the nervous system2
DESCRIPTION
TRANSCRIPT
Overlapping functions:• Sensory input• Integration• Motor output
Collect information from the environment• Internal and external• Also called the sensory input
Give the specific type of nerves found in the five basic senses
Processing of input to be interpreted and associated with the correct response
Processed in the CNS• Give an example of a correct response from
a stimulus
Conduction of signals from integration to effector cells
Signals are conducted by nerves• Nerve= tissue (composed of nerve cells)• Neuron= nerve cells
Through a combination of chemical and electrical signals
Structural and functional unit of the nervous system
Axon hillock
Dendrites Axons Axon hillock Myelin sheath Synaptic terminals Synapse Presynaptic cell Postsynaptic cell Neurotransmitter
Simplest- reflex arc• Reflex – an automatic response• Simplest reflex arc= two kinds of nerve cells
Motor and sensory Effector cells – muscle or gland Knee-jerk= more complex
Front thigh contracts/inhibition of back thigh Involves second circuit (three parts) Sensory neurons from quadriceps= synapse with
interneuron and motor neuron
Always in the CNS Also called association neuron, local
circuit neuron Multi-branched Always communicate with each other “memory”
Both are clusters of cell bodies Nuclei- found inside the CNS Ganglion- found outside CNS
Take note the difference in structures of sensory neuron, motor neuron and interneuron
Relate it to their function
Single presynaptic neuron to several postsynaptic neurons
Convergent of several presynaptic neuron to a single postsynaptic neuron
Circular path (memory)
Outnumber neurons 10:1• Then: glial cells do not participate in nerve
signalling• Now: presence of some synaptic
interactions Important in structural integrity of
the Nervous system Normal functioning of neurons
Embryo: radial glia- form tracks where neurons migrate
Mature CNS: astrocytes – structural and metabolic support; stimulate formation of tight junctions between cells lining the capillaries of the brain (blood-brain barrier
Glia that form myelin sheath around axons of neurons
Oligodendrocytes – CNS Schwann cells- PNS Myelin sheath- provides electrical
insulation
Change in the voltage across the plasma membrane of neurons
Caused by movement ions across the plasma membrane• Ion channels
All cells have voltage across theri plasma membrane
This membrane potential exists because of difference in ion concentration
Electrically polarized• Anion- inside• Cation-outside
Animal cell: normally -50V to -100V Resting state of neuron: -70V Resting potential- membrane
potential of an unstimulated neuron
Presence of special ion channels Selective permeability of the
membrane Anions do not diffuse readily outside
the membrane
Plasma membrane- lipid bilayer Lipid- not electrically charged Ions cannot dissolved readily (do not
diffuse easily) Presence of pumps or channels
Presence of more K+ channels than Na+ channels due to high permeability to K+
Ion channels do not determine the rate and direction of ion movement
Electrochemical concentration is responsible
-85mV- amount that can counterbalance concentration gradient of K• Called the equilibrium potential of
potassium
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All cells have membrane potential but only neurons and muscle cells can generate large amount of membrane potential• Excitable cells• Resting potential- membrane potential of
resting cell
Nerve impulses are the action potential generated in a cell that travel through pulse-like wave of voltage though membranes
Due to stimulus
Ungated ion channels- open all the time
Gated ion channels• Chemically-gated ion channel- stimulus:
chemical (e.g. Neurotransmitter)• Voltage-gated ion channel- stimulus: change
in membrane potential
Graded potential: change in membrane potential due to strength of stimulus• Hyperpolarization- membrane potential is
more negative• Depolarization- membrane potential is more
positive
Recall: muscle contraction at the cellular level
Threshold potential• Action potential- response when threshold
potential is met (generated only in axons)• Threshold potential usually -50mV to -55
mV• Hyperpolarization do not produce action
potentials
Voltage-gated ion channels are stimulated• Potassium- single voltage-sensitive gate
Closed- resting state Opens slowly in response depolarization
• Sodium- two voltage sensitive gates Activation- closed (resting state); opens rapidly
during depolarization Inactivation- open (resting state); closes slowly
in response to depolarization
Responsible in the restoration of the internal membrane potential • Due to low permeability to sodium • Inc permeability to potassium• Hyperpolarization- responsible for refractory
period
A domino effect• always in a forward direction due to
refractory period
Diameter of axon- faster if higher diameter• Current is inversely proportional to the cross
section of a wire Presence of Schwann cells
• Node of Ranvier- gaps between Schwann cells Ion channels are concentrated, extracellular fluid
is in contact with the axon membrane at the node Saltatory conduction
Unique cell junction that control communication• Between: 2 neurons, sensory receptors and
sensory neurons, motor neurons and muscle cell, neurons and gland cell
Two types• Electrical synapse• Chemical synapse
Less common than chemical synapse Cells are connected by gap junctions Allows action potential to spread
directly from presynaptic terminals to postsynaptic terminals
No loss of signal strength and delay Responsible for rapid movement
Presence of synaptic cleft (gap) Electrical signals not directly
transmitted Signal: electrical-chemical-electrical Presence of synaptic vesicles
containing neurotransmitter in the presynaptic axon
Presence of chemical-gated ions
Recall: structure of a neuron Presence of inhibitory and excitatory
synapses
Depolarization of the plasma membrane
Results from influx of positive ions (e.g. Na ions)
Depolarization may lead to an action potential if the threshold potential is met
This is called EXCITATORY POSTSYNAPTIC POTENTIAL or EPSP
Inc permeability of Cl and K ion channels
Hyperpolarization INHIBITORY POSTSYNAPTIC
POTENTIAL or IPSP
Depends on the type of receptor and ion channel at the receiveing end
Region where voltage-gated sodium channels open
Production of action potential: • Several synaptic terminals acting
simultaneously on one postsynaptic terminal
• Few synaptic terminal discharging neurotransmitter at a greater frequency Summation is produced
Temporal: frequency of chemical transmission is great; no time to return to resting potential
Spatial: presence of many presynaptic neurons stimulating one postsynaptic neuron• Both are present in IPSP and EPSP
Effects vary• Can be within a few millisecond• Can be longer due to signal transduction
pathways it enters• Can remain active over a long period of
time
One of the most common among vertebrate and invertebrate
Can be inhibitory or excitatory depending on the receptor
Excitatory in muscle cell In heart muscle: inhibitory
• Dec ability to create action potential through hyperpolarization
Derived from amino acids Function as transmitters in the CNS Imbalances result in different
disorders• Parkinson’s disease- low dopamine• Schizophrenia-high dopamine• Psychoactive drugs like LSD and mescaline-
produce hallucinatory effects by binding to receptors of serotonin and dopamine
Catecholamines- from tyrosine Dopamine
• Epinephrine and norepinephrine Also functions as hormones
Serotonin- from tryptophan
• Affects signal transduction pathway• Affects biochemical processes
GABA- gamma aminobutyric acid• Transmitter of most inhibitory synapses• Produces IPSPs by inc Cl permeability• Present in Valium
Glycine Glutamate Aspartate
Short chain amino acids Operate via signal transduction
pathways Substance P- mediates pain Endorphins- feel good hormone
• Analgesic; counters pain• Dec urine output• Also acts as a hormone
NO and CO as regulators Release of NO in penis
• Viagra inhibits enzyme that masks the effect of NO
Gaseous messengers are not store rather are synthesized