- Central Nervous System (CNS)
CNS
Center of the nervous system
Brain & Spinal cord
Integrates sensory information and initiates motor responses
Includes ascending and descending neuron
2. PNS
Outside the CNS
Cranial nerves & Spinal nerves
Includes afferent and efferent neurons
3.Basic function of nervous system
Sensation
Integration
Response
4.Sensation
Receiving information about the environment to gain input (conscious perception) about what is happening outside the body or inside the body
5.Taste and smell
Chemical senses
6.Touch
physical or mechanical stimuli
7.Sight
light stimuli
8.Hearing
Sound waves
9.Where could sensory stimulus from inside the body come from?
Sensory stimuli from inside the body could be from stretch receptors in organ walls or from the blood concentration of certain ions
Steps of integration
-Received stimuli from the senses is communicated to the nervous system where the information is processed
-The stimuli are compared or integrated with: other stimuli, past stimuli, or the state of the person at the particular time
-Specific responses will be generated
Response
Production of a response on the basis of stimuli perceived by sensory structures
2 forms of response
- 2 Forms
- Voluntary (Somatic Nervous System)
- Involuntary (Autonomic Nervous System)
Voluntary
contraction of skeletal muscle
Involuntary
contraction of smooth muscle, regulation of cardiac muscle, activation of glands
Somatic nervous system
Conscious perception
Voluntary motor response: contraction of skeletal muscle
Includes reflexes that happen without conscious decision & motor responses that become automatic as a person learns motor skills
Autonomic nervous system
Involuntary control of the body, regulates the organs systems usually for the sake of homeostasis but can also be due to an emotional state
Sensory input can be internal or external
Motor output is from smooth & cardiac muscle, & glandular tissue
enteric nervous system
Controls smooth muscle and glandular tissue in the digestive system
Part of the autonomic nervous system & part of the PNS
Can operate independently of the CNS
Glial cells
variety of cells that support the neurons & their activities
Neurons
most functionally important cells
Contain a cell body (soma) & processes called axons & dendrites
Neurons pt.2
Conduct nerve impulses to communicate information about sensations, produce movements, & induce thoughts
Have extreme longevity
Cannot divide, so cannot be replaced
High metabolic rate, cannot survive without oxygen
Neurons cell body (soma)
synthesizes proteins (Nissl bodies), many mitochondria
Dendrites
conduct nerve signals to the cell body
Axon fibers
sometimes covered by a myelin sheath, conducts nerve impulses away from the cell body
Axon hillock
area between cell body & axon
Axon terminal
ending of axon branches, contain mitochondria & vesicles (with neurotransmitters)
synaptic end bulb
enlargement at end of axon terminal, connects with target cell at a synapse
input zone of neuron
Dendrites and cell body
Summation zone
axon hillock - incoming nerve impulses combine
Conduction zone
axon - contains many voltage-gated Na+ & K+ channels
Output zone
distal end of axon- contains many voltage-gated Ca2+ channels
Multipolar
Multipolar - single axon & many dendrites, most common (found in motor neurons)
Bipolar
Bipolar - 2 processes separated by cell body (rare: found in retina & olfactory mucosa)
Unipolar
Unipolar - 1 process with cell body off to the side, (found in sensory neurons of PNS)
4 major types of CNS
Astrocytes
Microglia
Ependymal cells
Oligodendrocytes
2 major types of PNS
Schwann cells
Satellite Cells
Oligodendrocytes
Found in the CNS
Hold nerve fibers together and produce the myelin sheath (phospholipid insulating cover)
1 cell myelinates many axons
Myelin sheath
Insulates, increases speed of action potentials, & holds neurons together
ONLY in the PNS, there are gaps in the myelin sheath: Nodes of Ranvier
Loss of myelin (demyelination) can cause a loss of sensation (numb) & motor control (paralyzed)
White matter
is myelinated
PNS: myelinated nerve
CNS: myelinated tract
Grey matter
cell bodies and dendrites are unmyelinated
PNS: ganglion (soma)
CNS: nucleus or ganglion (soma)
Astrocytes
Found in the CNS
Star shaped
Largest and most numerous
Connect to both neurons and capillaries, creating a frame-work
Transfer nutrients from the blood to the neurons
Trap leaked K+ & neurotransmitters
Form blood-brain barrier (BBB)
Repair damaged neural tissue
Microglia
Found in the in CNS
Least numerous and smallest glial cells
Phagocytic cells derived from monocytes
Small, usually stationary
In inflamed brain tissue, they enlarge move about and engulf pathogens, waste, & debris
Ependymal cell
Found in the CNS
Resemble epithelial cells
Form thin sheets, lining fluid-filled cavities (ventricles)
Aid in circulation of fluid with cilia
Shwann cells
Found in the PNS
Also called neurolemmocytes
Support nerve fibers and form myelin sheaths
Wrap around a portion of only one axon
Essential for nerve regrowth
Satellite cells
Cells that cover and support cell bodies (sensory & autonomic ganglia) that function like astrocytes but do not play a role in the BBB
Multiple Sclerosis (MS)
Autoimmune disease
Causes inflammation & destruction of myelin in the CNS
As the insulation (mylin sheath) is destroyed, scarring becomes obvious
"Multiple scars" are found in the white matter
Symptoms include both somatic & autonomic deficits in muscle control
Guillian-Barre Syndrome
Demyelinating disease of the PNS
Also an autoimmune reaction
Sensory & motor deficits are common
Autonomic failures can lead to heart rhythm changes, or drops in blood pressure, especially when standing up
