organization of the Nervous System lecture 1..pptx

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1. ORGANIZATION OF THE NERVOUS SYSTEM lecture 1 By: Gideon Gachihi , MSc, MPH Department of Clinical medicine MKU

2. Functions of the Nervous System THE NERVOUS SYSTEM- is a rapid communication system of the body, (carrying electrochemical impulses). Functions of the nervous system:- SENSATION - in internal & external environment INTEGRATION - of sensory information (brain and spinal cord) CO-ORDINATION - of voluntary & involuntary actions REGULATION - of peripheral structures & systems HIGHER FUNCTIONS – like memory, drives, emotions and learning -

3. Components of the nervous system it has two components:- central nervous system (CNS): It is Composed of the brain and spinal cord (approx 95% body's neural tissue) performs integration & co-ordination of nervous activity Higher functions like learning and memory 2. peripheral nervous system (PNS) all of neural tissue outside cns links CNS with all of body tissues & outside world

4. Subdivision of the nervous sytem

5. Cellular components of the nervous system The nervous system has two cellular components:- 1. The neurons : These are the structural and functional units of the nervous system. characteristics of neurons : Excitability i.e. ability to be activated. Conductivity i.e. ability to propagate the impulse The functional domains of neurons :- receptive domin :- comprises of the dendritic system Integration domin :- is the trophic centre and consists of the cell body Transmission domin:- is made of the axon Neuroglial cells they are involved in support and protection within the nervous system

6. Types of neurons based on location and function: (a) Interneuron- located entirely within CNS, they integrates functions in CNS (b) Sensory(afferents): - (conduct impulses from sensory receptor to CNS)- (c) Motor (Efferent nerves):- (conduct impulses from CNS to effector organ) - the information in this nerves is conducted very fast. categories of motor nerves:- ( i ) somatic motor nerves - stimulates skeletal muscles (ii) autonomic motor nerves- can be general or secreto -motor (a) general nerves that affects smooth and cardiac muscle, (b) sympathetic secretomotor nerves – to small glands eg sweat glands ( c) parasympathetic secretomotor nerves- to the big glands and salivary glads ( eg sub mandibular , parotid, lingual etc terms in the nervous system Nerve - bundle of nerves Ganglion- bundle of nerve cell bodies outside of CNS Nucleus - within CNS Tract - connects regions of CNS

7. Morphological classification of neurons Neurons can be classified as: 1. unipolar neurons :- found in the cortex and spinal cord 2. bipolar neurons: form inter-neurons 3. pseudounipolar:- most sensory nerves 4. multipolar neurons :- most common in the CNS 6. Golgi type I neurons :-have long axons like one meter long- most motor nerves 7. Golgi type II neurons : are star shaped comonly found in the cortices

8. Structures of neurons sensory motor retina

9. Structure of a neurone/Parts of a neuron Cell body- contains the nucleus and other organelles Dendrites- transmit electrical impulses TO the cell body Axon- transmits impulse AWAY from the cell body axons can be several feet long “ Axon hillock” is located near the cell body nerve impulses originate there Myelin is wrapped around the axon of many neurons >>In periphery, myelin is produced by Schwann cells >>In CNS , it is produced by oligodendrocytes

10. Structures of neurons sensory motor retina

11. Classification of neuroglial cells They are classified based on their location: Those found in the CNS Oligodendrocytes- do myelination in the CNS Epidymal cells- line the ventricles and Central canal of the spinal cord Cholloidal cells: produce CSF Astrocytes:- two type (protoplasmic and fibrocystic astrocytes) support the nerve cells Stellate cells:- form scaffolding for nerves Microglial cells :- for immune responses Those in the PNS Schwan cells:- do myelination in the peripheral nerves

12. Functional classification of nerves Direction of the impulses Afferent nerves :- carry sensations to the CNS Efferent nerves :- carry sensations fron the CNS to effector organs Types of sensations Sensory nerves :- carry sensory information Motor nerves:- carry motor sensatios to the effector organs

13. Receptors and Sensory pathways A sensory stimulus can evoke a reflex response; or be transmitted to the cerebral cortex in a system comprising of at least three neurons. The primary neuron /1 st order neurone:- is either the receptor itself or synapses with the receptor. This ends in the spinal cord or brain stem, depending on origin. The second order neuron commences from the spinal cord/brain stem and usually ends in the thalamus . The third order neuron commences from the thalamus and ends in the cerebral cortex

