what type of glia helps to synchronize the activity of axons

Chapter i Lecture Nerve Cells and Impulses

Modified: 2020-09-02

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We will kickoff from the bottom of the nervous system, that is nosotros'll start past looking at neurons and other building blocks of the system. This chapter covers one part of the puzzle, the part Within the neuron. The next chapter will cover what happens BETWEEN neurons.

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MODULE 1.one

Neurons and Glia

• The human nervous arrangement comprises two kinds of cells

• Neurons: my grad school instructor was disappointed after he disected his offset brain. All he had left was a pile of mush, neurons, glia, and other cells. The brain has no hidden organs like the body does (center, stomach, liver, etc.). It's the organization of those billions of neurons that makes the difference. The number of neurons per person varies merely information technology is around 86 billion

• Glia: several types and serve dissimilar functions: astrocytes, microglia, oligodendrocytes, Schwann cells, and radial glia

• Number of neurons: LOTS 86 billion (merely an estimate). Cortex: 16B, Cerebellum: 69B, Residuum of Brain: <1B, Spinal Cord: 1B

The Structures of an Animal Jail cell

Like other cells in the trunk, neurons incorporate the following structures and have their ain distinctive shape: (p. xx, neurons stained to appear dark)

• Membrane: separates the inside of the cell from the outside environs

• Nucleus: contains the chromosomes

• Mitochondria: performs metabolic activities and provides free energy that the cells requires

• Ribosomes: sites at which the prison cell synthesizes new protein molecules

• Endoplasmic reticulum: network of sparse tubes that transports newly synthesized proteins to their location

Motor and Sensory Neurons (p. twenty)

• A motor neuron

  • Has its soma in the spinal cord

  • Receives excitation from other neurons

  • Conducts impulses along its axon to a muscle or gland.

  • Has myelin sheath (run across saltatory conduction below)

• A sensory neuron

  • Is specialized at 1 end to be highly sensitive to a detail type of stimulation (bear on, low-cal, sound, etc.)

A Vertebrate Motor Neuron

A Vertebrate Sensory Neuron (we'll discuss Sensation and Perception subsequently in the course.

Components of All Neurons

• Dendrites: Branching fibers with a surface lined with synaptic receptors responsible for bringing information into the neuron. Some too comprise dendritic spines that further branch out and increase the surface area of the dendrite. The greater the surface surface area of the dendrite, the more than information it tin receive

• Soma/cell body: Contains the nucleus, mitochondria, and ribosomes. Responsible for the metabolic work of the neuron. Covered with synapses on its surface in many neurons

• Axon:Thin cobweb of a neuron responsible for transmitting nerve impulses toward other neurons, organs, or muscles. May have a myelin sheath, an insulating material that contains interruptions in the sheath known as nodes of Ranvier.

• Presynaptic terminals: Presynaptic terminals at the stop points of an axon release chemicals to communicate with other neurons

Afferent, Efferent, and Intrinsic

• Afferent axon: refers to bringing data into a construction (i-style highway INTO the CNS)

• Efferent axon: refers to conveying information away from a structure (one-way highway FROM the CNS)

• Interneurons or intrinsic neurons are those whose dendrites and axons are completely independent inside a single structure (e.thou., the thalamus, p. 21)

Variations Among Neurons

• Neurons vary in size, shape, and function

• The shape of a neuron determines it connectedness with other neurons and its contribution to the nervous system

• The function is closely related to the shape of a neuron

  • Example: Purkinje cells of the cerebellum branch extremely widely within a single plane

The Various Shape of Neurons

a) Purkinje cell (only found in cerebellum), b) sensory neurons from skin to spinal cord, c) pyramidal cell of motor area of the cerebral cortex, d) bipolar cell of retina of the centre, east) Kenyon prison cell from a honeybee (p. 22)

Types of Glia

• Astrocytes

  • Help synchronize the activity of the axon by wrapping around the presynaptic terminal and taking up chemicals released by the axon

