Science Guide Notes

Incorrect answers:

Which lobe of the cerebrum is responsible for pain?

• parietal lobe

Which of the following is the location where the CSF is reabsorbed by the body in the ventricular system?

• meninges

At what membrane potential does a neuron voltage-gated sodium channels open?

• -55mV

What is the process called when multiple EPSPs (or IPSPs) occur in rapid succession at the same synapse and are summed up over time?

• temporal summation

A voltage-gated potassium channel named for its lag in returning the neuron membrane potential to rest following after hyperpolarization is known as which of the following?

• delayed rectifier

What is the receptive field for a sensory receptor?

• the place where a stimulus can be detected by a sensory receptor

Which of the following lists the approximate membrane potential that the falling phase of the action potential stops at during the undershoot?

• -80mV

The map of receptive fields in the LGN is the same as it was in which of the following presynaptic regions?

• retina

The __________________ has a dual effect, both exciting the direct pathway while simultaneously inhibiting the indirect pathway.

• substantia nigra

According to the USAD Science Resource Guide, most input to V1 goes to which layer?

• layer 4C

Which membrane helps the tips of the stereocilia of the hair cells create the back-and-forth movement of the cilia required for sound transduction?

• Tectorial membrane

Which of the following are known as the bundles of axons that connect the brain to the face and head?

• cranial nerves

Which of the following is the point where the axons from both eyes come together?

• optic chiasm

What is the primary route for visual information to leave the eye?

• optic tract

Which of the following is a common method to evoke sensations in a missing hand or arm for amputee patients?

• touching the face

Which of the following neurotransmitters is under-produced by weak or malfunctioning neurons resulting in Parkinson’s disease?

• dopamine

Critical periods can be seen not just in visual development, but also in other areas, such as auditory language development. Critical periods in development are short and usually take place during which part of one’s development in life?

• within the first few weeks to months of life

What is the main advantage of magnetic resonance imaging (MRI) over electroencephalogram (EEG)?

• Higher spatial resolution

Which of the following causes the alignment of hydrogen atoms in the body during an MRI scan?

• introduction of strong magnetic field

Scientists used the method of transcranial magnetic stimulation for what purpose?

• to confirm findings from lesion studies and imaging studies

Introduction

• neuroscience = study of nervous system

• main role of nervous system = communication

• more than a thousand disorders of the brain

• field is relatively new so won’t know everything

Section 1

• multidisciplinary nature = molecular bio, cell bio, physiology, chem, physics, psych, comp sci, cognitive sci, and math

Neurons

• neuron = nerve cell

• found in nearly all species of animals

• new neurons can’t be made easily as old ones die off

• most neurons humans will have are in place when we’re born

• connections between neurons continue to change

Parts of the Neuron

• DNA in nucleus

• nucleus found in cell body/soma (other organelles also located here

• contains cytosol and separated from outside by neuronal membrane

• dendrite = receiving part (word comes from Greek word for “treelike”)

• dendrite bring in input, function like antenna

• generally have one long axon (serves as the “wire” transmitting output signals from neuron)

• axon is of uniform thickness and can extend as far as several feet in mammals (half length of your entire body)

• some axons branch to other regions of brain/body

• no organelles in the axon

• myelin formed by glial cells, which insulate axon (aids in sending of signals)

• myelin is similar to plastic/rubber insulation on a copper wire (serves to speed up conduction of neuron’s electrical signals

• small gaps in myelin = nodes of ranvier (allow axon inside to gain access to fluid outside which is important for sending electrical signals)

• axons begin at soma specifically axon hillock and end at slightly wider sections called axon terminals

• axon terminal = where signal/neurotransmitter is released to next neuron

• signal received by dendrites on other neuron

• point where a neuron connects w/ another = synapse

• neurons are not directly connected

• axon terminal of presynaptic neuron is filled with neurotransmitters

• neurotransmitters produced in soma then sent down neuron

• when time is right neurotransmitters float across synaptic cleft and bind to specialized receptors on postsynaptic neuron

