Nerves, Neurones and Synapses - Neurology PDF

Nerves, Neurones and Synapses  Nervous Systems Central Nerves - bundles of neurones ○ Brain ○ Spinal cord Peripheral ○ Somatic ○ Enteric (controls the digestive system) ○ Autonomic ◆ Symp...

Nerves, Neurones and Synapses  Nervous Systems Central Nerves - bundles of neurones ○ Brain ○ Spinal cord Peripheral ○ Somatic ○ Enteric (controls the digestive system) ○ Autonomic ◆ Sympathetic ◆ Parasympathetic Bioelectricity Membrane potential ○ Results from ion gradients Some cells: membrane potential changes in response to stimuli - Excitable cells ○ e.g Neurones Nerve impulses - changes in membrane potential that travel down nerves Ion Gradients Cell membrane highly permeable to ions This allows formation of ion gradients ○ Membrane potential Resting membrane Allows electrical signalling and excitability potential = -70 m/v Membrane potential is the basis of: ○ Neurotransmission ○ Muscle contraction ○ Secretion ◆ hormones, neurotransmission, digestive enzymes, mucus and surfactant ◆ Immune responses Transport across cell membranes Passive diffusion Active Transport Facilitated diffusion Ion Gradients Membrane potential - due to unequal ion distribution This gradient important for speedy transmission Calculating membrane potential Nernst Equation describes memb potential at eqm, Potential of 1 ion Sum every ion to get total Em Also take account of ion permeability Neurones highly specialised cells Transmit info as electrical signals (nerve impulses or action potentials) APs only travel one way - from dendrites -> axons  Action Potential electrical impulses formed by ions moving into neurone Signal received at dendrites => dendritic depolarisation ○ Ligand gated ion channels ○ Metabotropic channels This depolarisation opens voltage-gated sodium channels (Nav) Action Potential propagation Action Potential detail stimulus = electrical or mechanical or chemical This has to be strong enough to reach threshold potential ○ Enough depolarisation to open rst Nav Myelination CNS ○ Oligodendrocytes Peripheral NS ○ Schwann cells Axons in most vertebrae are myelinated (mostly covered in fatty substance - myelin) Only Nodes of Ranvier are exposed AP ‘jumps’ from one node to next Much faster neurotransmission Much more energy ef cient Saltatory Conduction fi fi  Repolarisation Summary: Membrane potential is key to all cell - Sum of all ion gradients Excitable cells - depolarise in response to stimuli - Neurones, skeletal muscle, hormone secreting Neurones transmit electrical signals Synapses - point of communication between neurones - electrical signal converted to chemical - Neurotransmitter release  Synapses Where one neurone meets another - cleft between them Electrical AP triggers release of chemical neurotransmitter (signal) Neurotransmitters from pre-synapse bind to receptors on postsynaptic neurone - triggering depolarisation. Neurotransmitter A messenger of neurological information from one cell to another! It is a chemical that is released from nerve cell that transmits an impulse from nerve cell to another or muscle, organ or other tissue. Types of neurotransmitter Excitatory: Glutamate Monoamines Acetylcholine Inhibitory: GABA - in brain Glycine - spine Endorphins - pain sensations Neuromodulators: Neuropeptides - neurotransmitter & hormone Endocannabinoids - cannabinoid system (activated by cannabis)  Neurotransmitter Receptors Ionotropic: - Glutamate (exc.), GABA, Glycine Faster Ion channels Metabotropic (G-protein coupled) - Monoamines (exc.), histamine, etc. Responses can have more diverse effects Some have both kinds - e.g glutamate and GABA Synaptic Transmission Multiple synapses onto same dendrite ○ Can be excitatory or inhibitory Wether neurone res is a sum of all synaptic inputs fi  Nervous Tissue Cells Neurones; Unipolar - sensory (one end sensory receptor - other end is post-synapse that releases neurotransmitter) Bipolar - 1 neurone - neurone Multipolar - lots of inputs Glial cells (50-80% of brain) Astrocytes Microglia Oligodendrocytes Glial Cells > neurones Provide supporting roles Vital for neuronal health Neuronal immune system CSF production (chronic fatigue syndrome) Astrocyte > neurones Astrocytes and microglia make up Support neurone brain immune system BBB (blood brain barrier) Communication NT waste Scar tissue formation - in brain injury Tripartite Synapse Astrocytes form this They can form many of these synapses maintains synapse integrity Oligodendrocyte myelination of CNS neurones Attacked by immune system in MS Microglia Brain-speci c immune system ○ A type of macrophage Potential role in many diseases ○ MS - they destroy the myelin (from chronic microglial in ammatory activity) ○ amyotrophic lateral sclerosis ○ Alzheimer’s disease - produce pro-in ammatory cytokines => chronic state of neuroin ammation - this has shown to worsen neurodegeneration. ○ Parkinson’s disease - similar case