Lesson 4 - Neurons and Nerve Impulses-1

Neurons and Nerve Impulses Neuropsychology 2011 Lesson 4:

Neurons and Nerve Impulses Neuropsychology 2011 Lesson 4

The Cells of the Nervous System:

The Cells of the Nervous System The human nervous system is comprised of two kinds of cells: Neurons Glia The human brain contains approximately 100 billion individual neurons. Behavior depends upon the communication between neurons.

The Cells of the Nervous System:

The Cells of the Nervous System Neurons are information-conducting units Have many characteristics in common with other body cells Also have specialized characteristics that make it possible to: Receive information from other cells Send electrical impulses by relying on changes in chemical charges on its cell membrane

The Cells of the Nervous System:

The Cells of the Nervous System Like other cells in the body, neurons contain the following structures: Membrane Nucleus Mitochondria Ribosomes Endoplasmic reticulum

The Cells of the Nervous System:

The Cells of the Nervous System The membrane - a structure that separates the inside of the cell from the outside environment. The nucleus – a structure that contains the chromosomes. The mitochondrion - structure that performs metabolic activities and provides energy that the cells requires.

The Cells of the Nervous System:

The Cells of the Nervous System Ribosomes - sites at which the cell synthesizes new protein molecules Endoplasmic reticulum – network of thin tubes that transport newly synthesized proteins to their location

Slide 8:

Fig. 2-2, p. 29

The Neuron’s Structure:

The Neuron’s Structure Many types of neurons all with distinctive shape which is suited for its function: to receive, conduct and transmit signals All neurons have the following major components : Dendrites – receive incoming information from other neurons Soma – the cell body which contains the machinery for common cellular functions Axon – sends information to other neurons Presynaptic terminals – site for getting information from previous neurons

The Cells of the Nervous System:

The Cells of the Nervous System Neuron cells are similar to other cells of the body but have a distinctive shape. A motor neuron has its soma in the spinal cord and receives excitation from other neurons and conducts impulses along it axon to a muscle. A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation (touch, light, sound, etc.)

Slide 11:

Fig. 2-6, p. 31

Slide 12:

Fig. 2-5, p. 30

The Neuron’s Structure:

The Neuron’s Structure Neurons may have 1 to 20 dendrites – each may have many branches Dendrites increase the surface area of the neuron. Surface area is further increased by sub-branches and by small protrusions called dendritic spines that cover each branch. The greater the surface area, the more information a neuron can gather

The Cells of the Nervous System:

The Cells of the Nervous System Dendrites are branching fibers with a surface lined with synaptic receptors responsible for bringing information into the neruon . Some dendrites also contain dendritic spines that further branch out and increase the surface area of the dendrite. Dendrite shape of dendrites vary and depend upon varying inputs.

Slide 15:

Fig. 2-7, p. 31

The Neuron’s Structure:

The Neuron’s Structure Each neuron has a single axon – only one output – which extends out of the cell body at axon hillock (little hill) The axon may have branches called axon collaterals which usually emerge from it at right angles Towards its end, axons may divide to smaller branches called teleodendria

The Neuron’s Structure:

The Neuron’s Structure At the end of each teleodendrion is a knob called an end foot or terminal buttons The end foot of one neuron sits very close to the dendritic spine of another neuron but does not touch. The space between the two is called the synapse Axons are insulated with myelin , which helps speed the rate of information transfer

The Cells of the Nervous System:

The Cells of the Nervous System Cell body/ Soma - contains the nucleus, mitochondria, ribosomes, and other structures found in other cells. Also responsible for the metabolic work of the neuron.

The Cells of the Nervous System:

The Cells of the Nervous System Axon - thin fiber of a neuron responsible for transmitting nerve impulses toward other neurons, organs, or muscles. Some neurons are covered with an insulating material called the myelin sheath with interruptions in the sheath known as nodes of Ranvier.

The Cells of the Nervous System:

The Cells of the Nervous System Presynaptic terminals refer to the end points of an axon where the release of chemicals to communicate with other neurons occurs.

The Cells of the Nervous System:

The Cells of the Nervous System Terms used to describe the neuron include the following: Afferent axon - refers to bringing information into a structure. Efferent axon - refers to carrying information away from a structure. Interneurons or Intrinsic neurons are those whose dendrites and axons are completely contained within a single structure.

