Psyco

INTRODUCTION TO PSYCHOLOGY

CLASS #4

Dr. Charles-Etienne Benoit

 Evidence for the existence of healed trepanations (cranial surgery) from Ensisheim, Alsace, France.

 Trepanation: a surgical intervention in which a hole is made into the human skull to treat health problems related to intracranial diseases or release pressured blood buildup from an injury.

 More than 1,500 trepinated skulls from the Neolithic period have been uncovered throughout the world.

Mesolithic brain surgery (5100 BC)

Lillie, 1998

 17th Century B.C., is a document that contains the earliest reference to the brain.

 It may have been a manual of military surgery that describes 48 cases.

 The papyrus is a scroll of 4.68 meters.

 It is written right-to-left in the Egyptian cursive form of hieroglyphs.

 The red ink highlights the patients’ ailments and their prognoses.

 The word “brain” occurs only 8 times in ancient Egyptian, 6 of them on these pages.

 The papyrus describes here the symptoms, diagnosis, and prognosis of two patients with compound fractures of the skull.

 It compares the surface of the brain to “those ripples that happen in copper through smelting”.

Second part of the papyrus

 Egyptians saw the preservation of the body after death as an important step to living well in the afterlife.

 It takes about 70 days to completely mummify a dead body.

 The first step is to push a sharp rod up the nose and into the brain.

Mummification

Anubis

The Brain

With blood vessels Without blood vessels Around 1.3 - 1.4 kg

1 trillion neurons trillions of "support cells"

Cellular architecture

Camillo Golgi

1843 – 1926

Lombardy, Italy

 He experimented with metal impregnation of nervous tissue, using mainly silver.

 The Golgi method would stain a limited number of cells at random in their entirety and was published in 1973.

The first illustration by Golgi

Cellular architecture

Santiago Ramon y Cajal

1852 – 1934

Naverre, Spain

 He was the first to identify the unitary nature of neurons.

 He articulated the neuron doctrine that state that the nervous system is made up of individual cells.

Cajal and Golgi shared the nobel prize for their work in 1906.

Afferent inflow to the mammalian cortex

Today’s Lecture

 A look at the nervous system

 Brain cells  Neurons

 Glia

 Neuronal signaling  Chemical

 Electrical

The nervous system

 The nervous system is generally divided into two main parts.

 The central nervous system includes the brain and spinal cord.

 The peripheral nervous system, comprising the sensory and motor nerves and associated nerve cell ganglia (groups of neuronal cell bodies), is located outside the central nervous system.

The autonomous nervous system

 The autonomic nervous system is involved in controlling the involuntary action of smooth muscles, the heart, and various glands.

 The sympathetic system uses the neurotransmitter norepinephrine, and the parasympathetic system uses acetylcholine as its transmitter.

 The two systems frequently operate antagonistically.

The ventricules

 The brain is immersed in a fluid called cerebrospinal fluid.

 It is produced in the lateral ventricles and in the third ventricle.

 It provide structural support and is essential in brain regulation.

Brain cells

 The nervous system is composed of two main classes of cells:

 Neurons

 Glial cells

 Neurons are the basic signaling units that transmit information throughout the nervous system.  As Ramon y Cajal deduced, neurons take in information, by changes in their activity

levels, pass it along to other neurons.

Neuron’s structure

 Typical neuron has four morphologically defined regions:  (1) the cell body

 (2) dendrites

 (3) axon

 (4) presynaptic terminals

 An axon convey electrical signals over distances ranging from 0.1 mm to 2 m.

 These electrical signals, called action potentials, are initiated at a specialized trigger region near the origin of the axon.

Type of neurons

 Unipolar cells have a single process emanating from the cell. Different segments serve as receptive surfaces or releasing terminals. Unipolar cells are characteristic of the invertebrate nervous system.

 Bipolar cells have two types of processes. The dendrite receives electrical signals and the axon transmits signals to other cells.

 Pseudo-unipolar cells carry somatosensory information to the spinal cord. During development the two processes of the embryonic bipolar cell fuse and emerge from the cell body as a single process that has two functionally distinct segments. Both segments function as axons; one extends to peripheral skin or muscle, the other to the central spinal cord.

Type of neurons

 Multipolar cells have a single axon and many dendrites. They are the most common type of neuron in the mammalian nervous system.

 Three examples illustrate their large diversity.

 Spinal motor neurons innervate skeletal muscle.