  • Responsible for dilating claret vessels to bring more than nutrients into brain areas with heightened activity

• Microglia

  • Remove waste product material, viruses, and fungi from the brain

  • As well remove dead, dying, or damaged neurons

• Oligodendrocytes (in the brain and spinal string) and Schwann cells (in the periphery of the body)

  • Build the myelin sheath that surrounds and insulates sure vertebrate axons

• Radial glia

  • Guide the migration of neurons and the growth of their axons and dendrites during embryonic development

(When embryonic evolution finishes, most radial glia differentiate into neurons and a smaller number differentiate into astrocytes and oligodendrocytes.)

Shapes of Various Glia Cells (encounter folio 23 for caption)

The Claret-Encephalon Barrier

• A mechanism that surrounds the brain and blocks nearly chemicals from entering

  • The allowed system destroys damaged or infected cells throughout the body

  • Considering neurons in the brain by and large do not regenerate, it is vitally of import for the claret brain barrier to block incoming viruses, bacteria, or other harmful material from entering

  • Most viruses fail to penetrate claret-encephalon barrier just some can: chicken pox and shingles; genital herpes

  • Glucose and blood-brain barrier

How the Blood-Encephalon Bulwark Works (come across p. 24 for caption)

• Agile Transport

  • The protein-mediated process that expends free energy to pump chemicals from the blood into the encephalon

    • Glucose, certain hormones, amino acids, and a few vitamins are brought into the brain via active transport

• The blood-brain bulwark is essential to wellness, but can pose a difficulty in allowing chemicals such equally chemotherapy for brain cancer to laissez passer the bulwark

Nourishment of Vertebrate Neurons

• Vertebrate neurons depend well-nigh entirely on glucose

  • That sugar that is i of the few nutrients that tin pass through the blood-encephalon barrier

• Neurons need a steady supply of oxygen

  • 20 percent of all oxygen consumed by the body is used by the brain (if your body sent you an itemized bill for oxygen utilise, the encephalon would use the well-nigh)

• The body needs a vitamin, thiamine, to use glucose

  • Prolonged thiamine deficiency leads to death of neurons every bit seen in Korsakoff's syndrome, a event of chronic alcoholism

  • Korsakoff'southward syndrome is marked by severe memory impairment

MODULE one.ii

The Nerve Impulse

• The electric message that is transmitted down the axon of a neuron

  • Does non travel directly downwards the axon, only is regenerated at points along the axon and so that it is non weakened

• The speed of nervus impulses ranges from less than one meter/second to 100 meters/2d

  • A bear on on the shoulder reaches the brain more quickly than a touch on the foot

  • Neural conduction velocity is major factor in all beliefs and cannot exist made faster (a biological limit) (an analog is the transmission delay when watching live news on TV. Ballast and correspondent must wait for sound to travel a long distance. And then, back to the body, a larger animal's neurons may have to travel further, and thus take longer to arrive.)

• The brain is not ready up to annals small differences in the fourth dimension of arrival of touch letters

• However, in vision, movements must exist detected as accurately as possible (we'll talk over farther in Vision chapter)

• The properties of impulse control are well adapted to the exact needs for information transfer in the nervous system

The Resting Potential of the Neuron

• Messages in a neuron develop from disturbances of the resting potential

  • At residuum, the membrane maintains an electrical gradient known as polarization

  • A difference in the electrical charge inside and outside of the cell (remember of a car bombardment. its two poles (anode and cathode) are polarized. But (and don't ever do this!), if you lot contact both poles at the same time with a conductor such as a wrench, the battery will depolarize. Electricity, at 12 volts, is now being conducted from one pole to the other and sparks will fly!

