Chapter 4: The Brain and Behaviour Flashcards
Define NEURONS:
- Neurons: specialised cells which are the basic building blocks of the nervous system
- Link together in circuits
- Can vary greatly in size and shape
- Function to receive, process and send messages
- Neurons generate electricity that creates nerve impulses
- Release chemicals that communicate to other neurons, muscles and glands
What are the three main parts of the neurons?
- Cell Body
- Dendrites
- Axon
Explain the CELL BODY:
o Cell body
Also “soma” contains biochemical structures needed to keep the neuron alive, nucleus carries genetic information that determines how the cell develops and functions
Surface of cell body has receptor areas that can be directly stimulated by other neurons.
Explain the DENDRITES
o Dendrites
Emerge from the cell body in branch-‐like fibres. They are specialised receiving units that collect messages from neighbouring neurons and send them on to the cell body where information is combined and processed.
Explain the AXON:
Conducts electrical impulses away from the cell body to other neurons, muscles or glands
Branches out at its end to form axon terminals which may connect with dendrites from numerous neurons
What are GLIAL CELLS?
Glial cells of the peripheral nervous system include SHWANN CELLS, which form the MYELIN SHEATH, and SATELLITE CELLS, which provide nutrients and structural support to neurons.
GLIAL CELLS PURPOSE?
• Supported in function by glial cells (Greek roots for “glue”)
o Surround neurons and hold them in place
o Manufacture nutrient chemicals that neurons need
o Absorb toxins and waste that would damage/kill neurons
o Send out long fibres to guide newly developed neurons to targeted places during prenatal development, as new neurons are formed through cell division
How do nerve impulses occur in 3 steps?
• How nerve impulses occur: (3 steps)
o 1. At rest, neuron has electrical resting potential due to – and + ions distributed inside/outside the neuron
o 2. Action potential or nerve impulse produced when stimulated – i.e. ions flow in and out through cell membrane and reverse electrical charge of resting potential
o 3. Original ionic balance is restored, neuron is again at rest
What is the RESTING POTENTIAL?
• RESTING POTENTIAL:
o 1. Neurons surrounded by body fluids – separated by membrane
o 2. Membrane a sieve with ion channels which allow some substances in, limit others
o 3. Nerve impulse = flow of ions
- ***** Outside membrane Na+ and Cl-‐, inside K+ and negatively charged protein ions
- ***** High Na+ concentration outside and high protein ion concentration inside
- ***** This creates a difference of 70mV and is the neurons resting potential
What is ACTION POTENTIAL?
• ACTION POTENTIAL:
o 1. -‐70mV to +40mV when the neuron is stimulated – action potential/nerve impulse
o 2. When neuron is stimulated, sodium channels open up
o 3. Sodium ions flood through channels attracted by negatively charged protein ions: creates state of depolarisation
o 4. Restores to resting potential when positively charged potassium ions flow out through channels
What is ABSOLUTE REFRACTORY PERIOD?
o 1. Period once impulse has passed a point along the axon
o 2. When potassium ions are flowing out
o 3. Membrane not excitable, cannot discharge another impulse
What is GRADED POTENTIALS?
graded potentials are when changes in the negative resting potential do not reach the -‐50mV action potential threshold required, and thus do not create an action potential at all
What is the MYELIN SHEATH?
- Structure and Purpose
• Myelin sheath: a whitish, fatty insulation layer derived from glial cells during development.
- Covers many axons that transmit through brain and spinal cord
- Interrupted at regular intervals by Nodes of Ranvier
- Axons lacking myelin sheath – “fuse-‐like” transfer of action potential
- Myelinated axons – Nodes of Ranvier allow more frequent depolarisation and thus higher electrical conduction speeds
How do neurons talk: STEP 1
Step 1
• Regulating ion flow in and out of the cell body and creates a resting membrane potential
- Meaning a voltage difference between the inside and outside of the neuron ****(-‐75 millivolts)
- Meaning a teeny-‐tiny battery
How do neurons talk: STEP 2
Step 2:
• Excitatory and inhibitory messages from other neurons (via dendrites) change resting potential
• If enough excitatory messages are received, resting potential will excess a threshold ****(~-‐55mv)
How do neurons talk: STEP 3
Step 3:
• This initiates a rapid depolarization at the axon hillock which creates a current that moves down the axon
• This current is called an action potential
Action Potentials:
• Depolarization and repolarization occurs in less than ***0.02 seconds
- Depolarization “overshoots” and repolarization “undershoots”
- This slows down the action potential and leads to a refractory period of about **0.0015 seconds
How do neurons talk: STEP 4 - The problem and Solution(s)
Step 4: the Problem
• For a number of reasons, the charge sent down the axon wouldn’t travel far enough or fast enough if not for some help
Step 4: The solution (PART I):
• One form of help comes form the oligodendrocytes (a type of glial cell) that form the myelin sheath
• The myelin sheath prevents the ions from escaping
Step 4: The solution (PART II):
• The Nodes of Ranvier act like the amplifiers that re-‐generate the action potential
How do neurons talk: STEP 5
Step 5:
• Ultimately, the current reaches the end of the axon
- While communication within the neuron is purely electrical this is not how it goes between neurons
- Arrival of action potential at ***presynaptic axon terminal triggers release of chemicals (neurotransmitters)
How do neurons talk: STEP 6
Step 6:
• Neurotransmitters enter the synaptic cleft and float through cerebrospinal fluid
• Some then bind with receptors on dendrites of adjacent neurons
How do neurons talk: STEP 7
Step 7:
• Depending on the neurotransmitter and the dendrite, two things can happen…
***** Excitatory post synaptic potential (ESSP): depolarizes postsynaptic neuron
**** Inhibitory post-‐synaptic potential (ISSP): hyperpolarizes postsynaptic neuron
What is the Synaptic Space
• Synaptic space: a tiny gap between the axon terminal and the next neuron
What are NEUROTRANSMITTERS?
• Neurotransmitters: chemical substances that carry messages across the synaptic space to other neurons, muscles or glands
5 STEPS in the PROCESS of CHEMICAL COMMUNICATION:
LIST
• 5 steps in this process of chemical communication:
- Synthesis
- Storage
- Release
- Binding
- Deactivation
Explain the 5 steps of CHEMICAL COMMUNICATIONS:
- Synthesis: transmitter molecules formed inside neuron
- Storage: molecules stored in synaptic vesicles, or chambers within axon terminals
- Release: when action potential comes down axon, vesicles move to surface of axon terminal and molecules are released into fluid-‐filled space between axon of presynaptic (sending) neuron and membrane of postsynaptic (receiving) neuron
• 4. Binding: molecules cross synaptic space and bind to receptor sites, large protein molecules embedded in the receiving neuron’s cell membrane. Transmitters will match only a particular receptor molecule in the postsynaptic membrane.
o Chemical reaction occurs during binding: two effects on receiving neuron
o Excitation or inhibition
**Excitation: sodium channels open and depolarisation, creating a graded potential or action potential
. ***Inhibition: potassium ions or negative chloride ions flow, increasing neuron’s negative potential and making it harder to fire the neuron.
• 5. Deactivation: once a neurotransmitter molecule binds to its receptor, it continues to excite or inhibit the neuron until it is deactivated or shut off.
o **some are broke ndown by other chemicals
o **some are deactivated by reuptake, where transmitter molecules are taken back into the presynaptic axon terminals
