Neurons n shit
- The nervous system functions alongside the endocrine system to communicate
- Contains brains, spinal chord and nerves
- Organized to detect changes internally and externally, evaluate, and initiate a response to said changes
Central Nervous System (CNS)
- Structural and functional centre
- Brain and spinal chord
- Evaluates and initiates response to sensory information
Peripheral Nervous system (PNS)
- Nerves that lie on outer region
- Cranial = originates from brain
- Spinal = originates from spinal chord
Somatic Nervous System (SNS)
Stimulus -> Somatic and special receptors -> Somatic sensory division (afferent) -> Somatic integration centers
Response <- Somatic effector (skeletal muscle) <- Somatic motor (efferent) <- Somatic integration centers
Autonomic Nervous System (ANS)
Stimulus -> Visceral receptors -> Visceral sensory division (afferent) -> Autonomic integration centers
Response <- Sympathetic division (efferent) <- Autonomic Integration Centers
<- Parasympathetic ^
Afferent Division – Incoming sensory pathways
Efferent – Outgoing sensory pathways
Systems (organs innervated)
- Somatic Nervous System (SNS)
o Carries information to somatic effectors (skeletal muscles)
o Somatic sensory division carries feedback to information to somatic integration centers in CNS
- Autonomic nervous system (ANS)
o Efferent division carries info to autonomic/visceral effectors (smooth muscles, cardiac muscles and glands
o Sympathetic division: prepares for immediate threats (FIGHT-OR-FLIGHT)
o Parasympathetic division; Coordinates normal resting activities (REST-AND-REPAIR)
o Visceral sensory division carries feedback to autonomic integrating centers in CNS
Cells of Nervous System
Glia (Neuroglia) – Support the neurons – 5 major types
- Astrocytes (CNS)
o Star shaped, largest and most numerous
o Cell extensions connected to neurons and capillaries
o Form a tight sheath around brain capillaries – With tight junction to endothelial cells form the blood brain barrier
o Astrocytoma – slow growing brain tumor
- Microglia (CNS)
o Small stationary cells
o In inflamed brain tissue, they enlarge and carry out phagocytosis
- Ependymal cells (CNS)
o Resemble epithelial cells and form thin sheets lining fluid filled cavities
o Aid circulation or produce fluid
o Ependymoma: Glioma tumor more common in children – 5 years survival post op
- Oligodendrocytes (CNS)
o Smaller than astrocytes
o Hold fibers together and produce myelin sheath
o Oligodendroglioma: tumor in anterior portion of brain often occur in forties – 10 year post op survival
- Schwann cells (PNS)
o Found only peripheral
o Support nerve fibers and form myelin sheath
o Gaps in myelin sheath are called nodes of Ranvier
o Neurilemma is formed by cytoplasm of Schwann cell wrapped around myelin – for nerve regrowth
o Neuronal sheath is myelin sheath + neurilemma (Schwann wrapped on axon)
o Satellite cells – Schwann cells that cover and support cell bodies
Neurons – Excitable cells that initiate and conduct impulses that make nervous system functions possible
- Dendrites
o One or more on Neurons branching from cell body
o Conduct nerve signals to cell body
o Distal ends = receptors
o Dendritic spines = small protrusions on dendrites of brain neurons (connection points for axons of other neurons)
- Axon
o Single extending process attached to axon hillock (covered by myelin sheath)
o Conducts nerve impulses away from cell body
o Distal tips = telodendria (ends in synaptic button)
o Axon varicosities: Swelling that makes contact with another cell
- Cytoskeleton
o Microtubules and microfilaments and bundles of neurofilaments (neurofibrils)
o Allow rapid organelle transport
§ Vesicles – mitochondria
§ Motor molecules shuttle neurotransmitter containing vesicles from soma -> terminal buttons
o Functional regions of neuron
§ Input zone – dendrites and cell body
§ Summation zone – Axon hillock
§ Conduction zone – Axon
§ Output zone – telodendria and synaptic knobs
Classification of Neurons
- Multipolar
- Bipolar
- (pseudo)unipolar
Functional classifications
- Afferent (sensory) neurons
o Conduct impulses to spinal cord and brain
- Efferent (motor) neurons
o Conduct impulses away from spinal cord and brain to muscles/glandular muscles
- Interneurons
- Reflex arc
o Signal conduction route to and from CNS starting in receptors and ending in effectors
o Three-neuron arc is most common – Afferent, efferent and interneurons
§ Afferent – impulses to CNS from receptor
§ Efferent – Impulses from CNS to effectors
o Two-neuron arc is the simplest – Efferent and efferent neurons
o Synapse – signal from one neuron to another
§ Chemical synapse – common in adults / Located in junction of synaptic knob of one neuron and dendrites or cell body of another neuron
Nerves and tracts
Nerves – Bundles of peripheral nerve fibers held together by layers of connective tissue
- Epineurium: surrounds the complete nerve
- Perineurium: surrounds bundles of nerve fibers (Fascicles)
- Endoneurium: surrounds each nerve fiber
Tracts – Bundles of nerve fibers within the CNS
White matter
- Myelinated nerves (PNS)
- Myelinated tracts (CNS)
Gray matter
- Composed of cell bodies and unmyelinated fibers
- Nuclei (CNS)
- Ganglia (PNS)
Mixed Nerves
- Contain sensory and motor neurons
- Sensory nerves are mostly sensory neurons
- Motor nerves have mostly motor neurons
Nerve Impulses
Membrane potentials
