Nervous System
senses and responds to changes in the environment
Three classes of neurons:
afferent neurons, interneurons, efferent neurons
afferent neurons
Sensory neurons. detect stimulus and relay info about it to central nervous system
Stimulus-
form of energy that specific receptors detect
interneurons
In the spinal cord and brain, interneurons receive and process the sensory information and integrate a response
efferent neurons
Motor neurons (efferent neurons) delivers information away from the brain and spinal cord to the body’s effectors, which carry out the specified response
effectors
muscles, glands
Dendrites-
slender extensions of cell body that are input zones for info
Cell body neuron
contains nucleus, cytoplasm, most of the organelles
Axon-
slender and long extension that is neuron’s conducting zone (where it sends the message)
Terminal branches-
ending axon branches
Synaptic terminals-
ends of terminal branches which release neurotransmitters-
neurotransmitters
chemicals that transmit signal from one neuron to another
Synapse-
junction between a synaptic terminal of one neuron and the dendrites of another (between two neurons or between a neuron and an affector)
Myelin sheath-
insulating covering on axons of vertebrate nerve cells made of special cells called Schwann cells
Nodes of Ranvier + signals
gaps in sheath between successive Schwann cells. signals are electrical (ions) and to bridge
the gaps between synapses there are chemical signals
Nerve-
hundreds or thousands of neurons wrapped together
Resting neuron
polarized
polarized
cytoplasmic fluid is negatively charged
Resting membrane potential-
-70 millivolts
electrical response to stimulation in neurons
action potential–In neurons, this electric gradient will briefly reverse itself in response to stimulation–goes from - to +, travels from a neuron’s trigger zone (dendrites) to output zone (axon)
neurons at rest
hen a neuron is at rest, there are about 15 sodium ions in the fluid inside the cell for every 150 outside. There are 150 potassium ions inside for every 5 outside. The ions can only enter and leave the cell through transport proteins
Passive ion channels
permit the passage of specific ions; Potassium channels are the most common- potassium ions leak out of the cell, down their conc gradient. This increases the positive charge in the extra cellular fluid outside the cell.
Gated ion channels
closed when the neuron is at rest and only open at certain voltages
Sodium potassium pump
actively transports sodium ions out of the cell and potassium ions into the cell. For every three sodium ions pumped out of the cell, two potassium pumped in. Therefore- more positive ions are pumped out than in, helping maintain negative resting potential and a higher concentration of K+ inside the cell and a higher concentration of Na+ outside the cell.
what happens when a stimulus occurs?
special voltage-activated Na+ ion gates open, and Na+ flows inside the neuron, making the neuron positively charged. The membrane becomes depolarized- membrane potential becomes less negative (closer to zero.)
graded response
A slight stimulus causes a graded response- does not spread far from point of stimulation
generating action potential
if the stimulus is intense enough to cause the voltage to reach –55 mV, this is called the threshold level, and an action potential, or an impulse, is generated. Depends on positive feedback mechanism- as Na+ flows into the cell, cytosol becomes more positively charged, which opens even more voltage gated sodium channels. self-propagating mechanism causes to continue. The membrane reaches zero potential and then spikes to +35 mV. At this point, the sodium gates close, membrane is once again impermeable to sodium.
action potential is self-propagating-
when one section of the membrane becomes positively charged, this stimulates the next area of the membrane to open the sodium gates and the next area will become positively charged.
Repolarization-
after spike to +35 and closing of sodium gates, membrane potential returns to resting potential. Voltage gated potassium ion channels slowly open, allowing potassium to leak out of neuron. Neuron becomes negative once again.
Hyperpolarization-
At repolarization, more potassium ions have moved out than necessary to establish the original polarized potential. Membrane is hyperpolarized- -80mV.
Four steps of stimulation–
Resting state (voltage activated channels are closed), depolarization (at threshold, Na channels open, and Na entering neuron causes further depolarization, generating action potential) repolarization (Na channels close, K open and diffuse out of cell, returning negative charge on inside,) return to resting state. (channels close)
Refractory period-
period during which the neuron will not respond to a new stimulus. Membrane is polarized, but the Na+ and K+ are on the wrong sides of the membrane; sodium potassium pumps return them to resting potential location; Once ions are returned to resting potential location, neuron is ready for another stimulus.