Sensory receptor steps
If the stimulus is strong enough to reach threshold an action potential will travel down the axon
When it reaches the end bulbs it releases a neurotransmitter
The neurotransmitter binds to receptors on the target neuron, which then has its own action potential
The 2nd neuron synapses with a neuron in the thalamus, which then sends the information to the sensory cortex
Response to stimuli from brain
In cerebral cortex, information is processed (integrates the stimulus with your emotional state & memories)
A plan is developed & the motor cortex will send out a command to your skeletal muscles
An upper motor neuron synapses with a lower motor neuron
The lower motor neuron releases Ach onto the muscle fiber & the action potential begins the muscle contraction
Electricity parts
Current = Voltage
Resistance
Voltage is the potential difference
Ions differences create voltage
Resistance is provided by the plasma membrane
All living cells maintain a difference in the concentration of ions across their membranes
Protein channels
Channels are selective to which ions they allow through
Mechanically-gated:
open in response to physical deformation of receptor (sensory), when pressure is applied channels open & ions enter the cell
Chemically-gated (ligand-gated):
open when the right chemical binds, ions then cross the membrane changing its charge. Also called ionotropic receptors because it allows ions to enter or leave the cell.
Voltage -gated channels:
open and close in response to changes in membrane potential, Normally the inner portion is negative & when it becomes less negative ions cross the membrane
Leakage channels:
opens & closes at random, contribute to the resting membrane voltage of excitable membranes
Membrane potential
Potential = distribution of charges across a cell membrane
Measured in millivolts (mV)
Compares the inside charge to the outside charge
The membrane surface has a slight difference in ion charges & this allows neurons & muscle cells to generate electrical signals
Resting Membrane Potential (RMP)
Polarized Membrane:
plays most important role
Leaky channels allow K+ to leak out and lower amounts of Na+ to leak in
Magnitude of potential difference: measured in millivolts (-70mV)
Maintained by Na+/K+ pumps
sodium potassium pump
Pumps 3 Na+ out of cell & 2 K+ into cell
Works against concentration gradient
ATP required for energy to run pump
Cells become polarized
Depolarization
starts with the opening of Na+ channels
rushes into the cell, changing the membrane potential to become less negative
Membrane potential reaches +30mV
Repolarization
Na+ channels close & K+ channels open
leaves the cell, voltage moves back towards -70mV
Hyperpolarization
K+ channels are slow to close & the membrane potential go below -70mV
What initiates action potential
Ligand-gated channels open when a neurotransmitter binds to it
Mechanically-gated channels open when a physical stimulus affects a sensory receptor
Voltage-gated channels open when the membrane potential is -55mV
All of Nothing Principle
A stimulus either triggers an action potential or it does not
Once threshold is reached an action potential will occur!!
The magnitude is the same no matter how it reached threshold
absolute refractory period
when the membrane is depolarizing
Will not respond to a stimulus, no matter how strong
relative refractory period
when the membrane is repolarizing
Membrane will respond only to a very strong stimulus
Coding for stimulus intensity
All action potentials are alike
Strong stimuli generate impulses more often
Frequency gives strength not amplitude of individual impulses
Propagation of an action potential
Continuous conduction"
Action potential starts in the beginning of the axon
Contains a high density of Na+ channels, rapidly depolarizes
As depolarization spreads, more Na+ channels open & it travels down the axon
Never moves backward
Faster Speed: large diameter, presence of myelin sheath, & increased temperatures
Saltatory conduction
Occurs in myelinated axons
Action Potentials occur only at Nodes of Ranvier
Flow under myelin sheath to next node
"Leaps" from node to node
Faster & saves ATP (less surface area to pump Na+/K+ across)
graded potentials
Local changes in membrane potential
Usually occur in dendrites
Amount of change determined by the size of the stimulus
Can be either depolarizing or hyperpolarizing
or Ca2+ enter (depolarize) or
or Cl- leave the cell (hyperpolarize)
generator potential
Develop in dendrites
In sensory neurons:
Both free nerve endings & those that are encapsulated
receptor potential
Graded potentials in their membranes result in the release of neurotransmitters at synapses
In sensory neurons: Both taste cells & photoreceptors of the retina
Happens only in cells that act as receptors that communicate with sensory neurons
post synaptic potential
Graded potential in the dendrites of a neuron that is receiving synapses from other cells
Can be depolarizing or hyperpolarizing
EPSP) Excitatory post synaptic potential:
causes cell to move towards threshold
(IPSP) Inhibitory post synaptic potential:
causes cell to move away from threshold
Summation
Summate = add together
Occurs at the axon hillock (except for sensory neurons)
Combined effects of graded potentials
If membrane depolarizes, the membrane will reach threshold
Can be either spatial or temporal
electrical synapses
Cells joined by gap junctions
Allows electrical currents to flow between cells
Found in cardiac muscle & single-unit smooth muscle (shown)- remember, intercalated discs in cardiac muscle have gap junctions and desmosomes!
