Wednesday, May 1, 2024

Central Nervous System (CNS)

  1.  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.



We have an expert-written solution to this problem!

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.



We have an expert-written solution to this problem!

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.



We have an expert-written solution to this problem!

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.



We have an expert-written solution to this problem!

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.



We have an expert-written solution to this problem!

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.

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