• method of signaling across synaptic cleft is highly customizable

The Morphology of Neurons

• morphology/shape may vary depending on what job the neuron has

• various physical forms of neurons

• computer modeling shows dendritic arbor relates to the region that where neuron can get info and how that neuron can send signals to other neurons

• motor neurons = movement commands to muscles

• motor neurons: soma at one end, surrounded by small set of dendrites, large axon going to muscle

• sensory neurons = sensory input from body to brain

• sensory neurons: dendrites on both ends connected by long axon w/ a cell body on a branch in the middle

• interneurons = connect other neurons within the nervous system

• interneurons = small, no long axons, no large complex dendritic arbors

Grey Matter and White Matter

• central nervous system divided into 2 types of tissues (grey and white matter bc of the way they look)

• grey matter = cell bodies and dendritic arbors of neurons

• white matter = myelinated axons

• myelin = fatty tissue that looks white w/out stains

• central nervous system tend to be clustered

• cell bodies and dendrites in same location, axons run along same pathway

• clusters of axons are lighter than clusters of cell bodies and dendrites which appear a darker grey or tan depending on if brain is preserved or fresh

• brain = grey outside and white inside

• spinal cord = grey inside and white outside

Glia

• non-neuronal cells found in nervous system

• new glia cells produced throughout the life

• types: astrocytes, oligodendrocytes, Schwann cells, and microglia

Oligodendrocytes and Schwann Cells

• both myelinating glia (form myelin sheath)

• oligodendrocytes = myelinate axons in central nervous system (brain and spinal cord)

• Schwann cells = myelinate axons in the peripheral nervous system (tissues, skin, muscles, internal organs, etc.)

Astrocytes

• most common type of glial cell

• named bc star shaped

• regulate chemical makeup of extracellular fluid in space between cells

• removes excess signaling molecules and maintains proper balance of ions

• also can react to tissue damage

• send out long extensions to wrap around blood vessels in brain to form part of blood brain barrier

• blood brain barrier keeps substances in blood stream from entering brain (preventing infections)

• blood brain barrier makes it difficult for nutrient molecules to nourish brain cells and to deliver drugs to help treat diseases or disorders of nervous system

• once thought glia were passive (acting like glue)

• glial cells have active role in modifying neural activity by helping modulate signaling in the synapses

• astrocytes can even alter signals that neurons send and receive plus help to keep those neurons nourished and supported

Microglia

• smaller than other glial cells

• immune cells of the brain

• important bc central nervous system + brain are isolated from body’s immune system since there is a blood brain barrier

• clear away pathogens and damaged/dead neurons, ingesting and destroying them like macrophages in body’s immune system

• evidence shows hyperactivated microglia cause inflammation in brain (may play a role in some neurodegenerative diseases, such as Alzheimer’s disease)

Anatomy of the Central Nervous System

• shapes/forms of cells and structures in the nervous system are often directly related to their functions

The Meninges

• central nervous system = encased in bone (brain and spinal cord)

• 3 layers of meninges surround central nervous system and protect it bc tissues don’t come into direct contact with the nervous system

• dura mater: latin for “tough mother”, thick and leathery, outermost layer

• epidural space is outside dura mater between it and the skull, which contains fats to absorb shock (can be useful site to inject meds)

• arachnoid membrane: just inside dura mater , long stringy components like spider webs, usually right next to dura mater but if there’s trauma then blood can pool into subdural space

• subarachnoid space: just below arachnoid membrane, larger, filled w/ cerebrospinal fluid (cushions and protects brain), allows blood vessels room to access brain (deliver nutrient and oxygen to neurons and glial cells)

• pia mater: innermost layer, latin for “gentle mother", very thin, follows every bump and groove of brain closely, keep cerebrospinal fluid outside brain, allow blood vessels to pass thru and access the brain

The Lobes of the Brain

• cerebrum (largest and most notable component of human brain) is made up of two symmetrical cerebral hemispheres (left and right side)

