temp., vibration proprioception-sense of position)) B. Visceral sensation (pain or discomfort) Other type of senses Conscious sense Unconscious sense (BP, peristalsis)

10. Motor response Somatic motor responses (controlled by us) Visceral/Autonomic motor response (uncontrollable) 2 types Sympathetic Nervous system Stimulate on fight and flight response Two kind of neurons preganglionic and postganglionic neurons Originate from thoracolumbar division of spinal cord (T1-L2/L3) Parasympathetic Nervous system Stimulate on rest and digest or feed and breed Originate from central nervous system as cranial nerves and sacral division (S2-S4) of spinal cord (Craniosacral outflow)

11. Structure of Neuron Neuron is a fundamental unit of nervous system that generate electrical signals called action potential which allows them to quickly transmit information over long distances It consist of 3 parts – Cell body, Dendrites and Axon Dendrites Multiple extensions that extrude from cell body of neuron that communicate with other neurons to collect information or receive information Increase the surface area of the neuron and act as sensory receptor. They tend to taper The dendrites have ligand gated ion channels and G protein coupled receptor for the production of EPSP (excitatory postsynaptic potential) and IPSP on its surface

12. Cell Body/Soma/ Cyton A compact structure that contains nucleus and in cytoplasm have Nissl’s granules (present in both soma and dendrites but not in axon) Its function is to synthesize and release proteins that are important for neuronal growth and regeneration of axons and could be neurotransmitter, enzymes etc. Axon Hillock is junction between soma and axon Axon Large single extension that conducts the processed information from neuron to the axon terminal in the form of action potential (depolarization and repolarization wave) Kinesin is a special protein present inside the axon that transfer these enzymes, neurotransmitters and certain protein from the cell body towards the terminal axon

13. Dynein are proteins that transport in opposite direction i.e. from synaptic end to cell body and this transport is called as Retrograde Axonal transport (nerve growth factors, mitochondria) Apart from this axonal transport, various infections like polio, rabies, varicella zoster, herpes simplex etc. are using this machinery for their movement to cell body from axon terminal and back to it Voltage gated ion channels (Sodium and potassium) They have special insulating substance called myelin Axon terminal Have voltage gated calcium channel Synoptic knobs contains neurotransmitter that are released to carry out the concerned function in response to cacium Reuptake of neurotransmitters Myelin sheath an insulating layer around the axon and is not continuous to increase the saltatory conduction of neuron. Schwann cells in PNS and Oligodendrites in CNS wrap over the axon to form myelin sheath

14. Structural classification of neurons Unipolar are found in skin Bipolar in retina and olfactory cells Pseudo-unipolar in dorsal root ganglia of spinal cord, 5 th cranial nerve nuclei Multipolar in most part of the body

15. Functional Types Sensory neuron carries the afferent information to the brain they are mainly pseudounipolar General visceral afferent neurons (from viscera) General Somatic afferent neurons (from locomotor system) Special senses afferent neurons (from eyes and ears etc.) Motor neurons carries motor impulses from brain or spinal cord to tissue level. They are of two types upper & lower motor neurons General visceral efferent neurons (towards viscera) General Somatic efferent neurons (towards locomotor system) Interneurons connect sensory neurons to mo tor neurons

16. Supporting cells of brain There are six glial cell in nervous system Four in CNS and two in PNS CNS PNS Astrocyte Satellite cell Oligodendrocyte Schwann cell Microglia Ependymal cell

17. Astrocytes (Star cell) Astrocyte are star shaped glial cells in brain and spinal cord. It interacts with up to 2 million neurons at a time. They are the most abundant cells in the brain Have central body and some processes with foot plates in the end 2 types Fibrous astrocyte Long processes mostly unbranched and have vascular feet. Found in white matter of brain & spinal cord Protoplasmic astrocyte Short processes but are branched and have abundant of organelles and cytoplasm Seen in grey matter of brain and spinal cord

18. Functions Scaffold (covering neurons forming a meshwork around it) The number of astrocytes are more as compared to neurons Ratio of astrocytes to neurons is 5:1 The astrocytes are present in CNS in the form of network They interlink with the help of footplates Reservoir of Glycogen In case of decrease ATP production in neurons, the astrocytes feeds neuron (contain glycogen and are capable of gluconeogenesis and glycogenolysis) and provide nutrients Recycle neurotransmitters Excessive neurotransmitters are reuptake by axonal endings but at one point it is saturated Astrocytes now takes the charge and takes the excessive neurotransmitters and further supply it back to the neurons

19. Regulate neuronal communication by developing & stabilizing synapses (thrombosaponins I & II, Hevin , Glypicans 4 & 6 – help in recruiting ampa receptors that makes synapses more active) Promote myelinating activity of oligodendrocytes Forms BBB The foot processes of astrocytes binds the junction of endothelium and prevent passage of various substances forming blood brain barrier Mostly lipid soluble substances are permeable to this barrier Protein are not allowed to enter as they may stimulate any neuron There are places in the brain where blood brain barrier is absent is Area Postrema in medulla is devoid of BBB because in presence of toxins it stimulate chemotactic centre in midbrain and causes vomiting Osmoreceptors around the hypothalamus checks hydrogen ion level, electrolyte imbalance and accordingly stimulate posterior pituitary Hypothalamic-Pituitary axis