Slide 22:

Fig. 2-8, p. 32

The Cells of the Nervous System:

The Cells of the Nervous System Neurons vary in size, shape, and function and in the length and branching of their axons, and in complexity of their dendritic processes. The shape of a neuron determines it connection with other neurons and contribution to the nervous system. The function is closely related to the shape of a neuron .

The Neuron’s Structure:

The Neuron’s Structure Simplest sensory neuron , a bipolar neuron consists of a cell body with a dendrite on one side and an axon on the other (d on next page) Somatosensory neurons project from sensory receptors to spinal cord – modified so dendrite and axon are connected, which speeds transfer of information doesn’t go through cell body) (b on next page) Motor neurons in brainstem project to facial muscles and those in spinal cord project to muscles of the body (c on next page)

Slide 25:

Fig. 2-9, p. 32

The Cells of the Nervous System:

The Cells of the Nervous System Glia are the other major components of the nervous system that exchange chemicals with adjacent neurons. Astrocytes helps synchronize the activity of the axon by wrapping around the presynaptic terminal and taking up chemicals released by the axon. Microglia - remove waste material and other microorganisms that could prove harmful to the neuron.

Slide 27:

Fig. 2-10, p. 33

Slide 28:

Fig. 2-11, p. 33

The Cells of the Nervous System:

The Cells of the Nervous System (Types of glia continued) Oligdendrocytes & Schwann cells build the myelin sheath that surrounds the axon of some neurons. Radial glia - guide the migration of neurons and the growth of their axons and dendrites during embryonic development.

The Cells of the Nervous System:

The Cells of the Nervous System The blood-brain barrier is a mechanism that surrounds the brain and blocks most chemicals from entering. The immune system destroys damaged or infected cells throughout the body. Because neurons in the brain generally do not regenerate, it is vitally important for the blood brain barrier to block incoming viruses, bacteria or other harmful material from entering.

The Cells of the Nervous System:

The Cells of the Nervous System Active transport is the protein mediated process by which useful chemicals are brought into the brain. Glucose, hormones, amino acids, and vitamins are brought into the brain via active transport. Glucose is a simple sugar that is the primary source of nutrition for neurons. Thiamine is a chemical that is necessary for the use of glucose.

Slide 32:

Fig. 2-12, p. 34

Cell Membrane:

Cell Membrane Extracellular fluid and intracellular fluid (cytoplasm) – mainly water in which salts and other chemicals are dissolved Concentrations of dissolved substances are different Bilayer (phospholipid) which regulates movement of substances in and out of cell

Slide 34:

Fig. 2-3, p. 30

Cell Membrane:

Cell Membrane Protein molecules can ionize in water Proteins embedded in the cell membrane act as gates and transportation systems to allow select substances to pass Protein’s function is a property of its shape or ability to change shape

Cell Membrane:

Cell Membrane Categories of proteins: Channels : shape allows the proteins to create channels or holes through which substances pass. Different proteins with different size holes allow different substances to enter/leave a cell Gates : some proteins can change shape. Some gates work by changing shape when another chemical binds to them, so protein acts like door lock. When appropriate “key” comes, locking device changes and becomes “activated”. Gates may also change in response to changes in environement (i.e. electrical charge or temperature changes)

Cell Membrane:

Cell Membrane Pumps: in some cases the protein acts as a pump, a transporter molecule that requires energy to move substance across the membrane

Neuron’s Electrical Activity:

Neuron’s Electrical Activity 1936 Cambridge University – Hodgkin and Huxley examined electrical activity of Giant Squid axons (which could be kept live and functioning) NT in a synapse interacts with postsynaptic neuron to either send information to other neurons or inhibit information from being passed Activity relies on the balance of ions between the inside of the neuron (intracellular) and the outside of the neuron (extracellular)

The Nerve Impulse:

The Nerve Impulse A nerve impulse is the electrical message that is transmitted down the axon of a neuron. The impulse does not travel directly down the axon but is regenerated at points along the axon. The speed of nerve impulses ranges from approximately 1 m/s to 100 m/s.