• cerebrum has wrinkly appearance

• gyri (gyrus) = bumps on brain

• sulci (sulcus) = grooves or folds

• outermost layers of cerebrum are called cerebral cortex, huge proportion of the cell bodies or some of the brain’s neurons are found here

• if ironed and spread out it’d have a large surface but it’s wrinkled to fit in our skull

• patterns of gyri and sulci are similar among everyone so we can identify specific locations on the surface and divide the brain up into lobes

• four lobes: frontal, parietal, occipital lobe, and temporal

• frontal: largest, located in front (just behind forehead), regions related to complex thinking or cognition (prefrontal cortex), back edge contains motor cortex (plans and sends instructions for movement)

• parietal: just behind frontal lobe, process sense of touch, temp, pain, and body position, important for understanding spatial relationships

• occipital: back of cerebrum, process and analyze visual info from eyes

• temporal: sides of head, near temples, auditory sound and speech processing, music, memory, and object recognition, several important regions of grey matter found here under the cortex

• cerebellum: behind and underneath cerebrum, wrinkly cortex of its own, latin for “little brain” (miniature cousin of brain), contains more neurons than cerebrum, concerned w/ motor system (learn to make smooth and coordinated movements and to learn new kinds of movement)

Subcortical Structures

• below cerebral cortex

• following regions found in both left and right hemisphere

• amygdala: found in temporal lobes, Greek word meaning “almond” because of its appearance, major component of the limbic system, studies on neural basis of emotional memory often focus on amygdala and circuitry found within its subregions of grey matter or nuclei

• hippocampus: long-term memory, spatial navigation, part of limbic system, takes name from mythological version of sea-horse (upper body of horse and lower body of fish) since sea-horse shaped, among first areas to suffer cell death in Alzheimer’s disease leading to memory deficits, popular target for scientists to study in order to understand how circuitry of brain can change based on experience, damage to both hippocampi results in a subject being unable to create new long-term memories or change short-term memories into most stable long-term memories (memory consolidation) which is a favorite topic of Hollywood movies such as Memento and 50 First Dates

• thalamus: very center of each hemisphere, relay station between body and brain, all input from body and sense must pass here before going to the brain except smell which has a direct projection to amygdala and doesn’t stop here, plays a role in consciousness, emotional behaviors, motor control, and cognition

• hypothalamus: below thalamus and neighbors with pituitary gland which release important growth hormones, works to mediate action of pituitary gland and many unconscious functions of the brain that are critical for homeostasis, functions carried out by hypothalamus (regulating body temp, blood pressure, hunger and thirst, sleep and wakefulness, etc.) are crucial to survival, provides link between hormones release by pituitary gland and endocrine system and the many targets of those hormones in the nervous system

• basal ganglia: cluster of nuclei found in center of hemispheres around thalamus, structures comprising basal ganglia are caudate nucleus, putamen, globus pallidus, subthalamic nucleus and substantia nigra (technically located in midbrain) are also included, helps planned voluntary movements get started and prevents unwanted or excess movement from occurring, Parkinson’s disease has neurons of substantia nigra degenerate and die off causing problem in main circuit of basal ganglia, plays other roles like motor learning, cognition, behavior, and emotion

The Brainstem

• located very bottom of brain and connects brain to spinal cord

• thalamus is scoop of ice cream on the ice cream cone that is the brainstem

• control critical functions like breathing, heart rate, coordination, and reflexes

• three subdivisions of brainstem: midbrain, pons, and medulla

• midbrain: topmost part, contains nuclei to help regulate eye movements and pupillary light reflex, regions that help integrate visual info from eyes with auditory info from the inner ear into map of environment

• pons: latin word meaning “bridge”, connects midbrain and medulla, contains nuclei for coordinating facial movements, chewing muscles, hearing, and balance

• medulla: aka medulla oblongata (long and rectangular), bottom-most part, contains nuclei for relaying touch sensations form the face, swallowing food, vomiting, regulating blood pressure, heart rate, breathing, etc.

