PSA523 Nervous System Mechanics PDF

15/02/2023 PSA523 2: Mechanics of the nervous system Dr Clare Holley [email protected] 1 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron Action potential Reading 2 Last week 1: The brain through the ages What can you remember? 3 1...

15/02/2023 PSA523 2: Mechanics of the nervous system Dr Clare Holley [email protected] 1 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron Action potential Reading 2 Last week 1: The brain through the ages What can you remember? 3 1 15/02/2023 Lecture objectives  Describe the structure of the nervous system  Understand why ions are important and recognise their distribution inside and outside of the neuron  Identify the characteristics of an action potential  Explain how action potential signal moves along an axon 4 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron Action potential Reading 5 The nervous system (NS) Network of neurons in the brain, spinal cord and periphery  Central Nervous System (CNS) • Brain and Spinal Cord  Peripheral Nervous System (PNS) • Nerves (Cranial and Spinal) • Ganglia (a mass of nerve cell bodies) 6 2 15/02/2023 CNS: Brain 7 CNS: Cerebrum 8 CNS: Spinal cord  Continuous with brain stem  Long conical structure  Thickness of adult’s little finger  Mediates information transmission between brain & body below the neck 3 major functions:  Coordinating certain reflexes  Conduit for sensory and motor information 9 3 15/02/2023 CNS: Spinal cord  Protected by vertebrae (24 vertebra)  Core of grey matter surrounded by white matter Dorsal Ventral 10 CNS: Spinal cord  Spinal nerves split into dorsal & ventral roots before entering spinal cord  Afferent neuron axons enter cord in dorsal root & terminate in dorsal horn  Efferent neurons have a cell body in ventral horn & axons leave cord in ventral root Reproduced with permission from Pearson Education From Physiology & Behavior (9th Ed.) by N Carlson 11 PNS: Function Afferent neurons Receptors Efferent neurons CNS Effectors  Connects CNS to limbs & organs via cranial and spinal nerves  Conveys info from environment to CNS (afferent neurons)  Conveys messages from CNS to muscles and glands (efferent neurons) 12 4 15/02/2023 PNS: Nerves  Neuron axons grouped into bundles  Only present in the PNS  43 pairs • 12 cranial nerve pairs • 31 spinal nerve pairs 13 PNS: Cranial nerves  12 pairs • 10 → brainstem • I & II → forebrain  Information between the brain and body above the neck • Exception: Vagus nerve 14 PNS: Spinal nerves  31 pairs  Each pair is associated with a particular segment of spinal cord  Named dependent on vertebral level they attach  Spinal nerves can contain sensory & motor fibres Reproduced with permission from Pearson Education From Physiology & Behavior (9th Ed.) by N Carlson 15 5 15/02/2023 Divisions of the PNS Peripheral Nervous System Somatic Nervous System Sympathetic Nervous System Autonomic Nervous System Parasympathetic Nervous System Enteric Nervous System 16 Somatic NS  Voluntary control of body movement  Receives sensory information and controls spinal nerves that innervate skin, joints & muscles • Afferent neurons carry sensory info from skin (sensory neuron) • Efferent neurons control skeletal muscles via (motor neuron)  Neurons are excitatory 17 Autonomic NS  Controls involuntary functions and internal environment  Afferent neurons carry sensory info from internal organs to CNS  Efferent neurons control smooth muscle, cardiac muscle & glands  Neurons are excitatory or inhibitory  Has three further sub-divisions: 1. Sympathetic Nervous System, 2. Parasympathetic Nervous System and 3. Enteric Nervous System 18 6 15/02/2023 ANS: Sympathetic NS (SNS)  Any responses for activities which expend energy  Coordinates Fight or Flight response 19 ANS: Parasympathetic NS (PSNS)  Activities involved with increase in the body’s supply of stored energy  Coordinates Rest and Relax response 20 ANS: Enteric NS (ENS)  The “second brain”  Lines your gastrointestinal tract from oesophagus to rectum  Main role is controlling digestion • swallowing • release of enzymes • control of blood to facilitate nutrient absorption 21 7 15/02/2023 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron • Structure and general function • Movement of ions • Resting potential Action potential Reading 22 Neurons  Transmit information to other neurons, muscle of gland cells  80% of neurons are in the brain 23 Neuron structure Dendrites Axon Terminal/ Terminal Boutons Axon Cell body/soma Structure and general function • Recommended reading pp. 31-34 24 8 15/02/2023 1: Sensory neurons  Part of PNS  Contain sensory receptors for detecting sensory changes  Sends information about these changes to CNS  Cell body in PNS, axon enters CNS (axon terminals located in CNS) 25 2: Motor neurons  Part of PNS  Synapses to skeletal muscle to command movement or onto glands to release hormones  Relays signal from CNS to PNS  Dendrites & cell body in CNS, axon enters PNS 26 3: Interneurons  In CNS  Receives info from sensory neurons  Sends info to motor neurons  Integrate / change signal • Integrate: inputs from multiple afferent neurons – average signal • Change: Interneurons can provide excitatory or inhibitory signals 27 9 15/02/2023 Neuronal membrane  Made of two layers of lipid molecules  Lipid molecules • Hydrophilic (water loving) heads • Hydrophobic (water hating) tails  Barrier: water soluble molecules cannot pass through  Particularly impermeable to ions hydrophilic "head" region hydrophobic "tail" regions hydrophilic "head" region Author Jerome Walker 28 Fluid environment  Fluid environment containing ions • Intracellular fluid • Extracellular fluid Cations (+ve) Anions (-ve) Sodium (Na+) Organic ions (A-) Predominantly extracellular Only intracellular Potassium (K+) Chloride (Cl-) Predominantly intracellular Predominantly extracellular 29 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron • Structure and general function • Movement of ions • Resting potential Action potential