Chapter 4- Neuronal function in the Nervous System

Channels in the cell membrane

1. help maintain the concentration gradient 2. may be voltage- or chemically-gated.

Multipolar, Pseudounipolar, Bipolar

A) multipolar cells from cerebellar cortex B & C) Multipolar cell from cerebral cortex D) pseudounipolar cell of the posterior root ganglion. E) bipolar cell from the retina F) bipolar cell from olfactory epithelium

concentration gradient

A. fluids across a membrane move from a higher to lower concentration B. the concentration of sodium ions in the interstitial fluid balances the concentration gradient C. protein ions in the intracellular fluid are impermeable to sodium ions; they cannot diffuse across the membrane. D. interstitial fluid contains a higher concentration of water than intracellular fluid E. sodium ions do not diffuse across the cell membrane

Implications

All of our neurologic functions are a function of either *hard-wired* connections (synapses) among neurons or connections that are *acquired* on the basis of experience. ~ Vegetative functions largely hard-wired at birth. (baby has ability to feel pain, light touch, see, hear) ~ Higher functions (language, memory, perception, feelings , emotions, etc.) are a function of genetically-programmed development or experience. (obtained by new synapses, have the capacity but not present at birth)

Anatomy Individual Neuron cells

Cell body Axon Dendrites Synaptic bouton or terminal

Neurons

Cells specialized for *receiving* and *transmitting* electrical impulses to and from: 1. each other (have the most connections) 2. sensory receptors (light, sound, touch) 3. and/or effector organs (muscle & glands)

A neural impulse is transmitted across the synaptic cleft primarily through ___ means.

Chemical

Ions:

Cl- is choride K+ is potassium NA + is sodium P- is Protein

Synapses

Connection between neurons or between neurons and effector organs or sensory organs. ~neurons communicate with each other.

Once thershold is reached

Reaching threshold causes neuronal membrane to have a *temporary* but significant increase in permeability to Na+, which diffuses inward (in response to both the concentration gradient and the electrical gradient until the cross-membrane electrical potential is reversed. ~ membrane characteristics change the flow inside by opening up to Sodium (Na+) found outside. (+/-) ~ reverse polarity is *brief* inside becomes + recharged.

Saltatory conduction:

Saltatory conduction in myelinated fibers. ~ myelin > bare > myelin> bare. ~ helps transmit faster. Current flows from one node of Ranvier (a 1- micrometer space between myelinating glial cells) to the next and brings the membrane to threshold at that location. ~ current jumps from one end to another

Synaptic bouton

Synaptic vesicles containing a neurotransmitter. The chemical in form of liquid that sends message.

Saltatory conduction is found in:

We find the nodes of Ranvier, saltatory propagations and faster conduction in: Motor cells of the PNS for *striated muscles, fine touch and proprioceptive cells* of the PNS, and many neurons in the CNS. striated muscles: voluntary and do our moving, need to move faster than smooth muscles (digestive) Proprioception- sense of position

Types of Neurons

based on cell shape, each neuron contains cell nucleus and one to a dozen projections. These projections receive stimuli and conduct neural impulses. 1. Multipolar (for this class) 2. Pseudounipolar 3. Bipolar

Bipolar

cell body has two extensions, right in the middle of the axon. - unique to auditory & visual. Found directly in ear or retina.

Pseudounipolar

cell body only has 1 extension purpose: to maintain vitality (keep alive) not to receive or send information. location: found only in 1 type of location- sensory neurons sent to the brain or spinal from the outside world. (touch, hearing, vision) ~ once they reach brain no longer pseudounipolar...

depolarizes

cell rapidly shifts from negative to positive.

Multipolar

coming off the cell body there are multiple extensions.

dendrites vs bouton

dendrites receive info bouton send info

effectors organs

e.g. neurons go down and synapse with muscle fibers or glands that does the work.

Sensory receptors

e.g. retina specialized receptor for light; a receptor at the base of the hair initiates nerve in the sensation of touch.

Graded vs Action potentials

graded potentials: are not self-propagating, become smaller as they go further away from the point of origin. Action potentials: are self-propagating, just as big in the end from where they started. Reference: think water ripple.

Allan Schore

has linked behavior and human development with the brain.

interstitial

in between cells

Saltatory propagation

is much faster than non-saltatory propagation.

Once AP has been generated

it travels to the neuron's bouton.

What is faster light touch or pain?

light touch works faster than pain. AP is slower for Pain because it does not have myelin therefore does not go through saltatory conduction.

axons

longer single fiber that conducts nerve impulses away from the neuron to other part of the CNS, glands, or muscles.

Neuron definition from textbook

or nerve cell, is the basic anatomic and functional unit of the nervous system, underlying all neural behavior, including speech, language, and hearing.

The relative amount of voltage in an electrical field is the definition of:

potential

Myelin cells

specialized cells that insulate the axons so they conduct their impulses faster.

The point of contact from one axon to another neuron is the

terminal bouton

During resting potential

the ionic difference across the membrane is at a steady state

IPSP

Inhibitory postsynaptic potential results from *outward* diffusion of K+. ~ opposite of EPSP: synapse the transmitter does one and only one thing, it pauses the channel for potassium to open up. ~ moves out because of concentration gradient. Potential becomes more negative.