• all pathways made of many axons that go between brain and body must pass here, so brainstem contains regions of both grey and white matter

The Spinal Cord

• other main section of central nervous system

• encased in vertebrae that make up the spine

• located below brainstem and serves to conduct info between brain and body (ex. touch, pain, temp, and body position from body and motor commands from brain

• butterfly-shaped section of grey matter and white matter on the outside

• cell bodies in grey matter help relay sensory info into and out of the many pathways that run up and down the spinal cord in the white matter

• cell bodies also responsible for spinal reflexes (soma does all work that brain isn’t even needed at all)

• one long continuous structures but divided up into sections

• cervical spinal cord: top section, innervates arms, neck, and shoulders

• thoracic spinal cord: next section after cervical, carries info to and from the chest and torso

• lumbar spinal cord: next section after thoracic, innervates the hips and fronts of the legs

• sacral spinal cord: next section after lumbar, innervates buttocks, backs of the legs, and genitalia

• nerves run between each vertebra and carry info to and from that section of spinal cord

• enlarged in both cervical and lumbar areas bc increased amount of skin and muscle those regions must target

The Ventricular System

• series of open holes/cavities inside central nervous system

• filled with cerebrospinal fluid (CSF) which is the same fluid that brain floats in

• each cerebral hemisphere contains one large lateral ventricle, connected near left and right thalamus at third ventricle (just along midline of the brain)

• narrows and becomes cerebral aqueduct in top of brainstem (in midbrain) then widens again to become fourth ventricle in pons and medulla

• in spinal cord CSF can be found in narrow tube in the middle called the central canal

• at bottom of spinal cord, CSF leaves inside of central nervous system and joins the fluids outside the brain in the meninges

• spongy material called choroid plexus lines each of the ventricles and produces new CSF, old CSF is reabsorbed by the body via the meninges in a continuous cycle

• CSF, a salty fluid, helps the brain float preventing it from being on floor of skull and deforming and damaging neurons (buoyancy also protects brain from impacts to head)

• CSF is continuously flushing thru central nervous system which is helpful for getting rid of old signaling chemicals so they don’t build up and for washing away toxins/infectious particles

The Anatomy of the Peripheral Nervous System

• Peripheral Nervous System consists of nerves/axons and some clusters of cell bodies called ganglia that leave central nervous system to relay info back and forth between brain and body

• helps brain stay in touch w/ physical environment and allows us to respond to it

• not protected by bone so more easily damaged, but can also more easily regenerate

• also not protected by blood brain barrier so more open to infection and toxins

• divided into somatic and autonomic nervous system

The Somatic Nervous System

• composed of all the axons leaving and entering the spinal cord that bring info to and from the tissues of your body

• also part of this system are the nerves (axons) that are found above the spinal cord that bring motor commands to the muscles of the head and neck or bring sensory input from those regions

• under voluntary control, we are conscious of the sensations and movements

The Autonomic Nervous System

• controls involuntary (unconscious) responses to regulate all kinds of aspects of the brain and body

• Greek word “autonomia” meaning “independence”

• functions w/out conscious effort

• specifically regulates the functions of our internal organs

• both divisions of the autonomic nervous system (sympathetic and parasympathetic) target the same organs, but have opposing effects and use different chemical signals

The Sympathetic Division

• responsible for the “fight or flight” response (activated by adrenal glands producing stress response and readying body to respond to it)

• same neural circuits and signals are activated in all types of cases

• each circuit starts w/ a neuron in central nervous system (generally found in thoracic and lumbar areas of spinal cord) whose axon extends out of vertebral column

• first neuron quickly synapses onto another neuron’s cell body in a specialized chain of ganglia located right next to the spine called sympathetic chain

• send signal to all organs very quickly with just one input

• sympathetic chain neuron’s axon leaves chain ganglion and travels to a specific target organ to yield a response