 Pyramidal cells have a roughly triangular cell body; dendrites emerge from both the apex and the base. Pyramidal cells are found in the hippocampus and throughout the cerebral cortex.

 Purkinje cells of the cerebellum are characterized by a rich and extensive dendritic tree that accommodates an enormous synaptic input.

Multipolar neurons

Neuron’s physiology

Overview

Intra-cellular transport

Mitochondria

 The most prominent roles of mitochondria are to produce the energy of the cell (ATP) through aerobic respiration and to regulate cellular metabolism.

 Have a double-membrane of lipid layers.

 Krebs cycle occurs in the matrix.

Citric acid cycle (Krebs cycle)

 A series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into adenosine triphosphate (ATP) and carbon dioxide.

Synapse

 Synapses are specialized regions that permit chemical or electrical signaling between neurons.

 The nerve cell transmitting a signal is called the presynaptic cell; the cell receiving the signal is the postsynaptic cell.

 Information is transferred across synapses from one neuron to the next, or from a neuron to a non-neuronal cell such as those in muscles or glands.

Synaptic contact

 Postsynaptic contact can occur on:  the cell body

 the dendrites

 the axon

Glial cells

 Glial cells are support cells and greatly outnumber neurons.

 2 to 10 times more glia than neurons in the vertebrate central nervous system.

 Although they arise from the same embryonic precursor cells, they do not have the same membrane properties as neurons.

 They are not electrically excitable and are not directly involved in electrical signaling.

 Glia in the vertebrate nervous system can be divided into two major classes: microglia and macroglia.

Microglia

 Microglia account for 10 - 15% of all cells found within the brain.

 They act as the first and main form of active immune defense in the central nervous system.

 Microglia in the brain are derived from Bone Marrow. Entering the central nervous system early in development, they reside in all regions of the brain throughout life.

Macrophage

 Microglia constantly scavenge the CNS for plaques, damaged or unnecessary neurons or infectious agents.

 Microglia can also become macrophages, clearing debris after strokes or other degenerative neurological disorders.

 Macrophage engulfs and digests cellular debris, foreign substances, microbes or anything else that does not have the type of proteins specific to healthy body cells on its surface.

Astrocytes

 They constitute nearly half the number of brain cells.

 Owe their name to their irregular, roughly star-shaped cell bodies.

 The astrocytes help to create a barrier, called the blood–brain barrier (BBB), between the tissues of the central nervous system and the blood.

 Astrocytes help nourish surrounding neurons by releasing growth factors and regulate the potassium (K+) concentration in the space between neurons.

Astrocytes support synaptic signaling

 The processes of astrocytes are intimately associated with both presynaptic

and postsynaptic elements.

 They prepare the surface of the neuron for synapse formation and stabilize newly formed synapses.

Blood-brain barrier

 The BBB limits which materials in the blood can gain access to neurons in the nervous system.

 Astrocytes ensheath small arterioles and capillaries throughout the brain.

 The barrier is largely the result of tight junctions between endothelial cells and cerebral capillaries, a feature not shared by capillaries in other parts of the body.

Glia insulating the axons

 Insulate: to separate from conducting bodies by means of nonconductors so as to prevent transfer of electricity.

 To increase the speed by which the electricity is conducted in axons.

 This happen because they are wrapped in an insulating sheath of a lipid substance, myelin.

In the CNS In the PNS

Myelination

Grey and white matters

 White matter refers to areas that are mainly made up of myelinated axons.

 Grey matter is a major component of the CNS, consisting of neuronal cell bodies, dendrites and axons (myelinated and unmyelinated), glial cells, synapses, and capillaries.

The colors

 The color arises mainly from the whiteness of myelin. Grey matter has a very light grey color which come from capillar

Organization of neurons in the CNS

 Neurons can be organized in clumps called nuclei (not to be confused with the nucleus inside each neuron).

 Sheets called layers which are most commonly found in the cortex.

 The cell bodies of glial cells are located in the white matter (e.g., oligodendrocytes), and in the cortex.

Functional organization of neurons

Neuronal signaling

The voltage difference across the neuronal

membrane in the resting state is typically − 70 millivolts (mV) inside,

which is known as resting membrane potential .

Membrane potential

 It results from the separation of net positive and net negative charges on either side of the membrane.

 The excess of positive ions outside the membrane and negative ions inside represents a fraction of the total number of ions inside and outside the cell at rest.

Membrane

It is formed by a series of lipid (fat) molecules

oriented with the heads facing outwards, and the

tails facing inwards.