to Alzheimer’s disease Neuroin ammation is largely mediated by communication between microglia and astrocytes Also a vital role in brain/ synapse development fl fl fi fl  Neuromuscular junctions Specialist synapse at either skeletal or smooth muscle Presynaptic neurone synapses —> muscle cell Skeletal muscle NMJ acetylcholine neurotransmitter signal is ended by acetylcholinesterase Ionotropic (ligand gated ion channel) receptor ○ Nicotinic acetylcholine receptor Summary Synaptic structures enable neurotransmitter release and detection Neurotransmitters can be excitatory or inhibitory - Sum of all synaptic inputs determines firing of neurone Most of the brain is glial cells - Play supporting roles to neurones NMJ is a specialised synapse - Skeletal muscle - ionotropic - Smooth muscle - metabotropic  The Somatic NS Part of peripheral NS Controls movement of skeletal muscles Ascending and Descending tracts Ascending: relay information from spinal cord —> sensory cortex Descending: relay information from motor cortex —> spinal cord Sensory Neurones Sense touch, scent, pain, etc.. Relay info to spinal cord and brain Enter spine at the dorsal horn, via dorsal root Unipolar neurones - cell body is at the dorsal root ganglion Myelinated  Motor Neurones Relay nerve impulses from spine to trigger contraction of skeletal muscle ○ Exit spine via ventral root Alpha motor neurone Multipolar Myelinated NMJ: Acetylcholine Binds to and activates Nicotinic acetylcholine receptor Ionotropic Drugs & Toxins targeting NMJs Botulinum toxin - Inhibits acetylcholine secretion ○ Flaccid paralysis (weakness or paralysis in the proximal limb muscles) ○ paralysis of diaphragm - cant breathe Organophosphates - Inhibit acetycholinesterase ○ Muscle spasms  Myasthenia Gravis Autoimmune disease ○ autoantibodies to acetylcholine receptors (85%) ○ Or muscle receptor tyrosine kinase (15%) Double vision and ptosis are distinguishing features Treatment Main symptomatic treatment is cholinesterase inhibitors ○ increase acetylcholine levels in synaptic cleft ○ Increases activation of remaining AcH receptors that aren’t damaged Similar in action to organophosphates Spinal Control of movement - Re exes Voluntary movements initiated in brain Many aspects of movement are controlled at spine ○ Stretch re ex from muscle spindles (knee jerk) ○ Inhibitory feedback from golgi tendon organs ○ withdraw re ex ○ Reciprocal inhibition of extensor and exor muscles no brain participation fl fl fl fl  The ve components of re ex arc Muscle Spindle + stretch re ex muscle spindles - specialised muscle bres surrounded by a capsule inside skeletal muscle Proprioceptors sense muscle length —> activate sensory neurones (Ia axons) Stretch re ex Increase muscle tension to counter stretching Vital for bearing body weight, etc.. fi fl fl fl fi  Golgi tendon organs + tension these organs sense tension Proprioceptors between muscle and tendon sense tension —> activate sensory neurones (type Ib) Contracts the muscle against resistance a Autogenic inhibition type Ib axons - synapse onto inhibitory spinal interneurones These inhibit & motor-neurones (using GABA/ glycine) & reduce muscle contactrion Regulates muscle tension within a normal range Imp for ne motor skills (e.g. gripping an egg without breaking it) Other proprioceptors: many others found in connective tissues of joints ○ especially in joint capsules Provide a lot of info about position, angle and movement of each joint ○ knowing where your hands are when eyes closed fi  Inhibitory interneurones vital for stretch re ex - stops opposing re exes Reciprocal inhibition ○ have to inhibit tricep (mostly) to contract the bicep Knee jerk re ex Combo of muscle spindle stretch receptor and reciprocal inhibition Excitatory interneurones Withdraw re ex - ipsilateral ○ stimulus and effector on same side of the body fl fl fl fl  Crossed extensor re ex maintain balance during withdraw re ex Painful stimulus on one foot => withdraw of foot and simultaneous contraction of the opposite leg muscles Contralateral ○ Stimulus and effector on different sides of body Brain control of movement complex brain network integrates sensory input and coordinates motor output fl fl  Basal Ganglia centre for controlling and initiating voluntary movement Cerebellum coordinates complex series of movement Inhibited by ethanol intoxication majority of neurones are small granule cells cerebellar granule cells = neurones in the rest of CNS Highly complex info processing centre Summary SNS coordinates and controls skeletal muscle Spinal control of muscles can be overridden by areas of brain movement Dedicated brain areas control voluntary movements by Muscle control can be coordinated by spinal cord w/ cooperation and collating sensory information out brain - Somatosensory cortex - Sensory (afferent) neurones - Motor cortex - Motor (efferent) neurones - Basal ganglia Me - Cerebellum Me - Reflexes