• targets include the heart (to increase heart rate and blood pressure), the lungs (to

  increase breathing rate), the digestive system (to halt production of bile), the secretory glands (to decrease saliva and tear production and increase sweat production), and the eyes (to dilate the pupil to allow more light into the eyes)

• these changes make organism more responsive to threats and less focused on general body maintenance and homeostasis

• effective but should only be activated for short periods of time

• long-term activation of the sympathetic nervous system can lead to chronic stress and damage to the organs and other systems of the body

The Parasympathetic Division

• responsible for “rest and digest”

• organism is focusing on digesting food, growth and cell division, immune responses, energy storage, and other aspects of maintaining homeostasis

• areas of the central nervous system that give rise to the neurons involved are the brain stem and sacral spinal cord

• the ganglia involved (where a neuron coming from the central nervous system meets the next neuron) are spread all over the body and are not all located in one place

• parasympathetic ganglia can usually be found very close to the organ that the second neuron in the sequence is targeting

• not efficient at sending quick signals to all organs at same time but instead is specialized for signaling organs individually to specialize the input to them

• keeps each of the systems of the body in balance

• parasympathetic system has inverse effect of the sympathetic nervous system

• sympathetic and parasympathetic nervous systems tend to be inhibitory or reduce the function of the other (goals are opposite and incompatible, but both are crucial for an organism’s survival

• each system also uses different chemicals to send signals to the target organs to help keep each system’s messages distinct

Section 2

• like many other types of cells neurons can send signals by releasing chemicals, but smth unusual is that they can also send electrical signals very long distances across the body

• electrical signals sent by the nervous system are mediated by the flow of charged atoms across the neuron’s cell membrane

• neuron might be in a particular state depending on which ions are either inside or outside the cell

• membrane potential is measure in millivolts

• resting potential and action potential function like the off and on states of a light switch

• membrane potentials are not fixed or static they are constantly changing

The Resting Membrane Potential

• electrical charge inside the neuron in resting state is called the resting membrane potential

• when neuron or other cell that can send electrical signals is not doing so, the cell is said to be at rest

• three main players that dictate the resting potential: cytosol, extracellular fluid, and the neuronal membrane

• cytosol and extracellular fluid are both mostly water w/ some charged ions dissolved in the water (sometimes properties are similar but also sometimes they’re different)

• neuronal membrane is a selective phospholipid bilayer (allows amount or concentration of ions to be different inside vs outside the cell bc ions cannot flow freely across membrane due to its selectivity

• membrane also allows the electrical charge of the liquid inside versus outside the cell to be different

• difference in concentration is key for communication

• difference in ion concentration and in electrical charge on the inside compared to the outside of the cell

• membrane acts like a wall preventing the ions from going into and out of the cell as they may like (instead ions must find an opening in the membrane to go thru)

• if there is an opening or channel for a particular ion to pass from one side of the membrane to the other, we say that the membrane is permeable to that ion

• passive transport is described as passive since it requires no expenditure of energy by the neuron

• two natural forces at play in a neuron that contribute to ion movement across the membrane are ionic concentration (or diffusion) and electrical charge (or potential)

Ionic Concentrations

• when learning about the states or potentials that a neuron might be it is crucial to understand the importance of ionic concentration

• if an ion is found in a high concentration in one area, it will tend to move, or diffuse, to an area of lower concentration of that particular ion

• a gradient is a measurement of how much something changes as you move from one region to another

• concentration gradient as a measurement of how the concentration of something changes from one place to another

Electrical Potentials

• a second force that is important to the passive transport process is electrical charge or electrical potential

• ions have either a negative or a positive charge

• positively charge particles will be attracted by negatively charge particles

• positively charged particles will be repelled by other positively charged particles

• ions in the extracellular fluid and inside the cell work like this and this force results in ions moving from one area to another

• an ion that is on one side of the membrane where there are many other like charges will be propelled by force to the other side, if the membrane is permeable to that ion

