HUBS1416 Introduction to the Nervous System PDF

Introduction to the Nervous System Advanced Human Bioscience HUBS1416 School of Biomedical Science and Pharmacy College of Health, Medicine & Wellbeing  Lecture Overview Part 1: Overview of the nervous system & the cells involved (neurons & glia) Part 2: Neuronal Communicati...

Introduction to the Nervous System Advanced Human Bioscience HUBS1416 School of Biomedical Science and Pharmacy College of Health, Medicine & Wellbeing  Lecture Overview Part 1: Overview of the nervous system & the cells involved (neurons & glia) Part 2: Neuronal Communication: Electrical & Chemical Signals Part 3: Divisions of the nervous system: CNS & PNS, autonomic & somatic  The Nervous System System of communication that is extremely rapid and highly complex Senses our environment (both external and internal) and responds to it Rapid (signals take only milliseconds) due to neurons generating electrical signals Complex due to the huge numbers of neurons involved and their interconnections The Nervous System Structurally divided into CNS (central nervous system) PNS (peripheral nervous system) Brain + Spinal cord Cranial nerves + Spinal nerves  Neuron structure is key to nervous system function Dendrites – Cell Body (Soma) – Axon – Axon terminals -  Integration in the nervous system Integration: “bringing together of different parts to make a whole” l Each neuron integrates many inputs (some inhibitory, some excitatory) l Response is either generation of or inhibition of an action potential l Allows the nervous system to process a huge amount of information very quickly, and produce appropriate responses A nerve and a neuron are NOT the same thing Neuron (or nerve cell) – a single Nerve – a bundle of many axons cell of the nervous system, with of neurons wrapped in an axon (or nerve fibre) connective tissue A section through a sciatic nerve The nervous system contains another type of specialised cell – Glial cells Glial cells provide a protective and supportive role to neurons Six types of Glia (neuroglia) l Four in CNS l Two in PNS  CNS Neuroglia l Astrocytes Largest and most numerous neuroglia Physically support neurons & repair damaged tissue Astrocyte processes wrap around capillaries and form the Blood Brain Barrier Help regulate ionic composition of ECF Remove excess neurotransmitters l Oligodendrocytes Contact exposed surfaces of neurons Form a ‘myelin sheath’ along axons in CNS Myelinated axons permit faster transmission of information 10  CNS Neuroglia Microglia Help protect CNS via phagocytosis of waste, debris and pathogens Macrophages of CNS: Increase in number when brain tissue is damaged or infected Ependymal cells Line the ventricles of the brain and central canal of the spinal cord Secrete cerebrospinal fluid (CSF) Monitor composition of CSF and help circulate it (cilia) 11  PNS Neuroglia Schwann Cells Multiple Schwann Cells form myelin sheath of PNS axons Also encase non- myelinated axons Satellite Cells Surround cell bodies of axons in the PNS Provide structural support Regulate passage of material between neuron soma and interstitial fluid Neurons are designed for communication Neuronal electrical signaling involves 3 events: 1. Generation of an action potential - creation of the electrical signal (depolarization) 2. Conduction of an action potential - getting the signal from one end of a neuron to the other 3. Synaptic transmission - passing the signal between one neuron and another or between a neuron and another cell 1. Generating the action potential l All cells have a membrane potential difference l Only neurons and muscle cells can rapidly change their membrane potential to create an electrical signal What is a membrane potential? The inside surface of the membrane has a negative charge compared to the outside surface of the membrane + + + + + + + + + + ++ + + + + + + + + + + + +  What causes the membrane potential? The cell membrane is selectively permeable and the intracellular and extracellular fluids have a different composition, which creates concentration gradients and differences in charge as a result K+ K+ There is a much higher concentration Na + of potassium ions Na + inside cells than outside There is a much higher concentration of sodium ions outside cells compared to inside Ions cross membranes by moving through channels Potassium Sodium ions ions Potassium Sodium channel channel When neurons are at rest, the sodium channels are mostly closed and potassium channels are mostly open, so potassium is able to move down its concentration gradient, and sodium is not When the cell is at rest, potassium moves out down its concentration gradient K+ K+ K+ Na+ K+ K+ Na K+ K+ + Na+ + K+ Na+ Na Na+ Na+ Since each potassium ion carries a positive charge, this means that the inside of the membrane ends up negative relative to the outside of the membrane