Nervous System Notes PDF

NERVOUS SYSTEM ​ Introduction ○​ Works tgt w endocrine system ○​ Aim → maintain homeostasis & provide quick & brief responses ​ Functions of nervous system ○​ Monitor’s body internal & external environments ○​ Integrate sensory information & follow up w commands (to effectors...

NERVOUS SYSTEM ​ Introduction ○​ Works tgt w endocrine system ○​ Aim → maintain homeostasis & provide quick & brief responses ​ Functions of nervous system ○​ Monitor’s body internal & external environments ○​ Integrate sensory information & follow up w commands (to effectors) ○​ Coordinate voluntary & involuntary responses of many other organ systems ​ Brief overview of organisation of nervous system ○​ Consists of CNS & PNS ○​ CNS: brain + spinal cord → receive & process sensory info as well as conduct signal from brain to effector ○​ PNS: all neural tissues outside CNS → has afferent & efferent divisions → means sensory receptors & effectors ​ Anatomical divisions of nervous system ○​ Central nervous system: ​ Brain + spinal cord ​ Integrates, processes & coordinates sensory input & motor commands ○​ Peripheral nervous system: ​ Includes all neural tissue outside CNS ​ Carries info between CNS & rest of body (from body to CNS the from CNS to effectors) ​ Functional divisions of the peripheral nervous system (PNS) ○​ Afferent division ​ Carries sensory info from sensory receptors to CNS ​ Receptors are sensitive sensory structures detecting changes in environment/ specific stimuli ​ AKA sensory neurons ○​ Efferent division ​ Carries motor commands from CNS to effectors eg muscles/ glands ​ Target organs & tissues respond by doing something thus the name effectors ​ AKA motor neurons ​ FURTHER DIVIDED INTO: ​ SOMATIC NERVOUS SYSTEM ​ ○​ Controls skeletal muscle contractions ○​ Under voluntary control ​ AUTONOMIC NERVOUS SYSTEM ○​ Under involuntary control ○​ Provides involuntary regulation of smooth muscles, cardiac muscles & glandular secretions ○​ Further classified into: ​ Sympathetic division: ​ Fight or flight response  ​ Parasympathetic division: ​ Rest or digest ​ Nervous system cells ○​ Contains 2 kinds of cells ​ Neurons ​ Basic functional unit ​ Send & receive signals ​ Neuroglia (glial cells) ​ Support & protect neurons → due to myelinated sheath ​ Does not send or receive signals ​ Structure of neurons (multipolar) ○​ Cell body (soma) ​ Large round nucleus w prominent nucleolus ​ Absence of centrioles → neuron cells cannot divide, regenerate or be replaced if lost to injury/ disease ​ Numerous mitochondria → produce energy for cellular activities ​ Clusters of RER & ribosomes → produce neurotransmitters & form Nissl bodies ​ Nissl bodies makes neural tissue appear grey → gray matter ○​ Axon hillock ​ Thickened region near axon ​ Starting point for generation of action potential ​ Transmission of nerve impulses ○​ Dendrites ​ Many, short & branched ​ Sensitive to stimulation ○​ Axon ​ Single & long ​ Transmits electrical signal ​ Has branches (aka collaterals) ending in axon terminals ​ Communicated w other cells via synapses ​ Structural classification of neurons ○​ Multipolar neurons ​ Most common in neurons in CNS → eg all motor neurons to skeletal muscles ​ Multiple dendrites ​ 1 long axon ○​ Unipolar neurons ​ Found in every sensory neuron of PNS ​ Dendrites & axons are continuous ​ Cell body pushed to 1 side ○​ Bipolar neurons ​ Small & rare ​ Found in special sensory organs (sight, smell & hearing)  ​ 1 dendrite & 1 axon ​ Functional classification of neurons ○​ Sensory neurons (multipolar neurons usually) ​ AKA afferent neurons ​ Carries info from sensory receptors to CNS ​ Has 2 types: ​ Somatic sensory neurons ○​ Monitor external environment ​ Pain & thermoreceptors ​ Visceral sensory neurons ○​ Monitor internal environment ​ Physiological environment of body ○​ Motor neurons (unipolar neurons) ​ AKA efferent neurons ​ Carries instructions from CNS to effectors ​ Has 2 types: ​ Somatic motor neurons ○​ Innervate skeletal muscles ​ Visceral motor neurons ○​ Innervate (supply w nerves) cardiac & smooth muscles & glands ○​ Interneurons (bipolar neurons in the eyes and ears) ​ Located in brain & spinal cord ​ Connects sensory & motor neurons ​ Responsible for: ​ Distribution of sensory info ​ Coordination of motor activity ​ Also involved in: ​ Memory, planning & learning ​ Neuroglia ○​ Supporting cells of nervous system ○​ Makes up half of vol of CNS ○​ Found in both CNS & PNS ○​ Types of neuroglia in CNS: ​ Astrocytes ​ Largest & most numerous neuroglia ​ FUNCTIONS: ○​ Forms structural framework for CNS ○​ Secrete chemicals maintaining blood-barrier (blood-brain barrier isolates brain tissue from general circulation) ○​ Repair damaged neurons  ​ Oligodendrocytes ​ Have cytoplasmic extensions wrapping around axons → forms myelin sheath → the myelin sheath speeds up electrical impulses due to its insulation ​ Microglia ​ Are WBCs migrated to the brain forming microglia → phagocytic cells (phagocytic cells derived from WBCs) ​ Smallest & rarest ​ Cleans up cellular debris, waste products & pathogens (phagocytosis) ​ Ependymal cells ​ Are simple cuboidal cells lining the central canal of spinal cord & ventricle in brain (cavities in brain filled with fluid, ependymal cells line the cavities) ​ Some regions of brain, secrete cerebrospinal fluid (CSF) & in other locations, help circulate CSF. (function) ○​ Types of neuroglia in PNS: ​ Satellite cells ​ Surround & support neuron cell bodies of PNS ​ Similar to astrocytes of CNS ​ Schwann cells ​ Form myelin sheath around peripheral axons ​ Similar to oligodendrocytes of CNS ○​ Myelination: ​ Increases speed of nerve transmission ​ Makes nerves appear white ​ White matter of CNS → contains myelin sheaths ​ Nodes of Ranvier ​ Gaps between adjacent myelin sheaths ​ Nerve conduction ○​ Two major functional properties of neurons ​ Irritability ​ Ability to respond to stimulus & convert it into nerve impulse ​ Conductivity ​ Ability to transmit impulse to other neurons, muscles/ glands ​ Resting membrane potential ○​ +ve & -ve charges separated by plasma membrane ○​ AKA transmembrane potential ○​ Membrane potential of undisturbed cell → resting membrane potential ​ RMP for neuron → -70millivolts (mV) ​ -ve voltage indicate inside of plasma membrane contain more -ve charge compared to outside ​ Information transfer between neurons & other cells ○​ Graded potentials ​ Are changes in membrane potential that cannot spread far from site of stimulation ​ Occurs in plasma membrane of all cells in response to environment stimuli ​ Triggers specific cell function ○​ Action potentials ​ Propagated change in membrane potential of entire plasma membrane ​ Only axon of neuron & skeletal muscle fibres have excitable membranes conducting action potential ​ Action potential meaning ○​ Action potential in an axon begins near axon hillock & travels through length of axon —> reaching axon terminal ○​ @ axon terminal —> activates synapse ○​ Action potential in neuron known as nerve impulse ○​ Generated by opening & closing of gated sodium & potassium channels in response to graded potential ○​ Membrane reaches level called “threshold” then action potential will appear. ​ All or none principle ○​ For any stimulus which brings the membrane to threshold, will generate action potential ○​ So, stimulus either generates action potential or does not produce any response. ​ Generation of nerve impulse / action potential ○​ Polarisation of resting membrane potential ​ RESTING STATE ​ External surface of neuron’s plasma membrane slightly +ve ​ Internal face slightly -ve ​ Chief extracellular ion is Na+ ​ Chief intracellular ion is K+ ○​ Stimulus initiates local depolarization ​ Stimulus changes permeability of local “patch” of membrane ​ Na+ ions diffuse rapidly into cell as Na+ ion much higher conc outside of cell ​ Changes polarity of membrane —> inside become more +ve whereas outside become more -ve (graded potential) ○​ Depolarization ​ When gated sodium channel open, speed up entry of Na+ into cell ​ Causes increase in +ve charge on inner surface of membrane shifting membrane potential to 0 millivolt (mV). ○​ Generation of action potential ​ If stimulus strong enough  ​ Depolarisation causes membrane polarity to be completely reversed ​ Action potential/ nerve impulse is initiated ​ Nerve impulse is an all-or-none response ​ Either propagated (conducted or sent) over entire axon or doesn’t happen at all. ○​ Propagation (widely spread) of action potential ​ Depolarisation of first membrane patch causes permeability changes in adjacent membrane ​ Events described in step 2 repeated ​ Stimulus changes permeability of local “patch” of membrane ​ Na+ ions diffuse rapidly into cell as Na+ ion much higher conc outside of cell ​ Changes polarity of membrane —> inside become more +ve whereas outside become more -ve (graded potential) ​ Action potential propagated rapidly along entire length of membrane ○​ Repolarisation ​ Almost immediately after Na+ ions rush into cell ​ Membrane permeability changes again ​ K+ ions diffuse out of cell into interstitial fluid ​ Restores -ve charge on inside of membrane & +ve charge on outside surface ○​ Restoration of initial conc of ions ​ Ionic conditions of resting state restored by sodium-potassium pump ​ 3 Na+ ions pumped out for every 2 K+ ions pumped back into cell ​ Propagation of an action potential ○​ Continuous propagation ​ Occurs in unmyelinated axons ​ Relatively slow ○​ Saltatory propagation ​ Occurs in myelinated axons ​ Much faster ​ Synapse ○​ Site where neuron communicated with another cell ○​ Synapse between neuron & another cell type —> neuroeffector junction ​ Neuromuscular junction: between neuron & muscle cell ​ Neuroglandular junction: between neuron & secretory cells ​ Transmission of an action potential ○​ Info moves form one location to another in form of action potential (nerve impulses) along axons ○​ At end of axon, info transferred from another neuron to effector cell ○​ Info transfer takes place via releases of chemicals —> neurotransmitters ​ Structure of a synapse ○​ Presynaptic neuron ​ Neuron on sending side of synapse ​ Axon terminals hold vesicles containing neurotransmitters ​ Neurotransmitters released and diffuse across synaptic cleft ○​ Postsynaptic neuron ​ Neuron on receiving side of synapse ​ Has receptors for neurotransmitters ​ Events occurring at the synapse ○​ Action potential arrives & depolarizes axon terminal ○​ Extracellular Ca2+ enters axon terminals & triggers exocytosis of acetylcholine (Ach) ○​ ACh diffuses across synaptic cleft ○​ ACh binds to receptors & triggers depolarisation of postsynaptic membrane  ○​ AcCh removed by acetylcholinesterase (AChE) TO TAKE NOTE: ​ K+ ions move out of cells during propagation action potential → repolarisation NERVOUS SYSTEM SAQs ​ Structure of neuron ○​ Cell body ​ Large round nucleus; prominent nucleolus ​ No centrioles → cannot divide, regenerate & be replaced if lost to injury/ disease ​ Numerous mitochondria → produce energy ​ Clusters of RER & ribosomes → produce neurotransmitters & form nissl bodies; make neural tissues appear grey (gray matter) ○​ Axon hillock ​ Thickened region near axon ​ Starting point for generation of action potential ○​ Dendrites ​ Many, short, branched ​ Sensitive to stimulation ○​ Axon ​ Single & long ​ Transmits electrical signal ​ Has branches (collaterals) ending in axon terminals → communicates with other cells via synapses ​ Classification of neuron ○​ Sensory neurons ​ Afferent neurons ​ Carries info from sensory receptors to CNS ​ Has 2 types ​ Somatic sensory neuron; monitor external environment ​ Visceral sensory neuron; monitor internal environment ○​ Motor neurons ​ Efferent neurons ​ Carries instructions from CNS to effectors ​ Has 2 types: ​ Somatic motor neurons; innervate skeletal muscles ​ Visceral motor neurons; innervate cardiac, smooth muscles & glands ○​ Interneurons ​ Located in brain & spinal cord ​ Connects sensory & motor neurons ​ Responsible for distribution of sensory info & coordination of motor activity ​ Also involved in higher functions like memory, planning & learning ​ Structure of spinal cord segment ○​ Each spinal segment have pair of: ​ Dorsal roots ​ Contains axons of sensory neuron ​ Dorsal root ganglia ​ Contains cell bodies of sensory neurons ​ Ventral roots ​ Contains axons of CNS motor neurons ​ Spinal nerves (dorsal & ventral roots join distally) ​ Mixed nerves ​ Both sensory & motor fibres ○​ Gray matter ​ Contains neuron cell bodies ​ Has projections → gray horns) ○​ Gray commissures ​ Contains axons crossing 1 side of spinal cord to other ​ Connects horns on either side of spinal cord ○​ White matter ​ Superficial ​ Divided into 3 columns containing bundles of sensory & motor nerve fibres tracts → Posterior, anterior, lateral white columns ○​ Posterior gray horn ​ Sensory nuclei ○​ Anterior gray horn ​ Somatic motor nuclei ○​ Lateral gray horn ​ Found in only thoracic & lumbar segments ​ Contains visceral (autonomic) motor nuclei ​ Structure of synapse ○​ Presynaptic neuron ​ Sending side of synapse ​ Axon terminal holds vesicles containing neurotransmitters ​ Neurotransmitters released & diffuse across synaptic cleft ○​ Postsynaptic neuron ​ Receiving side of synapse ​ Has receptors for neurotransmitters ​ Events occurring @ synaptic transmission ○​ Action potential arrives & depolarizes axon terminal ○​ Extracellular Ca2+ enters axon terminals & triggers exocytosis of acetylcholine (ACh) ○​ ACh diffuses across synaptic cleft  ○​ Ach binds to receptors & triggers depolarisation of postsynaptic membrane ○​ Ach removed by AChE (acetylcholinesterase) ​ Divisions of autonomic nervous system ○​ Sympathetic division ​ Preganglionic fibres leave thoracic & lumbar spinal segments ​ Ganglia near spinal cord ​ Short preganglionic fibres (opp of parasympathetic) ​ Long postganglionic fibres ○​ Parasympathetic division ​ Preganglionic fibres originate in brain stem & sacral spinal region ​ Ganglia located near/ within target organs ​ Long preganglionic fibres ​ Short postganglionic fibres