Ion Channels and Pumps

• ion channels are proteins that form specialized openings in the neuronal membrane through which ions can pass (these channels are like specialized gated in a fence)

• many types of ion channels are specific for a particular type of ion

• other types of ion channels might allow a variety of ions to pass through

• another way to classify types of ion channels is by what triggers them to open, a process known as gating

• channels that are gated can be opened and closed due to changes in the local environment of that area of the membrane

• generally, voltage-gated channels open when the voltage changes from the resting membrane potential

• ligand gated channels open when a particular molecule binds to a specific location on the channel

• binding of that specific molecule triggers a change in its shape or conformation, opening the channel and allowing ions to flow thru (chemical molecule is bound for a certain amount of time so eventually the channel will change again and close)

• intensity of the flow of ions thru the channel is based on the concentration gradient or the electrical potential (difference) across the membrane

• active transport is when an ion is actively moved or forced to go against either the concentration gradient and/or the electrical gradient (requires energy)

• main example of active transport in a neuron is a pump

• sodium-potassium pump is crucial in neurons for establishing resting membrane potential (it breaks down ATP to change its shape or conformation

• in ATP’s dephosphorylated state, binding 3 sodium ions form the cytosol and releasing any bound potassium ions, and in ATP’s phosphorylated state, binding two potassium ions from the extracellular fluid and releasing any bound sodium ions

• sodium-potassium pump exchanges sodium for potassium, always moving sodium out and potassium in (against their concentration gradients, setting up large concentration gradients when neuron is at rest)

• Na+ is majority outside and K+ is majority inside cell

• ion pumps are always working in the background of whatever else the cell is doing

• charge of a cell can be measured w/ a pointy electrode stuck inside the neuronal membrane, which is then compared with the ground electrode sampling of the charge from the extracellular fluid

• at rest, the electrical potential of most neurons is -60 to -80 mV (that much more negative than the outside of the cell due to potassium leak channels, which are always open)

Equilibrium Potential

• neuron’s resting potential is primarily determined by the movement of potassium across the membrane

• when only potassium leak channels open only K+ crosses the membrane and here the membrane potential is zero (same charge inside and outside the cell)

• Potassium ions leave thru open potassium channels (moving down the concentration gradient) taking its positive charge with it and making the inside of the cell more negative as more positive ions leave, resulting in positive charge building up outside cell membrane while negative charge builds up inside, creating the potential (the electrical difference between the inside and the outside of the cell

• electrical potential across the cell membrane eventually becomes high enough that the positive charge building up outside the cell (the electrical driving force) will start to repel the other positive potassium ions, driving some of them back into the cell via the same open channel

• when the electrical driving force is equal to the chemical driving force then the cell is at equilibrium (no net movement of ions bc for every K+ that leaves the cell, another one enters the cell)

• membrane potential at which a particular ion reaches equilibrium is referred to as its equilibrium potential

• each ion has a different equilibrium potential

• equilibrium potential for potassium is quite negative and since nearly all channels open at rest are potassium channels, the resting potential is also quite negative and close to the equilibrium potential for potassium

• the electrical potential and the chemical gradient are proportional to the difference between the inside and outside of the cell

The Nernst Equation

• the exact value of the equilibrium potential for a given ion can be mathematically calculated using the Nernst equation

• takes into account factors such as the charge of the ion in question, the temp of the cell, and the ratio of the external and internal concentrations of that ion

The Goldman Equation

• starts from the Nernst equation

• factors in the relative permeability of the cell to multiple different ions

• calculates what the membrane potential will be if there is permeability to more than one ion at a time

The Action Potential

• the electrical potential across the neuronal membrane when the cell is active and firing

• brief and drastic change in the membrane potential of a neuron

• lasting on the order of a few milliseconds, it’s caused by the rapid opening and closing of ion channels

• also referred to as spikes, nerve impulses, or APs

The Phases of the Action Potential

• before and after an action potential, a neuron is at its resting membrane potential (-60 to -80 mV)