Synaptic bouton or terminal

- found at the end of the axon. - structure that do the communicating with other cells (usually other neurons).

characteristic of an action potential

- it is propagated at a constant velocity - it is propagated at a constant amplitude - it begins at the axon hillock - it functions in an all-or-nothing manner False: A. neural information is signaled by the amplitude of the action potential

dendrites

- shorter and more numerous extensions off the cell body - receive signals from other neurons through synapse (contact with other neurons) by specialized receptors. ~ tree branch similarity

Neurons lecture notes:

- we have 50-100 billions neurons. - they are all connected in complex ways. - 1 can connect with hundreds or thousands other neurons. - they vary on individuals based on experiences, learning, genetics, etc... not all the same, however, some pathways at the same at a gross level such as moving your arm.

Graded Potential Threshold

-55mV usually at the axon hillock (base of axon)

Size of resting membrane potential (RMP)

-70mV : the inside of cell is negative compared to the outside

RMP

-70mV potential is maintained as a result of P- trapped intracellularly in combination to the semipermeability of the membrane to other ions. Sodium-potassium pump actively corrects for sodium that *infuses inward* in response to both: 1. the concentration gradient and 2. the electrical gradient. (+ attracts -) Sodium-potassium pump consumes energy to do its work!

Synaptic function (physiology)

1. "Action potential" arrives at bouton 2. Synaptic vesicles *migration* to and *fuse* with presynaptic membrane. 3. Neurotransmitter is *released* into the cleft and *diffuses* across the cleft to the postsynaptic membrane 4. The neurotransmitter temporarily changes the permeability (only picks up 1) of the postsynaptic membrane "Graded potential" occurs at postsynaptic membrane

Two types of graded potential

1. Excitatory postsynaptic potential (EPSP) 2. Inhibitory postsynaptic potential (IPSP)

Facts on composition or concentration of Ions:

1. Ions that are more concentrated inside: Protein anions (P-) and K+ ~ protein are big molecules can't easily get out. 2. Ions that are more concentrated outside: Na+ and Cl- 3. Semipermeable cell membrane Cl- > K+ > Na+ > P- ~ somethings get through easier: Chloride > Potassium > Sodium Ion > Protein (doesn't get through)

Intracellular concentrations

1. Negative charged proteins (P-) which cannot diffuse across membrane (molecules are too large) 2. Potassium ions (K+) - Diffuses down concentration (outward) gradient until the voltage electrical gradient and concentration gradients counterbalance each other. Diffusion is driven by having a lot K+ inside than outside. (Referred to as concentration grading e.g. blue food coloring in water will spread everywhere it can to equalize) On the other hand something is hold it in (the negative charge). The balance will end up with more K+ inside than outside.

Types of Cells in the Nervous System (CNS)

1. Neurons 2. Glial Cells

Anatomy of the synapse

1. Synaptic bouton: at the end of the axon. 2. Presynaptic membrane (before synapse) 3. Synaptic cleft (salt water space) 4. Postsynaptic membrane (after synapse)

how to get threshold:

1. Temporal and spatial summation of graded potentials (EPSPs and IPSPs) occurs at axon hillock. 2. if the summation reaches -55mV, an AP is generated. ~ EPSP and IPSP occuring at the about the same time and spatial position you get nothing. they cancel each other out. ~ You need to have less IPSP and more EPSP to get -55mV threshold.

Action potentials defined as:

A *self-propagating* impulse that travels down the length of an axon that has reached a "threshold" of -55mV (usually at the axon hillock) and that *depolarizes* each successive axonal segment to -55mV through current flow. length: toe to brain, axon may be 6 feet long before synapse occurs.

Graded potential

A *temporary*, graded change (small change) in the electrical potential across the membrane. Degree of change is a function of the amount and type of neurotransmitter secreted at a synapse. graded change- changes that are measured or calibrated. ~ synapse little bit of transmitter is released will cause a little change. ~ more transmitters released will cause bigger change.

Resting membrane potential

An electrical potential across the cell membrane that holds the power to generate an action potential that propagates itself down the length of the neuron. ~ electrical potential: difference in charge inside vs outside cell. +/- charge that comes from ions. e.g. salt into water reference = ions that give charge to vessel they are in.

EPSP

Excitatory postsynaptic potential results from *inward diffusion* of Na+. ~ transmitter release has a chemical effect that opens channels to sodium. Potential becomes *more positive* (or less negative). ~ -70 will go up towards positive direction: -60, - 50, -40, etc.

Cell bodies

Inside the cell body we find: *Organelles*: organs that make the cell viable and able to replicate itself. Function to keep cell alive & healthy. - Nucleus - Mitochondria

Ionic Concentration and Diffusion:

Intracellular & Extracellular concentrations

Genetic Programmed vs Experietial-based development

Our neurologic functions, including our language, memories, perceptions, feelings, and emotions, are all a function of the pattern of connections among 50 to 100 billion neurons, each of which as many as thousands of connections with other neurons.

Glial Cells

Non-neuronal cells that: 1. maintain homeostasis, (cleans up and maintains nice environment for cells) 2. form myelin, 3. and provide support and protection for neurons. Serve vital in function but no to communicate (helper cells)

restoring process

Once impulse starts the Sodium-potassium pump quickly works to restore RMP.

Extracellular concentrations

Sodium ions (Na+) Chloride ions (Cl-) More NA+ outside than inside. would like to concentrate gradient but an active force keeps most sodium out (pore) to maintain -70mV potential and keep neuron in a ready state to receive info or possibly transmit info to another neuron. If sodium goes in the RMP will reverse and make it less negative.

How to get to the next segment?

Successive segments of the axon are depolarized to threshold through *current flow* ~ the passing of electrons as can occur in any conductive medium such as water or copper wire. ~ speed of electricity