• The inside of the membrane is slightly more than negative with respect to the outside (approximately -70 millivolts)

• The resting potential of a neuron refers to the state of the neuron prior to the sending of a nerve impulse

The Membrane of a Neuron

• Forces Acting on Sodium and Potassium Ions

  • The membrane is selectively permeable, allowing some chemicals to pass more freely than others

  • Sodium, potassium, calcium, and chloride pass through channels in the membrane

• When the membrane is at rest:

  • Sodium channels are airtight

  • Potassium channels are partially airtight assuasive the boring passage of potassium

Ion Channels in the Membrane of a Neuron (p. 29)

Ion Channels

• The sodium-potassium pump is a poly peptide complex

  • Continually pumps three sodium ions out of the cells while cartoon ii potassium ions into the prison cell

  • Helps to maintain the electrical gradient

  • Uses agile transport (requires ATP)

Electric and Concentration Gradients

• The electrical gradient and the concentration gradient—the divergence in distributions of ions—work to pull sodium ions into the prison cell

  • The electrical gradient tends to pull potassium ions into the cells

• All the same, they slowly leak out, carrying a positive accuse with them

Sodium and Potassium Gradients for a Resting Membrane (p. 30)

The Activeness Potential

• The resting potential remains stable until the neuron is stimulated

  Membrare Resting Potential Video

  • Hyperpolarization: increasing the polarization or the difference betwixt the electrical charge of ii places (no activity potential)

  • Depolarization: decreasing the polarization toward zero

  • The threshold of excitation: a level above which whatever stimulation produces a massive depolarization (likewise see All or None Law below). Action potential but occurs when threshold is reached or exceeded.

• A rapid depolarization of the neuron

• The action potential threshold varies from one neuron to another, but is consistent for each neuron

• Stimulation of the neuron past the threshold of excitation triggers a nervus impulse or action potential.

• A neuron artificially stimulated in the middle of the axon will send action potential in both directions (and so, it's the organization of the nervous system, non the axon, that determines direction of action potential)

Video of Action Potential

Illustration: A string of firecrackers is a good illustration for the activity potential. From a distance, the explosions look continuous, but in reality the explosions are a series of discrete events.

Voltage-Activated Channels

Membrane channels whose permeability depends upon the voltage difference across the membrane

Sodium and potassium channels

When sodium channels are opened, positively charged sodium ions blitz in and a subsequent nervus impulse occurs

The Movement of Sodium and Potassium Ions During an Action Potential (p. 33) (imagine a series of these traveling downward the axon, ane later the other)

The Movement of Sodium and Potassium

• After an action potential occurs, sodium channels are quickly airtight

• The neuron is returned to its resting state by the opening of potassium channels

  • Potassium ions flow out due to the concentration gradient and take with them their positive charge

• The sodium-potassium pump later restores the original distribution of ions

Restoring the Sodium-Potassium Pump

• The process of restoring the sodium-potassium pump to its original distribution of ions takes fourth dimension

• An unusually rapid series of activity potentials can atomic number 82 to a buildup of sodium within the axon

  • Tin be toxic to a jail cell, but simply in rare instances such as stroke and after the employ of certain drugs

Blocking Sodium Channels

• Local coldhearted drugs block sodium channels and therefore prevent action potentials from occurring

  • Example: Novocain and Xylocaine

The All-or-None Law

• The all-or-none constabulary

  • States that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it

  • Action potentials are equal in intensity and speed within a given neuron

  • Think of a standard toilet: hit the handle become one flush (full tank, get same flush)

  • A neuron tin but burn down in one way

  • Information technology cannot requite a graded signal

    • My example is an erstwhile Dennis the Menace cartoon

    • In that cartoon Dennis'southward dad is carrying him home, and Dennis says: "Of course I heard you, you just did not seem mad enough however."

      • (If anyone finds that cartoon permit me know! I spent 2 hours looking for it final night!)

    • Obviously, Dennis'due south dad had been calling him: Dennis, Dennis, DENNIS!