• rising phase: first phase, membrane potential becomes more and more positive due to an influx of positive ions aka depolarization

• as membrane potential becomes more positive and it becomes closer to the charge outside the cell, the internal membrane potential approaches zero (charge inside = charge outside), so less polarization or difference between inside and outside of cell as the inside becomes more positive

• membrane potential continues becoming more positive (even after membrane potential is reached) until it becomes around +30 to +40 mV (this is called the peak or overshoot of the action potential)

• falling phase: membrane potential becomes more and more negative due to positive ions leaving the cell (aka repolarization or hyperpolarization), does not stop once reaches resting potential again but rather continues to become more negative before it stops around -80 mV (called the undershoot or afterhyperpolarization

• from there, there is a slower resetting of the membrane potential back to rest, largely due to the hard work of the sodium-potassium pump (this pump never stops working and has been operating this entire time, at this point the pumps role is especially vital)

Reaching Threshold

• what all the different types of stimuli have in common is that they work to raise the membrane potential to a point slightly more positive than when the neuron is at rest, which is called threshold (-65 to -50 mV)

• at threshold, voltage-gated sodium channels open, beginning the rising phase of the action potential

• once threshold is reached the action potential cannot be stopped

• phenomenon of either firing or not is referred to as the all-or-none potential

Voltage-Gated Sodium Channels

• at the membrane potential around -55mV the cell’s voltage-gated sodium channels open

• greater concentration of sodium outside than inside the cell creating a large chemical driving force for moving sodium ions to the inside of cell if possible

• large electrical potential, creating an electrical driving force, which also pushes positive ions (like sodium) into the cell (extracellular is more positive, so positive ions attracted to negatively charged cytosol)

• multiple forces acting on the sodium ions

• the potential eventually peaks at about +30 to +40 mV, at which time the voltage-gated Na+ channels become blocked or inactivated, and no more Na+ can get in

  • an inactivated voltage-gated ion channel cannot open regardless of what the surrounding membrane potential is

• absolute refractory period is the time during which it is impossible to initiate an action potential

Voltage-Gated Potassium Channels

• once potential reaches 0mV or so, voltage-gated potassium channels begin to open

  • these channels are relatively slow to change their conformation so they are not fully open till the peak of the action potential, at which point potassium can start to flow thru

• at the peak of action potential, the inside of the cell is 30mV more positive than the extracellular fluid, the reverse of the resting state

• each leaving potassium ion also removes a positive charge from the inside of the cells, rapidly hyperpolarizing the cell, making it more negative, and causing the falling phase

• these channels begin closing when the membrane potential reaches rest but do not fully close till a bit later

  • as a result potassium still has time to leak out and can make membrane potential more negative than the resting potential (around 80mV) and it’s referred to as hyperpolarization

• since this channel is slow to act and its job is to return the membrane potential to rest, it is referred to as a delayed rectifier

• more difficult to initiate an action potential during the undershoot bc more stimulus is required to get membrane potential up to threshold (referred to as relative refractory period)

• sodium-potassium pump is always working, but during relative refractory period this pump is critical to the restoration of the usual concentration gradients by moving sodium to the outside of cell and potassium to inside of cell

Action Potential Conduction

• action potential signal is first initiated at the base of the soma and the beginning of the axon border, at a zone called the axon hillock

• voltage-gated sodium and potassium channels are triggered to open at each subsequent section of the membrane as the signal travels down the axon

• when sodium enters via the next channels, those ions diffuse down to the next section and trigger the action potential all over again (this way signal does not die down but is constantly regenerated in order to maintain signal strength)

• propagation of the action potential along the axon only occurs in one direction

  • usually from the soma or axon hillock to the axon terminal at the end

• only the voltage-gated sodium channels in front of the action potential wave can be opened by the influx of sodium bringing the membrane potential to threshold, and the action potential wave continues in the forward direction only