14. Location of the cell bodies in the sensory path way 1 st order neourone /primary neurons - the dorsal root ganglia or cranial ganglia in the case of cranial nerves. 2 order neurons :- the spinal cord or brain stem . Their axons carry particular modalities of sensation and travel together and have a common origin and destination and are called tracts , they are named for origin to destination e.g. spinothalamic tract , spinaltectal , spinalreticular etc The modality of sensation therefore follows the pathway occupied by the tract e.g. the pain pathway. 3 Order neurone :- in the cerebral cortex

15. Motor Effector pathways There are two broad divisions of the effector pathways Somatic motor system Visceral (autonomic) motor system . The somatic motor pathway It comprises of two main components: The lower motor neurons:- comprises the nerves that make direct contact with the muscles. These form the final common pathway. Accordingly therefore, all spinal and motor cranial nerves are lower motor nerves. The upper motor neurons:- they influence the activity of the lower motor neurons and do not make direct contact with the target muscles. Their axons constitute the upper motor systems that occupy specific places in the nervous system, share origins and destinations. The bundles are also called tracts , and are named for origin to destination e.g. corticospinal , vestibulospinal , olivospinal , reticulospinal

16. Destruction of the upper and lower motor neurones Destruction or atrophy of lower motor neurons (in present context those of the ventral horn) results in: Flaccid paralysis of the affected muscles, Diminished or absent tendons reflexes, and Progressive atrophy of the muscles deprived of motor fibres. The “upper motor neuron lesion” after the acute effects have worn off are associated with the following signs:- Varying degrees of voluntary paralysis, which is most severe in the upper extremity, The sign of Babinski (up-turning of the great toe and spreading of the toes on stroking the sole), Spasticity with exaggerated tendon reflexes. Little or no muslce atrophy

17. histological features of neurons:- A) Nucleus : it is large and rounded with large central neucleoulus that reflect high degree of transcriprional activity b) Abundant rough endoplasmic reticulum: which synthesize the necessary proteins, and is visible as purple stained granules ( called nissl substance). NB> nissl body is present in the cell body the perikaryon and in dendrites and not the axon. c) Well developed golgi apparatus :- for production of secretary products d) Large number of mitochondria that supply the required energy e) Numerous lysosomes because of a high turn over of the cell membrane and other cell components f) The cytoskeleton :- it is highly organized to maintain the unique shape of these cells and particularly their long axons which may be up to one metre. It has the neurofilaments and intermediate filaments through out to act as internal scaffold to maintain the shape of the axon and the cell body. There are also organized pattern of microtubules for transportation of substances and organelles up and down the axons

18. Types of neurons: (a) Interneuron- located entirely within CNS, integrates functions in CNS (b) Sensory (from sensory receptor to CNS) (c) Motor (from CNS to effector organ):- are two types 1. somatic - stimulates skeletal muscles 2. autonomic - affects smooth and cardiac muscle, also glandular secretion Terms used Nerve- bundle of nerve fibers carrying a common impulses Ganglion - bundle of nerve cell bodies outside in the PNS of CNS Nuclei:- bundle of nerve cell bodies in the CNS Nucleus - within CNS Commissure:- buddle of nerve fibres connecting the opposite sides of the brain Tract - connects regions of CNS

19. Parts of a neuron Cell body- contains the nucleus and other organelles Dendrites - transmit electrical impulses TO the cell body Axon- transmits impulse AWAY from the cell body axons can be several feet long “ Axon hillock” is located near the cell body nerve impulses originate there Terminal boutons: are the terminal expanded ends of a neurone that make the synpase

20. Myelination of nerves Myelin is wrapped around the axon of many neurons In periphery, myelin is produced by Schwann cells In CNS, it is produced by oligodendrocytes

21. Myelination in CNS and importance of myelination Large axons are myelinated by Schwann cells or oligodendrocytes Gaps are left between the “wrappings” of each cell ( nodes of Ranvier ) Myelinated axons conduct nervous impulses more rapidly than unmyelinated In CNS, myelinated axons form “ white matter ” (Cell bodies and dendrites are gray matter )