The concentration gradients are maintained by active transport of sodium ions out of the cell and potassium ions in (i.e. against their concentration gradients) This requires a pump known as the Na+/K+ ATPase, and it is required for the long- term maintenance of membrane potentials What happens if sodium channels open? The membrane becomes more permeable to Na+ Sodium ions would move into the cell (down their concentration gradient) Na+ K+ K+ K+ Their positive charge K+ Na+ K+ means that the inside of the cell becomes more K+ K+ Na+ positive than the outside Na+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na + This process is known as depolarisation After depolarising, the membrane potential must return to normal rapidly, to be ready to pass on another signal … This process is known as repolarisation Repolarisation Na+ K+ K+K+ At the end of the signal, K+ K+ K+ sodium channels close, K+ Na + so sodium ions stop moving in Na+ K+ K+ K+ At the same time, Na+ K+ Na+ potassium channels Na + open again, and K+ K+ Na+ potassium ions move Na + out Na+Na+ Na+Na+ Repolarisation requires the movement of the potassium ions with their positive charges from the inside to the outside of the membrane to restore the resting membrane potential (inside negative)  2. Conduction of the action potential The role of the neuronal axon l Starts with depolarisation of the first section of the axon by opening sodium channels l A “wave” of depolarisation followed by repolarisation then moves rapidly along the axon as more and more sodium channels open. This process is known as nerve impulse conduction Role of myelin in conduction of action potentials Myelinated axons conduct their action potentials much faster than unmyelinated ones Why is this? Impulse conduction in un-myelinated axons the action potential travels along rapidly, but sodium channels have to be opened all the way along the axon to keep the action potential going. This channel opening takes time... Impulse conduction in myelinated axons Myelin prevents loss of ions out of the axon, meaning for the action potential to proceed along the axon the only sodium channels that have to open are located in the gaps between the myelin, this means the signal “jumps” along from one gap to the next. This is much quicker 3. Synaptic Transmission Involves chemical communication l To get the electrical message from one neuron to another, the first neuron (pre-synaptic) releases chemicals, neurotransmitters, that interact with the next neuron (post-synaptic) l The axon terminal of the first neuron comes very close to the dendrite of the next neuron, but it does not touch it, there is a small gap in between – the synapse l The neurotransmitters are released and move across the synapse to the next cell Synaptic Transmission - Communication between two neurons The three basic steps of synaptic transmission: 1. Neurotransmitter release: 2. Receptor activation: 3. Termination of the message: The arrival of the action The neurotransmitter The neurotransmitter is potential at the axon terminal diffuses across the inactivated by being transported triggers the release of synapse and binds to back into the axon terminal and neurotransmitter into the receptors on the by enzymatic breakdown synapse membrane of the next cell The nervous system can be divided into: Central and Peripheral Brain + Spinal cord Cranial nerves + Spinal nerves  Brain Externally, grey matter (cell bodies) visible, with internal white matter (axons) interconnecting different areas of the brain (more to come in CNS lecture) Spinal cord Externally, white matter (axons) visible, with grey matter (cell bodies) in the centre of the cord (more to come in CNS lecture)  Spinal and Cranial nerves 12 pairs of cranial nerves Each pair emerges from a different position in the brain and supplies a different structure or group of structures 31 pairs of spinal nerves each pair emerges from a different level of the spinal cord and supplies a different segment of the body  The nervous system can be divided into: Somatic and Autonomic The somatic division of the nervous system provides sensations from the The autonomic division of the nervous muscles, joints, tendons and skin & system provides sensation from the commands to the skeletal muscles smooth & cardiac muscles and glands & commands to smooth & cardiac muscles & glands Somatic and Autonomic can also be divided into Sensory and Motor  Autonomic sensory functions Stretch receptors in hollow structures like the bladder, gut, uterus Baroreceptors Chemoreceptors Somatic sensory functions  Special Senses Vision, hearing, taste, smell & equilibrium  Somatic motor functions Movement of all skeletal muscles  Autonomic motor functions Movement of smooth muscles and gland activity What is the relationship between the sensory (afferent) and motor (efferent) neurons in the nervous system?