the nervous system2

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