      • He was using a graded bespeak

    • Neurons cannot do that. They can only modify their rate of firing. Let me substitute the letter of the alphabet "D" for "Dennis" and wait at a neuron

    • Here is the early on signal D----------D----------D----------D----------D

    • Here is a afterwards betoken D-----D-----D-----D-----D-----D-----D

    • Here is the terminal signal D--D--D--D--D--D--D--D--D--D--D

    • The neuron changes its rate of firing and, thus, changes the information it is relaying

    • Notation that because of the refractory menstruation (come across beneath) the neuron cannot fire without some delay betwixt each action potential

• Action potentials vary from one neuron to another in terms of aamplitude, velocity, and shape

Refractory Periods

• In physiology, refractory means not responding

• Afterwards a neuron fires it cannot fire again for a short period of fourth dimension, that is its refractory period

  • There are 2 refractory periods in the neuron

    • Absolute Refractory Menstruation where no stimulus tin can make the neuron fire again

    • Relative Refractory Period where, following the Absolute Refractory Menstruation STRONGER than normal stimulation can make the neuron fire again, sooner than normal

    • The diagram below is from physiologyweb

• My metaphor is the "magic toilet"

  • imagine a toilet that always gives y'all a full affluent even when its tank is not full

  • a real toilet cannot do that even if you hit the handle harder

  • a magic toilet will affluent fully at some point in its filling if you hit the handle harder

In physiology, refractory ways non responding

• After a neuron fires it cannot fire again for a brusque menses of time, that is its refractory period

  • There are ii refractory periods in the neuron

    • Absolute Refractory Period where no stimulus tin can make the neuron fire once again

    • Relative Refractory Period where, post-obit the Absolute Refractory Menstruum STRONGER than normal stimulation tin make the neuron fire once more, sooner than normal

  • My metaphor is the "magic toilet"

    • imagine a toilet that always gives you a full affluent even when its tank is non full

    • a real toilet cannot exercise that fifty-fifty if you hit the handle harder

    • a magic toilet will flush fully at some point in its filling if you hit the handle harder a real toilet will not

    • but, a "one-half full" neuron volition fire fully when more strongly stimulated during the relative refractory period

    • see the Integration: All or none and summation link below for more details

After an activeness potential, a neuron has a refractory period during which fourth dimension the neuron resists the product of some other activeness potential

• The absolute refractory period: the first part of the period in which the membrane cannot produce an activeness potential

• The relative refractory period: the second part, in which it takes a stronger than usual stimulus to trigger an activeness potential

Propagation of an Activity Potential

In a motor neuron, the action potential begins at the axon hillock (a swelling where the axon exits the soma)

Propagation of the action potential: the transmission of the action potential downward the axon

The action potential does not directly travel downwards the axon

The Myelin Sheath

• The myelin sheath of axons are interrupted by brusque unmyelinated sections chosen nodes of Ranvier

  • Myelin is an insulating material equanimous of fats and proteins

  • At each node of Ranvier, the action potential is regenerated by a chain of positively charged ions pushed forth by the previous segment

An Axon Surrounded by a Myelin Sheath (p. 35)

Saltatory Conduction

• The "jumping" of the action potential from node to node (Latin: saltare = "to jump" in English)

  • Provides faster conduction of impulses than unmyelinated axon

  • Conserves free energy for the cell

• Multiple sclerosis: disease in which the myelin sheath is destroyed

  • Associated with poor musculus coordination and sometimes visual impairments

Saltatory Conduction in a Myelinated Axon (p. 35) (curt video)

Local Neurons

• Have brusk axons, exchange information with just close neighbors, and practice not produce action potentials

• When stimulated, produce graded potentials—membrane potentials that vary in magnitude and practise non follow the all-or-none police force

• Depolarize or hyperpolarize in proportion to the stimulation

• Hard to study due to their small size

• Most of our knowledge has come from the study of large neurons

Myth

But x per centum of neurons are active at any given moment

Truth

Yous use all of your encephalon, even at times when you might non exist using it very well

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Source: http://peace.saumag.edu/faculty/kardas/Courses/Physio/kalatchapter1.html

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