20. Glial scar (repairs the damage) In case of injury, the astrocytes reaches the affected site and prevents blood loss Homeostasis (balance ion concentration in the brain) Rich in hydrogen ions Astrocytes can efflux and influx hydrogen ion Potassium buffer Membrane of the neuron has potassium channel that leaks out the potassium Sodium-potassium ATPase to counter balance the loss where in there is influx of two potassium ion and efflux of 3 sodium ion Increase of potassium outside the cell, brings the potassium inside, exiting the neuronal cell In order to prevent this, astrocytes takes up the extra potassium and stores it

21. Microglia Microglia are the macrophages of the brain that broadly function to destroy pathogens and scavenge dead or dying cells They form the main form of active immune defence in CNS. They travel within the CNS & perform different job according to the need. Types: 1. Resting/Ramified microglia stays at one place and has a small cell body and a thin projections that r monitoring or sensing the local environment. (express to IL10 & TGFß ) 2. Reactive/activated microglia are highly motile (express to IL-1ß) 3. Phagocytic microglia Microglia dysfunction (area of research in Alzheimer’s disease)

22. Synaptic Pruning- Clean up xtra synapses in early childhood Receptors usually have sialic acid to protect them from phagocytosis but an enzyme released by microglia c/a neuraminidase cleaves it and through series of reaction leads to phagocytosis. It also chews neurons that have no CD200 or CD47 molecules. Scavenging property/ housekeepers Very sensitive to small change in the environment. Sense any foreign material, damaged cells, apoptotic cells, plaques, cleans dead cells or debris or engulf any foreign material Phagocytosis and promote inflammation During the inflammation or injury to brain there is rise in microglia that pull their branches and increase their cell body and multiply to increase its no. to fight against infection by secreting IFN-ɤ.

23. Cytotoxicity Apart from phagocytosis it releases variety of cytotoxic substances eg. ROS H 2 O 2, NO, that directly damages cell. Protease cytokine like IL1 causes demyelination of neuronal axons. Also IL1 inhibits IL10 and TGFß Antigen presenting cell and releases protein called cytokines (IL8) that help T cell to enter brain and fight infection. Promotion of repair Finally, they signal astrocyte to repair and heal the tissue, astrocyte releases IL10 and TGFß which is sensed by resting microglia that stops further inflammation

24. Ependymal cells Neuroepithelial lining of the ventricular system of the brain and the central canal of the spinal cord. Contains cilia and microvilli on its surfaces. Cilia helps in moving CSF while microvilli helps in reabsorption of CSF. Ependymal cells surrounded by blood vessels are called as Choroid Plexus that produces and regulates CSF. Neuro-regeneration: a recent study observed that ependymal lining might be a source of cells which can be transplanted into the cochlea to reverse hearing loss. SATELLITE GLIAL CELL These are the astrocytes of PNS

25. OLIGODENDROCYTES Signals are needed to send at farthest tissue within a given limit of time and myelin sheath helps in the same. From where does this myelin comes from? Again Glial cell – Oligodendrocyte are the last cells to form in our brain (25-30yrs). They produce certain proteins (BDNF, IGF1) that helps neuron grow and forms synapses. These cells have a special lipid membrane that helps in forming myelin sheath around neurons. This myelin sheath provide support and insulation to axons 1 oligodendrocyte extends its process to approx. 50 neurons SCHWANN CELLS Glia of PNS Wrap around axons in PNS i.e. does myelination

26. Na/K ATPase Leaky K ion Channel Leaky Na ion Channel Resting potential Na/K ATPase It efflux 3 Na ions and influx 2 K ions Net change is loss of 1 cation Responsible for minor change in electronegativity (approx. -5mV) Leaky Na ion channel Concentration of sodium ion is more outside the cell so there is less efflux of Na+ ion, thus less change in electronegativity Leaky K ion channel Potassium ion are usually present with anion Concentration gradient is also favourable Efflux of K+ ion results in generating electronegativity 3 Na+ 2 K+ K+ K+ K+ K+ Anions Anions Anions Anions Na + Na + Na + Na +

27. G- Protein coupled Receptor Ligand gated K/ Cl ion channel Ligand gated Na/Ca ion channel Graded Potential Ligand gated Na/Ca ion channel (excitatory postsynaptic potential) stimulate under the influence of glutamate Ligand gated K/Cl ion channel (Inhibitory postsynaptic potential) stimulate under the influence of GABA G Protein coupled receptor stimulate under the influence of protein, lipids etc. Na + Na + Na + -70mV ↓ -55mV

28. Voltage gated Ca ion channel Voltage gated K ion channel Voltage gated Na ion channel Action Potential Voltage gated Na ion channel (responsible for depolarization) Voltage gated K ion channel (responsible for repolarization) Voltage gated Ca ion channel (release of neurotransmitter) +30 ↑ -10mV ↑ -55mV ↑ -70mV +30 ↓ -10mV ↓ -55mV ↓ -70mV