Reading 30 10 15/02/2023 Movement of ions  Ions move because of: Concentration gradients (via diffusion) Electrical force (via electrostatic pressure) 31 Electrical polarity of neurons Neuron is polarised  At rest, neurons are -ve charged compared to extracellular fluid  -ve charge occurs if there are less +ve ions and/or more –ve ions inside cell  Whilst there is a difference in charge, an electrical force tends to move ions across the membrane Extracellular Neuron –70 mV 0 mV 32 Border guards Ion channels (leak channels): • Passive ion specific conduits • Selected ions rush down gradients of concentration & electric potential • Controlled by a gate Ion pumps: • Energy consuming • Active transport – against gradient • Maintains and builds gradients • Slower 33 11 15/02/2023 Net force Extracellular Fluid K+ Neuron -ve Na+ +ve → concentration gradient → electrostatic gradient 34 Potassium ions (K+) Diffusion  K+ highly concentrated in cell • K+ wants to move out of cell down concentration gradient  At rest, K+ leak channels allows K+ to leave neuron down concentration gradient  Inside cell becomes more –ve Electrostatic pressure • Not a lot of K+ moves out Ions will stop moving when opposing forces are equal (at equilibrium)  NB: this happens in a resting cell 35 Chloride ions (Cl-) Diffusion  Cl- highly concentrated outside cell  Cl- wants to move into cell down concentration gradient Electrostatic pressure  Inside of cell is -ve charged  Cl- also wants to move out of cell due to repel of electric charge Net force for Cl - = stay where it is 36 12 15/02/2023 Sodium ions (Na+) Diffusion  Na+ is highly concentrated outside cell  Na+ wants to move into cell down concentration gradient Electrostatic pressure  Inside of cell is -ve charged  Na+ also wants to move into cell due to electric charge attraction Net force for Na+ = move into cell Note: There are few sodium channels so ion movement is slight 37 Sodium/Potassium Pump How does extracellular Na+ remain greatest? 38 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron • Structure and general function • Movement of ions • Resting potential Action potential Reading 39 13 15/02/2023 Resting membrane potential  Two forces act on ions  Membrane is a barrier to ion movement  At rest membrane is permeable to K + so mainly K + ions move  K + ion movement stops once opposing forces reach equilibrium  Result: unequal distribution of positive & negative ions on the inside & outside of membrane Resting membrane potential  Difference in charge across membrane at rest = −70 mV 40 Summary of ion movement http://faculty.washington.edu/chudler/ap.html Author Looie496 41 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron Action potential • Characteristics • Triggering • Movement along a neuron Reading 42 14 15/02/2023 Phases of an action potential 1. Depolarisation: inside becomes more +ve 2. Repolarisation: inside becomes more –ve 3. Hyperpolarisation: more –ve than at rest Depolarisation +ve in cell Repolarisation +ve out cell Threshold Rest Rest Hyperpolarisation +ve still moving out 43 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron Action potential • Characteristics • Triggering • Movement along a neuron Reading 44 Neuron excitability  A stimulus causes small depolarisation (moves membrane potential towards 0 from -70 mV)  Size of this depolarisation is proportional to size of stimulus  If depolarisation reaches threshold (~ -55 mV) an AP occurs automatically 45 15 15/02/2023 Features of an AP  ‘all-or-nothing’ phenomena – AP only occurs if threshold is reached (at ~ -55 mV)  Large change in membrane potential (-70 to +30 mV)  Standard size & shape  Very rapid (1-4 ms)  Frequent (100’s per s) – depending on stimulus intensity 46 What regulates strength of a response?  APs are subject to an ‘all-or-nothing’ law  Strength of a response is not dictated by size of a single AP  Strength is a function of the ‘rate’ law  Rate of neural firing 47 Depolarisation  Stimulus causes a small amount of Na + to move into the cell  Na + is +ve charged → neuron becomes less –ve (slightly depolarised)  If depolarisation changes charge by +15 mV it activates voltage-gated channels in membrane 48 16 15/02/2023 Voltage-gated channels  Activated by changes in charge of membrane  Two types important for the AP: • Voltage-gated Na+ channels • Voltage-gated K+ channels Channels open Na+ & K+ rush over membrane Rapid changes in membrane potential 49 Voltage-gated action potential 1. Voltage-gated Na + channels open. Na + influx → more +ve 2. Na + channels become refractory at peak 3. Voltage-gated K + channels open. K + efflux → less +ve 4. Open K + channels allow outflow 5. Overshoot caused by slow closing K + channels 50 Resetting  The Na+/ K+ ATPase Pump moves 3 Na+ OUT & 2 K+ IN  The pump keeps Na+ conc. low in neuron  K+ also diffuse back in to neuron  This re-establishes resting membrane potential 51 17 15/02/2023 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron Action potential • Characteristics • Triggering • Movement along a neuron Reading 52 Movement along the neuron  Signal travels away from cell body towards axon terminals  No decay  Termed AP propagation 53 AP propagation  Na+ ions spread away from site of AP, change charge in nearby area of cell to be more +ve (depolarised)  Triggers another AP in this nearby area  Next AP occurs as previous AP starts to die out  APs are triggered one after another all the way to axon terminals  If axon branches, each branch continues the AP  AP stays the same size 54 18 15/02/2023 Refractory period Why is the refractory period important?  Prevents AP travelling backwards  Determines upper limit on AP frequency 55 Lecture outline Recap of last week Objectives Structure of the nervous system The resting neuron Action potential Reading 56 Coming up Workshop  When electrical transmission goes wrong Next week’s lecture  Dr Isobelle Kennedy  Supporting the brain (structures and cells)  Reading in preparation • Chapter 5, pp. 55-61 57 19