6 5 NEURONS AND SYNAPSES Neurons Neurons transmit

6. 5 NEURONS AND SYNAPSES

Neurons ■ Neurons transmit electrical impulses. ■ Two systems of the body are used for internal communications – The endocrine system ■ Consists of glands that release hormones. – The nervous system ■ ■ Consists of nerve cells called neurons. There about 85 billion neurons in the human nervous system. ■ Neurons help with internal communication by transmitting nerve impulses. ■ Nerve impulses are an electrical signal.

Short branched nerve fibers Transmit impulses between neurons Cytoplasm & nucleus Elongated nerve fibers Tips of toes/fingers to spinal cord Allows for salutatory conduction -jumping between node of Ranvier -Much quicker than continuous transmission.

Resting Potential ■ Neurons pump sodium and potassium ions across their membranes to generate a resting potential. ■ A neuron that is not transmitting a signal has a potential difference or voltage across its membrane. (resting potential). ■ This potential is due to the imbalance of positive and negative charges across the membrane. ■ Resting potential (-70 m. V).

Action Potential ■ An action potential consists of depolarization and repolarization of the neuron. ■ An AP is a rapid change in the membrane potential, consisting of two phases. ■ 1. Depolarization – a change from negative to positive – Due to opening of sodium (Na+) channels in the membrane, allowing sodium ions to diffuse into the neuron down the concentration gradient. – Raises membrane potential to about +30 m. V. ■ 2. Repolarization – a change from positive to negative – Closing of sodium channels and opening of potassium channels. – K+ ions diffuse out of the membrane. – Makes inside cell negative in relation to the outside of the cell. – Channel remains open until membrane has fallen to a potential close to -70 m. V. – This does not restore the resting potential as the concentration gradients have not be re-established. – This takes a few milliseconds. – *Like when you flush the toilet.

Propagation of action potentials ■ Nerve impulses are action potentials propagated along the axons of neurons. ■ A nerve impulse is an action potential that starts at one end of a neuron and is then propagated along the axon to the other end of the neuron. ■ The propagation of the AP happens because the ion movements that depolarize one part of the neuron trigger depolarization in the neighboring part of the neuron. ■ Nerve impulses always move in one direction along neurons, because an impulse can only be initiated at one terminal of a neuron and can only be passed on to other neurons/different cell types at the other terminal.

Local Currents ■ Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

DBQ

Synapses ■ Synapses are junctions between neurons and receptors or effector cells. ■ Synapses are junctions between cells in the nervous system. ■ Chemicals called neurotransmitters are used to send signals across synapses.

Synaptic transmission ■ When pre-synaptic neurons are depolarized they release a neurotransmitter into the synapse. ■ Synaptic transmission occurs very rapidly as a result of these events: – A nerve impulse is propagated along the pre-synaptic neuron until it reaches the end of the neuron and the pre-synaptic membrane. – Depolarization of the pre-synaptic membrane causes calcium ions to diffuse through channels in the membrane into the neuron. – Influx of calcium causes vesicles containing neurotransmitters to move to the pre-synaptic membrane and fuse with it. – Neurotransmitters diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane. – The binding of the neurotransmitters to the receptors causes adjacent sodium ion channels to open. – Sodium ions diffuse down the concentration gradient into the post-synaptic neuron, causing the post-synaptic neuron to reach the threshold potential. – An AP is triggered in the post-synaptic membrane and propagated on along the neuron. – The neurotransmitter is quickly broken down and removed from the synaptic cleft.

Acetylcholine ■ Secretion and reabsorption of acetylcholine by neurons at synapses. ■ Acetylcholine is used as the neurotransmitter in many synapses, including synapses between neurons and muscle fibers. ■ It is produced in the pre-synaptic neuron by combining choline (absorbed from the diet) with an acetyl group produced during aerobic respiration.

Threshold potential ■ A nerve impulse is only initiated if the threshold potential is reached. ■ Nerve impulses follow an all-or-nothing principle. ■ An AP is only initiated if the threshold potential is reached, because only at this potential do voltage-gated sodium channels start to open, causing depolarization. ■ At a synapse, the amount of neurotransmitter secreted following depolarization of the pre-synaptic membrane may not be enough to cause threshold potential to be reach in the post-synaptic membrane. ■ The post-synaptic membrane does not depolarize. ■ The sodium ions that have entered the post-synaptic neuron are pumped out by sodium-potassium pumps, returning the post-synaptic membrane to a resting potential.

Neonicotinoids ■ Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors. ■ Neonicotinoids are synthetic compounds similar to nicotine. ■ The bind to the acetylcholine receptor in cholinergic synapses in the central nervous system (CNS) of insects. ■ Acetylcholine does not break down neonicotinoids, so the binding is irreversible. ■ The receptors are blocked, so acetylcholine is unable to bind and synaptic transmission is prevented. ■ The insect is paralysis and dies. ■ Neonicotinoids are very effective pesticides. ■ Pros: Not highly toxic to humans and other animals. ■ Cons: The effects is has on honey bees and other beneficial insects.

Homework ■ Study for Quiz on 6. 5 ■ Research a drug and how it impacts synaptic transmission in the brain. ■ 1 page, at least 1 source ■ It can be a legal (prescribed/over the counter) drug or an illegal drug. ■ Due next class.