The Neuron’s Structure:

The Neuron’s Structure Neuronal flow of information = river flow of water Information flows from the dendrites to the cell body and axon like tributaries of a river Unlike a river, information received by dendrites does not just flow to the terminal buttons Neuron must collect and process information. It receives information from 100s or 1000s of dendritic spines and must summarize or average the information that flows through one axon.

The Neuron’s Structure:

The Neuron’s Structure Information traveling in neuron is a unidirectional electrical current In the axon, the electrical flow consists of discrete impulses When impulse reaches terminal buttons, one or more chemicals ( neurotransmitters ) is released into the synapse The chemical carries the message across the synapse to influence the electrical activity of the receiving cell – to excite or inhibit it - thus passing the message along Presynaptic neuron influences the postsynaptic neuron

The Nerve Impulse:

The Nerve Impulse The membrane of a neuron maintains an electrical gradient which is a difference in the electrical charge inside and outside of the cell.

Slide 43:

Fig. 2-13, p. 38

The Nerve Impulse:

The Nerve Impulse At rest, the membrane maintains an electrical polarization or a difference in the electrical charge of two locations. the inside of the membrane is slightly 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.

Neuron’s Electrical Activity:

Neuron’s Electrical Activity Neuron at rest maintains electrical charge of -70 millivolts (Scientists say charge outside is zero and charge inside is 70mV less than outside). This state is resting potential . This is energy that can be used at a later time. Four charged particles interact to produce the resting potential: Sodium (Na+), Potassium (K+), Chloride ( Cl -) and large protein anions (A-) Distributed unevenly across the membrane with more A- and K+ intracellular and Cl - and Na+ extracellular

Neuron’s Electrical Activity:

Neuron’s Electrical Activity Anions (A-) remain inside cell because there are no membrane channels through which they can leave – contribute to the negative charge inside the cell. This allows a transmembrane gradient. To balance this negative charge intracellularly , cells accumulate K+ ions inside as well. K+ ions pass through the membrane through open Potassium channels so about 20x as much is in the cell than out.

The Nerve Impulse:

The Nerve Impulse 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 closed. Potassium channels are partially closed allowing the slow passage of sodium.

Neuron’s Electrical Activity:

Neuron’s Electrical Activity This allows a transmembrane gradient – some K+ ions leave the cell because the internal concentration of K+ is much higher than external The efflux of even a small amount is enough to contribute to the charge across the membrane, with the inside being negatively charged relative to outside This does not seem to make sense but there is a limit on the number of K+ that accumulate inside because when intracellular concentration becomes higher than extracellular, K+ starts moving out of the cell

Neuron’s Electrical Activity:

Neuron’s Electrical Activity The high concentration of Na+ outside the cell relative to inside is maintained by the Na-K pump, a protein in the membrane that shunts Na+ out and K+ in A neurons membrane has thousands of pumps continuously working, each one exchanging 3 intracellular Na+ for 2 K+ with each pump action K+ is free to leave through open channels but closed Na+ channels prevent it from reentering At equilibrium, there are 10x as many Na+ outside as there are inside

Neuron’s Electrical Activity:

Neuron’s Electrical Activity Cl - contributes little to the resting potential. Cl - moves in and out of the cell through open chloride channels 3 aspects that contribute to resting potential: A- molecules remain inside the cell Gates keep out Na+ ions and channels allow K+ and Cl - to pass freely Na-K pump moves Na+ from extracellular fluid

Neuron’s Electrical Activity:

Neuron’s Electrical Activity Sodium (Na+) and Potassium (K+) At rest, extracellular fluid has high concentration of Na+, intracellular fluid has high concentration of K+ This uneven distribution is unique to the brain. It is promoted by: Permeability of membrane at ion channels . At rest, K+ easily crosses membrane, Na+ does not. With time enough of each ion can sneak in and distribution would be even Sodium-potassium pump imports K+ and exports Na+

Slide 52:

Fig. 2-14, p. 38

The Nerve Impulse:

The Nerve Impulse The sodium-potassium pump is a protein complex that continually pumps three sodium ions out of the cells while drawing two potassium ions into the cell. helps to maintain the electrical gradient. The electrical gradient and the concentration gradient (the difference in distributions of ions) work to pull sodium ions into the cell. The electrical gradient tends to pull potassium ions into the cells.

