Neurons and Synapses 6.5

The Nervous System Composed of cells called neurons. These are typically elongated cells that can carry electrical impulses at very high speeds. The nervous system can be broken down into two major parts: – Central Nervous System (CNS) Brain and Spinal cord – Peripheral Nervous System (PNS) Peripheral nerves that connect all parts of the body to the central nervous system.

The Nervous System

Nerve Impulse Neurons transmit messages in the form of nerve impulses. The impulses are electrical but involve the movement of charged ions, not electrons. Nerve impulse can travel along a neuron as high as 100 meters per second.

Neurons

Dendrites receive chemical signals and produce an electrical signal Axon transmits message Axon covered with a layer of fat called myelin Myelin does not completely cover the axon. Gaps are called nodes of ranvier Speeds up rate of impulse transmission Saltatory conduction

Resting Potential Neurons pump ions across their plasma membranes by active transport. Sodium is pumped out of the neuron and potassium is pumped in. Concentration gradients of both sodium(higher outside) and potassium(lower outside) are established across the membrane. The inside of the neuron develops a net negative charge compared with the outside. This is the resting potential across the plasma membrane of a cell that is not conducting an impulse.

Action Potentials An action potential is the reversal and then the restoration of the electrical potential across the plasma membrane of a cell as an electrical impulse passes along it. An impulse passing along the neuron allows sodium and potassium ions to diffuse across the membrane (through voltage gated ion channels).

Action Potential / Net Charges

Stage 1 in the Passage of a Nerve Impulse An action potential in one part of a neuron causes an action potential to develop in the next section of the neuron. This is from the diffusion of sodium ions between the region with an action potential and the region at the resting potential. If the potential rises above the threshold level, voltage gated channels open.

Stage 2 in the Passage of a Nerve Impulse : Depolarization Sodium channels open and sodium diffuse down the concentration gradient This reduces membrane potential and causes more sodium channels to open. Entry of positively charged sodium ions into the neuron develops a net positive charge. Depolarization of the membrane occurs reversing the membrane potential

Stage 3 in the Passage of a Nerve Impulse : Repolarization Potassium channels open after a short delay. Potassium ions diffuse out of the neuron down the concentration gradient through open channels. Exit of positively charged potassium ions develops a net negative charge inside the cell compared to outside. Repolarization is the restoration of the membrane potential to its original resting state.

Stage 4 in the Passage of a Nerve Impulse (refractory) Concentration gradients of sodium and potassium are restored. This is done through the removal of sodium from the neuron by active transport and the addition of potassium through active transport. This restores the resting potential of that part of the neuron. now again ready to conduct another nerve impulse. Sodium ions diffuse along inside the neuron from an adjacent region that has already depolarized and initiate depolarization.

Action Potential / Net Charges

Nerve Synapse A synapse is a junction between two nerves. The plasma membranes of either neuron is separated by a narrow fluid gap called the synaptic cleft. Messages are passed across the synapse in the form of chemicals called neurotransmitters. Neurotransmitters always pass in the same direction from the pre-synaptic neuron to the post-synaptic neuron.

Synaptic Transmission

Stage 1: action potential reaches the axon terminal of the pre-synaptic neuron Stage 2: calcium channels open and Ca++ ions diffuse into axon terminal Stage 3: Ca++ influx causes vesicles with neurotransmitters to fuse with the membrane, releasing their contents by exocytosis

Synaptic Transmission cont. Stage 4: neurotransmitters diffuse across the gap (synaptic cleft) and bind to receptors of the post-synaptic neuron Stage 5: binding of neurotransmitters on receptors cause sodium ion channels to open triggers an action potential Stage 6: neurotransmitters are either broken down or taken back up by the pre-synaptic neuron

Neural Transmission Both nerve impulses and synaptic transmissions have a threshold potential If this threshold is not reached, the impulse stops A typical post-synaptic neuron will be in communication with many pre-synaptic neurons

Acetylcholine Involved with contraction of muscles If acetylcholine triggers an impulse, the muscle contracts (black widow) If not enough acetylcholine, muscle does not contract (botox) Acetylcholinesterase is an enzyme that breaks down the neurotransmitters

Insecticide use Neonicotinoids are synthetic compounds that bind to acetylcholine receptors End up blocking the action of acetylcholine Works on insects more so than mammals Used as an insecticide Some controversy as to whether it is now disrupting some ecosystems (honeybees)