Mind and Brain (1)

Mind and Brain (1)

1. How a Nerve Impulse Travels

   • A nerve impulse is a sequence of small movements of atoms into and out of the axon of the neuron. Each movement is known as an 'action potential'.
   • Sodium has a positive charge. Inside the axon is a strong negative charge.
   • There are two passive forces that make these electrically charged atoms move.
     • Electrostatic Pressure
       • The negative inside the cell attracts all positive charged atoms into the cell.
       • The negative charge inside the cell repels all negatively charged atoms into the cell.
     • Pressure From the Concentration Gradient.
       • Atoms move from a location of high concentration to an area of low concentration.
   • Potassium
     • Despite the electrostatic pull on the positively charged Potassium atoms to enter the neuron, the concentration inside is so great that it overwhelms this.
     • Therefore, the overall pressure on Potassium atoms is to move out of the cell.
   • Sodium-Potassium Pump
     • There is a great pressure on Na+ atoms to enter the cell.
     • There is a great pressure on K+ atoms to exit the cell.
     • They do this with difficulty because the channels through the axon membrane are almost shut. Despite this, a few pass through.
     • The Sodium-Potassium pump fixes this and keeps the concentrations constant. It moves 3 sodium ions out and moves 2 potassium ions in.
   • Chloride
     • The pressure from the concentration gradient can be measured.
     • The pressure from the electrostatic from the force can be measured.
     • For Chloride, the two pressures are equal.
   • The Action Potential
     • 1. The threshold of excitation (-70 mV to -50 mV).
     • 2. Sodium channels open.
     • 3. Sodium rushes in at +50 mV.
     • 4. Potassium rushes out.
     • 5. Sodium channel closes, Potassium still comes in.
     • 6. Potassium channel closes.

2. The Major Features of a Neuron

   • External
     • Cell Membrane
       • The semipermeable membrane that encloses the neuron.
     • Dendrites
       • The short processes emanating from the cell, which receive most of the synaptic contacts from other neurons.
     • Axon Hillock
       • The cone-shaped region at the junction between the axon and the cell body.
     • Cell Body
       • The metabolic center of the neuron.
     • Axon
       • The long, narrow process that projects from the cell body.
     • Myelin
       • The fatty insulation around many axons.
     • Nodes of Ranvier
       • The gaps between sections of myelin.
     • Buttons
       • The endings of the axon branches, which release chemicals into synapses.
     • Synapses
       • The gaps between adjacent neurons across which chemical signals are transmitted.
   • Internal
     • Mitochondria
       • Sites of aerobic energy release.
     • Nucleus
       • The spherical DNA-containing structure of the cell body.
     • Endoplasmic Reticulum
       • A system of folded membranes in the cell body; rough portions play a role in the synthesis of proteins; smooth portions play a role in the synthesis of fats.
     • Cytoplasm
       • The clear internal fluid of the cell.
     • Ribosomes
       • Internal cellular structures on which proteins are synthesized.
     • Golgi Complex
       • A connected system of membranes that packages molecules in vesicles.
     • Microtubules
       • Tubules responsible for the rapid transport of material throughout neurons.
     • Synaptic Vesicles
       • Spherical membrane packages that store neurotransmitter molecules ready for release near synapses.
     • Neurotransmitters
       • Molecules that are released from active neurons and influence the activity of other cells.

3. Synapse

   • What Happens in the Synapse?
     • Memories are formed.
     • Learning occurs
     • Neurons link to one another.
     • Neural networks are formed.
   • Stages
     • 1. Calcium influx causes the vesicles to blend with the membrane.
     • 2. Neurotransmitters are released into the synaptic cleft (exocytosis).
     • 3. The neurotransmitters bind to receptors on the post-synaptic membrane.
     • 4. This leads to to a flow of ions into the next neuron. This flow of ions is called the post-synaptic potentials.
     • 5. Because these ions are atoms that have an electrical charge the electrical charge inside the next neuron changes slightly. 'Communication' between the two neurons has occurred.
     • 6. After the neurotransmitters have had their effect they are typically reabsorbed back into the neuron that released them (the pre-synaptic cell).
     • 7. One neurotransmitter, acetylcholine, is broken down and its components reabsorbed.
   • Release Pattern
     • Small molecule in pulses with each action potential.
     • Larger molecule neurotransmitters - gradually in response to general increase in Ca2+.
   • A Computational Unit
     • The electrical charge inside the next neuron will only significantly change if the effect of many post-synaptic potentials are added.
     • This will be most effective if they arrive at about the same time and in the same part of the next neuron.
   • EPSP or IPSP
     • If the effect of opening ion channels in the membrane of the post-synaptic neuron is to make it more positive then that post-synaptic potential is +.
     • At any time the post-synaptic neuron will be activated by both exitatory and inhibatory post-synaptic potentials.
     • Metabolic Receptors - can open ion channles from inside the cell. Slow developing, long lasting, varied in kind. Can activate a cascade of enzyme porduction.

4. Anatomy of the Nervous System

   • General Layout of the Nervous Sytem
     • Divisions of the Nervous System
       • CNS - composed of the brain and the spinal cord.
       • PNS - composed of the somatic nervous system and the autonomic nervous system.
         • Somatic Nervous System
           • Interacts with external environment.
         • Autonomic Nervous System
           • Regulates the body's internal environment.
   • Meninges , Ventricles and Cerebrospinal Fluid
     • Meninges
       • Made up of bone and three membranes.
       • The outer meninx is a tough membrane known as the dura mater
       • There is also the arachnoid membrane and the subarachnoid membrane. This contains many large blood vessels and cerebrospinal fluid.
       • There is also the pia mater which is on the surface of the CNS.
       • Protect the CNS and the brain.
     • Ventricles
       • The cerebral ventricles are the four large internal chambers of the brain.
       • There are the two lateral ventricles.
       • There is the third ventricle.
       • There is the fourth ventricle.
       • The subarachnoid space, central canal and cerebral ventricles are interconnected by a series of openings and form a single reservoir.
     • Cerebrospinal Fluid
       • Cerebrospinal fluid is produced by the choroid plexuses.
       • It protects the CNS. It fills the subarachnoid space.
       • It is continuously absorbed into the dural sinuses.
   • Blood-Brain Barrier
     • The cells of the blood vessel walls are tightly packed, forming a barrier to the passage of many molecules - particularly proteins and other large molecules.
     • It doesn't prevent the passage of all large molecules. Some large molecules are critical for normal brain function so are actively transported through cerebral blood vessel walls.