Neurons, Synapses, and Signaling Chapter 48

YOU MUST KNOW The anatomy of a neuron. The role of active transport in establishing the membrane potential of a neuron. How long-distance and short-distance signaling is done in neurons. The mechanisms of impulse transmission in a neuron. The process that leads to release of neurotransmitter, and what happens at the synapse.

Neuron organization and structure reflect function in information transfer The Neuron is the functional unit of the nervous system. It is composed if a cell body, which contains the nucleus and organelles. Dendrites which are cell extensions that receive incoming messages from other cells. Axons which transmit messages to other cells. Many axons are covered by an insulating fatty myelin sheath. This speeds the rate of impulse transmission. The synapse is a junction between two neurons (or a neuron and a muscle fiber or gland). Neuron is the basic structure of the nervous system that reflects function.

Neurotransmitters Neurotransmitters are chemical messengers released from vesicles in the synaptic terminals into the synapse. They will diffuse across the synapse and bind to receptors in the neuron, muscle fiber, or gland across the synapse, effecting a change in the second cell. Ex. Acetylcholine, dopamine, and serotonin. At the synapse, the transmitting neuron is the presynaptic cell and the neuron, muscle or gland that receives the signal is the postsynaptic cell.

Nervous System Central nervous system CNS consists of the brain and spinal cord. Peripheral nervous system (PNS) consists of the nerves that communicate motor and sensory signals throughout the rest of the body.

Sensory receptors collect information about the world outside the body as well as processes inside the body. Ex. The rods and cones of the eye; pressure receptors in the skin. Sensory neurons transmit information from the eyes and other sensors that detect stimuli to the brain or spinal cord for processing. Interneurons connect sensory and motor neurons or make local connections in the brain and spinal cord. Motor neurons transmit signals to effectors, such as muscle cells and glands. Nerves are bundles of neurons. A nerve can contain all motor neurons, all sensory neurons, or be mixed.

Ion Pumps and Ion Channels Membrane potential describes the difference in electrical charge across a cell membrane. The membrane potential of a nerve cell at rest is called its resting potential. It exists because of differences in the ionic composition of the extracellular and intracellular fluids across the plasma membrane. The concentration of Na+ is higher outside the cell, whereas the concentration of K+ is higher inside the cell. This gradient is maintained by sodium-potassium pumps. Changes in the membrane potential of a neuron are what give rise to transmission of nerve impulses (action potentials). A stimulus changes the permeability of the membrane to Na+, so the membrane depolarizes as Na+ enter the cell. This may result in an action potential.

Acton Potential An action potential (nerve impulse) is an all-or-none response to depolarization of the membrane of the nerve cell. In order to generate an action potential, a certain level of depolarization must be achieved, known as the threshold. Saltatory conduction which is the jumping of the nerve impulse between nodes of Ranvier (areas on the axon not covered by the myelin sheath), speeds up the conduction of the nerve impulse.

Neuron Communication The signal is conducted from the axon of a presynaptic cell to the dendrite of a postsynaptic cell via the synapse. Neurotransmitters are released by the presynaptic membrane into the synaptic cleft. They bind to receptors on the postsynaptic membrane and are then broken down by enzymes or taken back up into surrounding cells. There are two categories of neurotransmitters: excitatory and inhibitory. Excitatory neurotransmitters cause depolarization of the postsynaptic membrane, which brings the membrane potential closer to the threshold. Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic membrane, which moves the membrane potential further from the threshold.

An AP arrives, depolarizing the presynaptic membrane. The depolarization opens voltage-gated channels, triggering an influx of Ca2+ The elevated Ca2+ concentration causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft. The neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane. ** In this example, binding triggers opening, allowing Na+ and K+ to diffuse through.

Dopamine https://www.youtube.com/watch?v=lIJW9XLDE-I

Nervous System 49.2 The brainstem is made up of the medulla oblongata, pons, and midbrain. The brainstem controls homeostatic functions, such as breathing, swallowing, and digestion and conducts sensory and motor signals between the spinal cord and higher brain centers.

The Cerebellum helps coordinate motor, perceptual, and cognitive functions. The thalamus is the main center through which sensory and motor information passes to and from the cerebrum. The hypothalamus regulates homeostasis; basic survival behaviors such as feeding, fighting, fleeing, and reproducing; thermostat, thirst center, and circadian rhythms.

The cerebrum has two hemispheres, each with a covering of gray matter over white matter. Information processing is centered here, and this region is particularly extensive in mammals. The cerebral cortex controls voluntary movement and cognitive functions. The corpus callosum is a thick band of axons that enables communication between the right and left cortices.

Different regions….different functions