Chemical synapse
Involves the transmission of a chemical from one cell to another.
The release of acetylcholine is an example of a chemical synapse.
The synapse
connection between a neuron & its target cell
Ionotropic
ligand-gated
Metabotropic
involves a complex of proteins resulting in metabolic changes in the cell
nicotinic receptors
Causes depolarization
muscarinic receptors
Causes depolarization or hypopolarization
metabotropic receptors
Two common second messenger systems use cAMP and IP3 (inositol triphosphate)
Second messengers cause metabolic changes within the cell -hence the name metabotropic receptors
The effector protein catalyzes the second messenger
Alzheimer's disease
Progressive degenerative brain disorder that results in dementia
Due to misfolded proteins that form plaques & tangles that kill neurons
Brain shrinks
Parkinson's disease
Strikes mostly in the 50s & 60s
Degeneration of dopamine releasing neurons of the substantia nigra
Basal nuclei become overactive
Persistent tremor at rest, shuffling gait, & stiff facial expressions
Treated with L-dopa - but it becomes ineffective, deep brain stimulation can alleviate tremors
Huntington's disease
Fatal hereditary disorder
Strikes during middle age
Mutant protein accumulates in brain cells & the tissue dies
Massive degeneration of the basal nuclei & cerebral cortex
Produces "chorea" - wild, jerky, continuous flapping non-voluntary movements (hyperkinetic)
Disease is progressive & leads to mental deterioration
Usually fatal within 15 years
Huntington's Disease Chorea
Drugs used to treat it: block dopamine's effects
Creutzfeldt-Jakob disease
Human variant of prion disease - "mad cow disease"
What responses are generated by the nervous system when you run on a treadmill? Include an example of each type of tissue that is under nervous system control.
Running on a treadmill involves contraction of the skeletal muscles in the legs, increase in contraction of the cardiac muscle of the heart, and the production and secretion of sweat in the skin to stay cool.
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When eating food, what anatomical and functional divisions of the nervous system are involved in the perceptual experience?
The sensation of taste associated with eating is sensed by nerves in the periphery that are involved in sensory and somatic functions.
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Multiple sclerosis is a demyelinating disease affecting the central nervous system. What type of cell would be the most likely target of this disease? Why?
The disease would target oligodendrocytes. In the CNS, oligodendrocytes provide the myelin for axons.
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Which type of neuron, based on its shape, is best suited for relaying information directly from one neuron to another? Explain why.
Bipolar cells, because they have one dendrite that receives input and one axon that provides output, would be a direct relay between two other cells
Sensory fibers, or pathways, are referred to as "afferent." Motor fibers, or pathways, are referred to as "efferent." What can you infer about the meaning of these two terms (afferent and efferent) in a structural or anatomical context?
Afferent means "toward," as in sensory information traveling from the periphery into the CNS. Efferent means "away from," as in motor commands that travel from the brain down the spinal cord and out into the periphery.
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If a person has a motor disorder and cannot move their arm voluntarily, but their muscles have tone, which motor neuron—upper or lower—is probably affected? Explain why.
The upper motor neuron would be affected because it is carrying the command from the brain down.
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What does it mean for an action potential to be an "all or none" event?
The cell membrane must reach threshold before voltage-gated Na + channels open. If threshold is not reached, those channels do not open, and the depolarizing phase of the action potential does not occur, the cell membrane will just go back to its resting state.
The conscious perception of pain is often delayed because of the time it takes for the sensations to reach the cerebral cortex. Why would this be the case based on propagation of the axon potential
Axons of pain sensing sensory neurons are thin and unmyelinated so that it takes longer for that sensation to reach the brain than other sensations.
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If a postsynaptic cell has synapses from five different cells, and three cause EPSPs and two of them cause IPSPs, give an example of a series of depolarizations and hyperpolarizations that would result in the neuron reaching threshold.
EPSP1 = +5 mV, EPSP2 = +7 mV, EPSP 3 = +10 mV, IPSP1 = -4 mV, IPSP2 = -3 mV. 5 + 7 + 10 - 4 - 3 = +15 mV.
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Why is the receptor the important element determining the effect a neurotransmitter has on a target cell?
Different neurotransmitters have different receptors. Thus, the type of receptor in the postsynaptic cell is what determines which ion channels open. Acetylcholine binding to the nicotinic receptor causes cations to cross the membrane. GABA binding to its receptor causes the anion chloride to cross the membrane.