- All living cells maintain a difference in ion concentration
- Membrane potential: Slight excess of positive ions on the outside of the membrane and a deficiency of positive on the inside
- Difference of charge = potential (stored energy)
- Polarized membrane: membrane exhibiting membrane potential
- Measure of potential difference is in mV or V – sign of voltage indicates charge of inside polarized membrane
Resting membrane potential
- Maintained at -70mV by plasma membrane
- Excess of positive ions is caused by ion transport mechanisms and selective permeability
- Selective permeability maintain potential
- Sodium potassium pump
o Active transport of Na+ and K+ in opposite directions of each other
o Maintain potential and keep inside surface slightly negative
Local potentials
- Local potentials: Slight shift from the resting membrane in a specific region of the plasma
- Excitation: When stimulus triggers opening of additional Na+ channels and depolarizes membrane (potential moves towards zero)
- Inhibition: When a stimulus triggers opening of additional K+ channels increasing potential (Hyperpolarization)
- Local potentials = graded potentials / magnitude of deviation from resting os proportional to magnitude of stimulus
Action potential – Membrane potential of a neuron conducting an impulse (AKA nerve impulse)
Mechanism producing action potential
- When a good enough stimulus triggers Na+ gated channels to open – Na+ rush into the cell per diffusion and produce local depolarization
- Threshold is reached – VGC open and more Na+ enters
- All or none response
- VGC stay open for about 1ms before closing
- After Action potential peaks, membrane begins to move back to resting membrane potential (when K+ channels open) – Called repolarization
- Brief period of hyperpolarization as returns to resting potential
Refractory Period
- Absolute refractory: brief period of resistance to re-stimulation – Will not respond to stimulus no matter what (only for 0.5ms)
- Relative refractory: Time when the membrane is hyperpolarized and restoring potential – Only responds to very strong stimulus
Conduction of action potential
- At peak, plasma membranes polarity is reverse of resting potential
- Reversal allows current to flow from site of action potential to adjacent regions and trigger Na+ gates to open and thus triggers action potential
- Cycle repeats and doesn’t go backwards due to refractory period
- In myelinated fibers, action potential only occurs in nodes of Ranvier called Saltatory conduction
- Speed depends on diameter
Synaptic junctions
- Electrical synapses: Occur where cells joined by gap junctions allow an action potential to continue along postsynaptic membrane
- Chemical synapses: Occur where presynaptic cells release neurotransmitters across tiny gap to the postsynaptic cell, potentially causing an action potential there
- Synaptic knob: Tiny bulge at the end of a terminal branch of a presynaptic neurons axon that contains vesicle housing neurotransmitters
- Synaptic cleft: space between synaptic knob and plasma
- Arrangements of synapses
o Axodendritic: Axon signals postsynaptic dendrite – Common
o Axosomatic: axon signals postsynaptic soma – Common
o Axoaxonic: Axon signals postsynaptic axon – May regulate action potential of postsynaptic axon
- Plasma membrane of postsynaptic neuron has protein molecules that serve as receptors for neurotransmitters
Synaptic transmission sequence of events
- Action potential reaches synaptic knob and calcium floods into knob
- Increased calcium triggers release of neurotransmitter by exocytosis
- Neurotransmitter molecules diffuse across synaptic cleft and bind to receptor molecules to make ion channels open
- Opening of channels creates postsynaptic potential, either an excitatory postsynaptic potential or inhibitory postsynaptic potential
- Neurotransmitters action is quickly terminated by neurotransmitter molecules being transported back into the synaptic knob (reuptake)
- Additionally, could be metabolized by enzymes or taken up by nearby glia
Summation
- Spatial summation: Adding together the effects of several knobs being activated simultaneously and stimulating different locations on the postsynaptic membrane, producing action potential
- Temporal summation: when synaptic knobs stimulate a post synaptic neuron in rapid succession, effects can summate over a brief period to produce an action potential
Synapses and memory
- Memories are stored by facilitating synaptic transmission
- Short-term memoires may result from an axoaxonic facilitation of presynaptic axon terminal
- Intermediated long-term memory happens when serotonin blocks potassium channels in presynaptic terminal – prolongs action potential and increases amount of neurotransmitter released
- Long-term memories require structural changes at the synapse – More vesicles, more release sites, more presyn. terminals, more sensitive presyn. membranes
Neurotransmitters – AGAIN
Small-molecule neurotransmitters
- Acetylcholine
o Unique structure – Acetate (acetyl-coenzyme-A) with choline
o Acetylcholine is deactivated by acetylcholinesterase, choline molecules are released and transported back to presynaptic neuron to re-combine with an acetate
o Excitatory or Inhibitory role in various locations
- Amines
o Synthesized from amino acids
o Monoamines or catecholamines
o Found in brain and effect learning, emotions, and motor control (so I just don’t fucking have them)