The Nodes of Ranvier and Saltatory Conduction

• the myelin sheath speeds up action potential conduction by insulating the axon, the same way many electrical wires are insulated in plastic or rubber

• in those axons that are myelinated, the action potential is only regenerated at the Nodes of Ranvier

  • voltage-gated sodium and potassium channels are clustered at these nodes as these are the only places where ions can be exchanged

• conduction down a myelinated neuron is referred to as saltatory conduction, from the Latin word saltare, meaning “to jump,” as the message jumps or skips down the inside of the axon, from node to node, until it reaches the axon terminal

Synaptic Transmission

• the axon terminal of the presynaptic cell contains many mitochondria to help produce the ATP that powers so many important aspects of synaptic transmission and other cellular processes

• firm structure of the axon is created by long cytoskeletal elements called microtubules, but these cease as the axon terminal begins

• Norepinephrine is produced in a series of small bundles of neurons (or nuclei) in the brain, the main one is called the locus coeruleus (latin for blue spot)

• acetylcholine is the primary neurotransmitter found at the neuromuscular junction

• receptive field is where a stimulus can be detected by a sensory receptor

• retina layers seem to be inside out but it is so that the pigmented epithelium layer needs to be just outside of these cells in order to help support their function

• choroid brings oxygen and nutrients to the hard-working photoreceptor cells

• primates use a trichromatic (three cone) system

• bipolar cells are activated by glutamate and in turn synapse with the retinal ganglion cells

• axons of retinal ganglion cells exit out of the back of the eye as the optic nerve (a blind spot)

Neuroscience is a multidisciplinary field of study that explores the structure and function of the nervous system. It encompasses a wide range of topics, including the anatomy, physiology, biochemistry, and psychology of the nervous system. Here's an overview of key aspects of neuroscience:

1. Definition:

- Neuroscience is the scientific study of the nervous system, which includes the brain, spinal cord, and peripheral nerves.

2. Divisions of Neuroscience:

- Structural Neuroscience: Focuses on the anatomy and morphology of the nervous system.

- Functional Neuroscience: Explores the physiological processes and functions of the nervous system.

- Cognitive Neuroscience: Investigates the neural mechanisms underlying cognition, including perception, memory, and problem-solving.

- Computational Neuroscience: Involves the use of mathematical models and computer simulations to understand neural processes.

- Clinical Neuroscience: Addresses disorders and diseases of the nervous system and their treatment.

3. Components of the Nervous System:

- Central Nervous System (CNS): Includes the brain and spinal cord.

- Peripheral Nervous System (PNS): Comprises nerves and ganglia outside the CNS.

4. Neurons and Glial Cells:

- Neurons: Basic functional units that transmit signals in the form of electrical impulses.

- Glial Cells: Support and protect neurons, contributing to overall brain function.

5. Neurotransmitters:

- Chemical messengers that transmit signals between neurons, enabling communication in the nervous system.

6. Brain Structure:

- Cerebrum: Largest part of the brain, responsible for higher cognitive functions.

- Cerebellum: Coordinates motor functions and balance.

- Brainstem: Regulates basic physiological functions such as breathing and heartbeat.

7. Methods in Neuroscience:

- Electrophysiology: Measures electrical activity in the nervous system.

- Imaging Techniques: Includes MRI, CT, PET, and fMRI to visualize brain structure and function.

- Neurochemistry: Analyzes the chemical composition of the nervous system.

8. Neuroplasticity:

- The brain's ability to reorganize itself by forming new neural connections throughout life in response to experience and learning.

9. Neurological Disorders:

- Conditions affecting the nervous system, such as Alzheimer's disease, Parkinson's disease, epilepsy, and multiple sclerosis.

10. Ethical Considerations:

- The study of neuroscience raises ethical questions related to brain research, consciousness, and the use of neurotechnology.

This is a broad overview, and each subfield within neuroscience has its own intricacies and complexities. If you have specific questions or areas of interest, feel free to ask for more detailed information.