Nervous System Notes PDF

NERVOUS SYSTEM Dr Odunayo Azeez  Introduction The nervous system provides integration and control of all other systems. Nervous systems are fast-acting communication systems containing nerve cells (neurons) which carry information in the form of nerve impulses.  Introduc...

NERVOUS SYSTEM Dr Odunayo Azeez  Introduction The nervous system provides integration and control of all other systems. Nervous systems are fast-acting communication systems containing nerve cells (neurons) which carry information in the form of nerve impulses.  Introduction Sensation is the conscious or subconscious awareness of changes in the external or internal environment. Perception is the conscious interpretation of sensations – a function of the cerebral cortex. Helps us rationalize or make sense of information related to physical stimuli and to give meaning to it. Senses are modalities through which internal and external environments are perceived. To sense is to be able to perceive stimuli. Division of the nervous system Central Nervous System  The brain and the spinal cord Peripheral Nervous System  Groupsof neurons called ganglia and peripheral nerves  Provide pathways to and from the central nervous system for electrochemical signals (impulses).  Peripheral nervous system Composed of 2 divisions: Somatic  Provides sensory information (voluntary)  Transmitsimpulses to and from skeletal muscles – usually conscious actions. Autonomic  Motor system for involuntary viscera (smooth muscles, cardiac muscles and glands)  Autonomic nervous system is further divided into 3 subdivisions.  3 subdivisions of ANS: Sympathetic  participates in body’s response to stress; fight or flight Parasympathetic  returns body to resting state & conserves resources Enteric  controls smooth muscles of the gut  Neuron The basic functional unit of the nervous system. Composed of A typical motor - dendrites, neuron 13 - cell body and - axon. 1. Nucleus, 2. Perikaryon 3. Cell body, 4. Axon, 5. Dendrites, 6. Nissl bodies, 7. Axon hillock, 8. Myelin sheath, 9. Oligodendrocyte 10. Schwann Cell, 11. Skeletal muscle, 12. Neuromuscular junction and 13. Node of Ranvier.  Neuron Functionally classified into: Sensory (afferent neuron): Carries impulse from the periphery and receptors to the CNS for integration. Motor (efferent neuron): Carries response from the CNS to the effector organs. Interneuron: Located in the CNS for interpretation and responses.  Neuron Structurally, they are classified based on the processes into: Unipolar, Bipolar Multipolar  THE CENTRAL NERVOUS SYSTEM It is made up of - the brain, - spinal cord, - nerves (a group of neurons whose axons are travelling together) and - receptors (transducers that convert one form of energy to electrical signals, help in perception).  Glia Cells These are the other cells or accessory cells of the nervous system. Also called neuroglia. Neuroglia occupy half the volume of the brain. They do not initiate or conduct impulses but serve as sources of physical support for the neurons. There are four major neuroglia in the CNS and two in the PNS. Glia Cells CNS Astrocytes Oligodendrocytes Microglia Ependymal cells PNS Schwann cell Satellite cells  Astrocytes These are star-shaped cells which: Serve as glue that hold neurons together in proper spatial relation. Serve as scaffold to guide neurons to their destination during foetal development. Are responsible for the formation of blood-brain barrier. Aid repair of damaged tissues.  Oligodendrocytes These form insulative myelin sheath around the axons in the CNS. It usually have several elongated projections, each of which is wrapped around sections of the axon. This function is performed by Schwann cells in the PNS.  Microglia These are scavengers of the CNS. They are phagocytic macrophages delivered by the blood. They remain stationary or inactive until activated by inflammation or injuries.  Ependymal Cells These cells line the internal cavities of the CNS, i.e the ventricles of the brain and neural canal of the spinal cord. They contribute to the formation of cerebrospinal fluid. Glia cells have ability to divide, so most brain tumours are of glia origin, except when metastasis occurs from other parts of the body.  Astrocytes Oligodendrocyte Microglia Satellite cells Schwann cells  Action potential This is the brief reversal of membrane potential brought about by rapid changes in membrane permeability that spreads throughout the membrane in a non-decremental fashion. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and then ends with an almost equally rapid change back to the negative potential. To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber’s end. Action potential  Propagation of action potential Once an AP is initiated at the axon hillock, the impulse is automatically conducted throughout the neuron by one of the two methods: a) Conduction by local current flow b) Saltatory conduction. Propagation of action potential SYNAPSES A synapse is the junction between two neurons. Basically, a neuron-neuron synapse involves a junction between the axon terminal of the presynaptic neuron and the dendrites or cell body of the post synaptic neuron. Less frequently, axon-axon or dendrite-dendrite connections do occur. SYNAPSES  Types of Neurotransmitters Neurotransmitter is a chemical substance that acts as a mediator for the transmission of nerve impulse from one neuron to another neuron through a synapse. Classical NT are small rapid acting molecules. They include acetylcholine, catecholamines – epinephrine, norepinephrine and dopamine; serotonin, histamine, glycine, gamma aminobutyric acid (GABA), substance P and nitric oxide. Neuromodulators: These are slow acting chemical messengers that bind with neuronal receptors in non- synaptic sites. Types of Neurotransmitters  SOMATIC SENSATION Peripheral NS consists of nerve fibres that convey information between the CNS and other parts of the body. Afferent information about the internal environment does not reach the level of conscious awareness e.g. level of Ca2+ in the body fluid. Afferent information that reaches the level of conscious awareness are called sensory input. Include: - Somatic sensations – somaesthetic and proprioceptive - Special senses – vision, smell, taste and hearing.  SOMATIC SENSATION At the peripheral end of the afferent neurons are receptors which, by signal transduction converts different stimuli to AP that the CNS can interpret. Thus, receptors are signal transducers.  The Spinal Cord The spinal cord is made up of neuronal cell bodies called gray matter which forms an H or butterfly shape region. They gray matter is surrounded by outer white matter which is made up of bundles of nerve fibres grouped into columns called tracts. The Spinal Cord  The Brain Although the brain is a functional whole, it is organized into regions based on anatomical distinction, functional specialization and evolutionary development. It is divided caudo-cranially (rostrally) into: Brain stem: - Medulla and - Pons Cerebellum Forebrain - Diencephalon: Thalamus, Hypothalamus. - Cerebrum: Basal ganglia and cerebral cortex The Brain Autonomic Nervous System Differences between somatic and autonomic nervous system  Autonomic nervous system Each autonomic nerve pathway from the CNS to the effector organ consists of two neuron channels. Cell body of the first neuron (preganglionic neuron) is in the CNS. The axon synapses with the cell body of the second neuron in the ganglion. The axon of the second neuron (post ganglionic neuron innervates the effector organ.  Autonomic nervous system  Sympathetic Nervous system When stimulated, SNS prepares the body for actions Elevates blood pressure by generalized vasoconstriction. Bronchodilation. Vasodilation of blood vessels supplying skeletal muscle. Heart rate and force of contraction is increased. Increased sweating. Digestive and urinary activities are inhibited.  Parasympathetic nervous system It generally slows down activities that are enhanced by the SNS. Facilitates digestion and absorption - Increase in motility and glandular secretion. Involved in emptying of the bladder. Increases pupillary constriction and facilitates accommodation for near vision. Lowers the heart rate and force of contraction especially the atria.  Parasympathetic nervous system There are some exceptions to dual innervation of SNS and PNS - Innervated blood vessels (arterioles and veins) receive only sympathetic innervation. - Only blood vessels in the penis and clitoris have parasympathetic innervation. - Sweat glands only have sympathetic innervation. - Salivary glands have both innervation, but their activities are not antagonistic. The SNS produces thick copious saliva, rich in mucin while PNS produces watery saliva rich in enzyme and ions.