Slide 54:

Fig. 2-15, p. 39

Neuron’s Electrical Activity:

Neuron’s Electrical Activity NT in a synapse can open ion channels that allow rapid influx of Na+ and rapid efflux of K+ Intracellular space becomes more positive, so membrane potential moves from resting state (-70mV) to +50mV. This is called depolarization and is when an action potential occurs. Action potentials allow NTs to be released from terminal buttons Though action potentials occur entirely in one neuron, they result in NT release which allows communication between neurons.

Neuron’s Electrical Activity:

Neuron’s Electrical Activity As neuron becomes depolarized, K+ channels open and ions rapidly leave the neuron. The efflux triggers the closing of the sodium channels and eventually cell returns to resting state of -70mV, also called repolarization . (Occasionally, some additional K+ leaks out and then the membrane goes beyond -70mV, resulting in hyperpolarization .)

Neuron’s Electrical Activity:

Neuron’s Electrical Activity Action potential can not always be triggered. When the neuron is strongly depolarized, Na+ channels can not be opened Action potentials are “all or nothing”. Once the neuron is depolarized, Na+ channels open and an action potential occurs. These are always the same size.

Neuron’s Electrical Activity:

Neuron’s Electrical Activity Axons are not uniformly myelinated . There are small gaps known as nodes of Ranvier . So in myelinated neurons, ion channels and Na/K pumps occur only at the nodes. Depolarization jumps from one node of Ranvier to another where another action potential will take place. This is saltatory conduction and occurs for the entire length of the axon. Because action potential is actively propogated , transmission in myelinated neurons is faster than in other neurons.

The Nerve Impulse:

The Nerve Impulse An action potential is a rapid depolarization of the neuron. Stimulation of the neuron past the threshold of excitation triggers a nerve impulse or action potential.

The Nerve Impulse:

The Nerve Impulse The resting potential remains stable until the neuron is stimulated. Hyperpolarization refers to increasing the polarization or the difference between the electrical charge of two places. Depolarization refers to decreasing the polarization towards zero. The threshold of excitement refers to a levels above which any stimulation produces a massive depolarization.

Slide 61:

Fig. 2-16, p. 41

The Nerve Impulse:

The Nerve Impulse After an action potential occurs, sodium channels are quickly closed. 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.

The Nerve Impulse:

The Nerve Impulse Local anesthetic drugs block sodium channels and therefore prevent action potentials from occurring. Example: Novocain and xylocaine

The Nerve Impulse:

The Nerve Impulse The all-or-none law 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.

The Nerve Impulse:

The Nerve Impulse After an action potential, a neuron has a refractory period during which time the neuron resists the production of another action potential. The absolute refractory period is the first part of the period in which the membrane can not produce an action potential. The relative refractory period is the second part in which it take a stronger than usual stimulus to trigger an action potential.

The Nerve Impulse:

The Nerve Impulse 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 is the term used to describe the transmission of the action potential down the axon. the action potential does not directly travel down the axon.

Slide 67:

Fig. 2-17, p. 43

The Nerve Impulse:

The Nerve Impulse The myelin sheath of axons are interrupted by short unmyelinated sections called nodes of Ranvier. Myelin is an insulating material composed of fats and proteins At each node of Ranvier, the action potential is regenerated by a chain of positively charged ion pushed along by the previous segment.

Slide 69:

Fig. 2-18, p. 44

The Nerve Impulse:

The Nerve Impulse Saltatory conduction is the word used to describe this “jumping” of the action potential from node to node. Provides rapid conduction of impulses Conserves energy for the cell Multiple sclerosis is disease in which the myelin sheath is destroyed and associated with poor muscle coordination.

Slide 71:

Fig. 2-19, p. 45

The Nerve Impulse:

The Nerve Impulse Not all neurons have lengthy axons. Local neurons have short axons, exchange information with only close neighbors, and do not produce action potentials. When stimulated, local neurons produce graded potentials which are membrane potentials that vary in magnitude and do not follow the all-or-none law,. A local neuron depolarizes or hyperpolarizes in proportion to the stimulation.