Anesthetics
Anesthetics such as Novocain prevent voltage-activated sodium ion channels from opening- therefore neuron cannot transmit an impulse, so pain is not experienced
All-or-none response-
+intensity
only a stimulus strong enough to depolarize the membrane to its critical threshold level results in transmission of an impulse - no differences in strength of an impulse. Intensity of sensation depends on the number of neurons stimulated and on their frequency of discharge
impulses in neurons that are not myelinated
(invertebrates) continuous conduction
continuous conduction
the impulse travels in a smooth, progressive manner
neurons are myelinated in ?
vertebrates
Where are the channels that allow Na+ and K+ to pass located?
nodes of Ranvier
draw neuron
ok
Saltatory conduction
Action potential appears to jump from one node of Ranvier to the next- requires less energy and happens quicker
Presynaptic neuron-
neuron that ends at specific synapse
Postsynaptic neuron-
neuron that begins at that synapse
Electrical synapses-
presynaptic and postsynaptic neurons are connected by gap junctions- allows for impulse to be directly transmitted
Chemical synapses-
majority of synapses- presynaptic and postsynaptic neurons are separated by space called synaptic cleft. At this space, the electrical signal converted to chemical signal.
Acetylcholine-
neurotransmitter. released from motor neurons, triggers muscle contractions.
Norepinephrine-
primes the body for stress. neurotransmitter.
Dopamine, seratonin-
neurotransmitter. affect mood (imbalance can cause depression, ADD, schizophrenia)
Glutamate-
neurotransmitter. found in brain
Neurotransmitters
the chemical signal that conduct the signal from presynaptic to postsynaptic neuron– Neurotransmitter bind with receptors on postsynaptic cells. stored in synaptic vesicles.
synaptic vesicles
sacs in synaptic terminals
When action potential reaches a synaptic terminal-
Ca+ ions flow into synaptic terminal, causing vesicles to fuse with presynaptic membrane; Neurotransmitter molecules are released by exocytosis.
What happens when neurotransmitter molecules are released?
Neurotransmitter molecules diffuse across synaptic cleft and combine with receptors on post synaptic neuron, then an cause ion channels to open or activates a second messenger. Neurotransmitter remaining in synaptic cleft then is either broken down by enzymes or transported back into synaptic terminal.
study neurotransmitter diagram
ok
what kind of signals can neurotransmitters send?
excitatory or inhibitory. Same neurotransmitter can have different effects
Excitatory postsynaptic potential (EPSP)-
Na+ gates are open and membrane becomes depolarized- can cause an action potential
Inhibitory postsynaptic potential (IPSP)-
K+ gates open, and membrane becomes hyperpolarized. This makes it more difficult to generate action potential.
Central nervous system (CNS)-
includes brain and spinal cord
Peripheral nervous system (PNS)-
includes sensory receptors and nerves
Somatic division-
PNS. receptors and nerves concerned with changes in external environment
Autonomic division-
PNS. receptors and nerves that regulate internal environment
Sensory receptors
structures that detect information about changes in the internal or external environment. Sense organs contain the sensory receptors
Types of Sensory Receptors
Mechanoreceptors, Chemoreceptors, Photoreceptors, Thermoreceptors
Mechanoreceptors-
recognize mechanical energy-touch, pressure, gravity, stretching, movement
Chemoreceptors
recognize chemicals (tongue, nose)
Photoreceptors
recognize light (eyes)
Thermoreceptors
recognize heat (skin)
Sensory adaptation-
even if stimulus continues at same intensity, the frequency
of action potentials decrease. Some sensory receptors adapt slower- ex- receptors for cold and pain o Some adapt rapidly- ex- pressure and smell
Eye: Lens
Lens bends light rays and brings them to a focus on the retina. covered by a thin membrane called the cornea
curve of lens
The curved surface of the lens and cornea bends the light rays in a pattern that is upside down and reversed left-right
iris
Iris is a ring of smooth muscle that regulates the size of the pupil, thus regulating the amount of light entering.
Retina
Retina lines the posterior 2/3 of the eye. Contains photoreceptor cells: Rods/Cones
Rods-
125 million- function in dim light, not sensitive to color
Cones
Cones- 6.5 million- function in bright light, responsible for color vision
Optic nerve
passes out of eyeball and transmits signals to thalamus. Organization in the brain produces the sensation of sight.