Ion channel

 The membrane’s selective permeability to some ions, and the concentration gradients formed by active pumping, lead to a difference in electrical potential across the membrane; this is the resting membrane potential.

 The membrane potential, represented here by the positive charges outside the neuron along the membrane and the negative charges inside.

Ion channel modulation

Several types of stimuli control the

opening and closing of ion channels.

Important for action potential

Ion channel are proteins

 Ion channels are integral membrane proteins and composed of several subunits.

 Some channels contain auxiliary subunits that modulate the gating of the pore

Crystal structure of a channel

 Side view of the tetrameric voltage-gated K+ channel

Channel distribution

 Variants of a potassium channel are expressed in different regions of the brain (autoradiography).

Ion channel pump

 The Na+/K+ pump preserves the cell’s resting potential by maintaining a larger concentration of K+ inside the cell and Na+ outside the cell.

 The pump uses ATP as energy for active transport.

Adenosine triphosphate (ATP)

 Adenosine triphosphate (ATP) is a complex organic chemical that provides energy to drive many processes in living cells (muscle contraction, nerve impulse propagation, chemical synthesis).

 Found in all forms of life

ADP

Channels vs pumps

 Channels have a continuous aqueous pathway for ion conduction across the membrane. This pathway can be occluded by the closing of a gate.

 Pumps have two gates in series that control ion flux. The gates are never open simultaneously, but both can close to trap one or more ions in the pore.

Recording of action potential

 Their discovered the basis for propagation of nerve impulses.

 They made their discovery from the giant axon of the Atlantic squid.

 This discovery earned them the Nobel Prize in Physiology or Medicine in 1963.

Andrew Huxley

1917 – 2012

London, England

Alan Hodgkin

1914 – 1998

Oxfordshire, England

Giant atlantic squid

 Its axon is the largest axon known to science.

 They have a diameter of 1 mm.

Voltage clamp

 Used to measure the ion currents through the membranes of excitable cells, such as neurons, while holding the membrane voltage at a set level.

 Measure the membrane potential, and then change the membrane potential (voltage) to a desired value by adding the necessary current.

 This "clamps" the cell membrane at a desired constant voltage, allowing the voltage clamp to record what currents are delivered.

Action potential

 Voltage-gated ion channels located in the spike-triggering zone at the axon hillock, and along the extent to the axon, open and close rapidly, changing their conductance to specific ions and resulting in the action potential.

 The time course of changes in membrane voltage during an action potential, and the underlying causative changes in membrane conductance to Na+ (gNa) and K+ (gK).

All or none

 If the stimulation is not large enough, it will not occur.

 Once the threshold is reach, it will provide an action potential always of the same amplitude that will go through the entire neuron.

Refractory period

 This is known as the absolute refractory period. It is followed by the relative refractory period, during which the neuron can generate action potentials, but only with larger-than-normal depolarizing currents.

 The refractory period lasts only a couple of milliseconds and has as consequence that the neuron’s speed for generating action potentials is limited to about 200 action potentials per second.

Hyperpolarization state, the voltage-gated Na + channels are

unable to open, and another action potential cannot be generated.

Summation

 Sometime, simultaneous post-synaptic stimulations are necessary to reach the threshold to trigger an action potential.

Neuron conduction

The change in membrane potential along a neuronal process during electric conduction decreases with distance.

Node of Ranvier

 Node of Ranvier are periodic gap in the insulating sheath (myelin) on the axon of certain neurons that serves to facilitate the rapid conduction of nerve impulses.

 Action potentials in myelinated nerves are regenerated at the nodes.

Neuron conduction

Louis Ranvier

1835 – 1922

Lyon, France

 Ranvier was born and studied medicine at Lyon, graduating in 1865.

 In 1867, Ranvier entered the Collège de France.

 In 1875, he was appointed to its chair of general anatomy.

 In 1878, Ranvier discovered the nodes which received his name.

Gap junction

 Some neurons communicate via electrical synapses. They are very different from chemical synapses.

 In electrical synapses, no synaptic cleft separates the neurons. Instead, the neuronal membranes are touching at specializations called gap junctions.

 These channels form a continuous bridge that provides a direct communication path between the two cells where it provide an instantaneous signal transmission.

Properties of synapses

Properties of synapses

Charcot-Marie-Tooth disease

 A demyelinating disorder caused by a single mutations (connexin gene) expressed in the Schwann cell that blocks gap-junction.

That’s it for today!

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