22. Repair of nerves in CNS and in PNS Schwann cells can help repair damaged nerves in PNS Capacity for repair is much better in the PNS than in CNS In fetal brain, neurotropins promote neuron growth Mulerian degeneration

23. The Blood Brain Barrier(BBB) Astrocytes are the Most common glial cell in CNS and They Form blood-brain barrier This barrier help with ion uptake and with neurotransmitter uptake Astrocytes have many glucose transport carriers, which help move glucose from blood to brain -Blood-brain barrier (BBB) Why the Blood brain barrier Probably due to effects of astrocytes on brain capillaries Everything must move into brain by diffusion and active transport Many substances (including therapeutic drugs) cannot cross BBB hence the brain tissues remain highly protected

24. The autonomic nervous system It consists of two broad complementary parts i.e. sympathetic and parasympathetic . A third component, the enteric nervous system , for GIT control has recently been recognized as an independent entity. In parasympathetic system both preganglionic and postganglionic fibres travel with cranial nerves (III; VII; IX; X) except for a small sacral component. It is said to be craniosacral . In the sympathetic system, the preganglionic fibres arise from the thoracic and lumber segments and travel with the ventral roots.This system is thus called t horacolumbar . Postganglionic fibres may then reach the target in one of the four ways: Directly Join nearby spinal nerve Join nearby cranial nerve Form a plexus around nearest artery

25. Receptors and Sensory pathways A sensory stimulus can evoke a reflex response ; or be transmitted to the cerebral cortex in a system comprising of at least three neurons. The primary neuron is either the receptor itself or synapses with the receptor. This ends in the spinal cord or brain stem, depending on origin. The second order neuron commences from the spinal cord/brain stem and usually ends in the thalamus . The third order neuron commences from the thalamus and ends in the cerebral cortex

26. Summary on the electrical activity of neurons to always remember The nervous system is comprised of the central nervous system (brain, spinal cord) and the periphery (cranial and spinal nerves) Periphery is divided into autonomic and motor neurons. 2. Cells of the nervous system are glial cells and neurons. Neurons conduct nervous impulses, glial cells “support” neurons. 3. Myelination affects the speed at which impulse is delivered.

27. 4. Neurons conduct electrical and chemical signaling . Action potential starts at a very small area of the membrane and is conducted along the length of the membrane. Action potential rises with Na influx and falls with K efflux. 5. Speed of transmission is affected by (a.) presence of myelin, and (b.) the diameter of the neuron. (faster in larger neurons) 6. Neurotransmitters deliver signals across synapses.

28. 7. Sometimes signal is excitatory, sometimes inhibitory. Excitatory: receptors serve as ion channels, depolarizes, brings closer to threshold. Inhibitory: causes hyperpolarization A given synapse is always one or the other. Some act as second messengers (more long-term effects). 8. Neurotransmitters are typically small fast- acting molecules . Neuropeptides are larger and slower-acting “neuromodulators” (many hormones are so classified) learning, motivation, response to stress, etc .

29. synapses Synapse- is a connection between a neuron and a second cell i.e From presynaptic to postsynaptic neuron and involves Release of neurotransmitters (chemicals) There are two types of synapses 1. chemical synpses- the most abundant in the body 2. electrical synapses- found only in specialized areas eg In smooth muscle and heart gap junctions Components of a synapse: Presynaptic membrane Synaptic clefts Post synaptic membrane Neurotransmitter substance receptors

30. Summary The nervous system is comprised of the central nervous system (brain, spinal cord) and the periphery (cranial and spinal nerves) Periphery is divided into autonomic and motor neurons. 2. Cells of the nervous system are glial cells and neurons. Neurons conduct nervous impulses, glial cells “support” neurons. 3. Myelination affects the speed at which impulse is delivered.

31. 4. Neurons conduct electrical and chemical signaling. Action potential starts at a very small area of the membrane and is conducted along the length of the membrane. Action potential rises with Na influx and falls with K efflux. 5. Speed of transmission is affected by a.) presence of myelin, and b.) the diameter of the neuron. (faster in larger neurons) 6. Neurotransmitters deliver signals across synapses.