The Nervous System 2 2023-24 PDF

The Nervous System 2 Human Physiology 1 The Nervous System The nervous system consists of two major parts: (1) Central Nervous System (CNS) consisting of the brain and the spinal cord (2) Peripheral Nervous System (PNS) that contains nerves that transmit information to and f...

The Nervous System 2 Human Physiology 1 The Nervous System The nervous system consists of two major parts: (1) Central Nervous System (CNS) consisting of the brain and the spinal cord (2) Peripheral Nervous System (PNS) that contains nerves that transmit information to and from the CNS Together, these organs are responsible for the control of the body and communication among its parts. Nervous Tissue The majority of the nervous system is tissue made up of two classes of cells: neurons and neuroglia. 1. Neurons Excitable nerve cells Transmit electrical signals 2. Neuroglia or just glia Means “nerve glue” Supporting Cells Neurons Classes of Neurons Afferent neurons: Known as sensory neurons, transmit sensory signals to the CNS from receptors in the body. Efferent neurons: Transmit signals from the CNS to effectors in the body such as muscles and glands. Interneurons: Form complex networks within the CNS to integrate the information received from afferent neurons and to direct the function of the body through efferent neurons. Neuroglia Also known as glial cells, act as the “helper” cells of the nervous system. Each neuron in the body is surrounded by anywhere from 6 to 60 neuroglia that protect, feed, and insulate the neuron. Neuroglia Supporting cells = neuroglia (“nerve glue”) or glial cells CNS Astrocytes Oligodendrocytes Microglia Ependymal cells PNS Schwann cells Satellite cells https://www.youtube.com/watch?v=AwES6R1_9PM Neuroglia (CNS glial cells) Astrocytes Star shaped; the most numerous Involved in metabolism & synapse formation Microglia Contains Phagocytes are cells that protect the body by ingesting (phagocytosing) harmful foreign particles, bacteria, and dead or dying cells Ependymal cells Line the cavities of CNS and spinal cord; cilia Oligodendrocytes Produce myelin sheaths in CNS Neuroglia (PNS glial cells) Satellite cells Surround neuron cell body Schwann cells Form myelin in PNS Nerves Neurons bundle together to form nerves Some nerves may be only a few neurons, and others may be hundreds or thousands The myelin sheath may insulate axons by surrounding it There may be some gaps in the myelin sheath called nodes of ranvier Impulses jump from one node to the next, increasing the speed impulses travel Nerve Impulse route Dendrite through cell body axon hillock axon axon terminal Nerve Impulse route How does this happen?  The Nerve Impulse A nerve impulse is an electrical signal that travels along an axon. A nerve impulse is a signal that travels from one neuron to the next and finally to an end organ eg. Group of muscle fibers or back to the CNS There is an electrical difference between the inside of the axon and its surroundings, like a tiny battery. An Action Potential An action potential is a brief change in electrical conditions at a neuron’s membrane that occurs when a neural signal arrives; it is what happens when we say a neuron “fires.” Polarized Because there is a potential difference across the cell membrane, the membrane is said to be polarized. If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized. If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized. Resting Membrane Potential (RMP) At rest the electrical charge inside the cell is negative relative to the outside of the cell. This means that the intracellular fluid is relatively negative to the extracellular fluid. This difference is known as the RMP ~ -70mV The Nerve Impulse Formation of an action potential: The formation of an action potential can be divided into five steps. (1) A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. (2) If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes The Nerve Impulse (3) At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close. (4) The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire. (5) The K+ channels close and the Na+/K+ transporter restores the resting potential. The Nerve Impulse When the nerve is activated, there is a sudden change in the voltage across the wall of the axon, caused by the movement of ions in and out of the neuron. This triggers a wave of electrical activity that passes from the cell body along the length of the axon to the synapse Changes in this electrical potential are signals to receive, transmit and integrate information within and between cells Resting Membrane Potential (RMP) Depolarisation Phase Repolarisation Phase Resting Membrane Potential (RMP) Resting Membrane Potential (RMP)  https://www.youtube.com/watch?v=oa6rvUJlg7o  https://www.youtube.com/watch?v=oa6rvUJlg7o https://youtu.be/kY8FEq0teOs Synapses Junctions between neurons Information is passed (usually chemically) - Unidirectional Presynaptic (toward synapse) vs postsynaptic (away from synapse): most neurons function as both Synaptic cleft (tiny gap) Synapses Neurons do not physically touch one another; instead they are separated by a gap called a synapse. Neurotransmitters released from the terminal bulb diffuse into the synapse, just as they do at the neuromuscular junction. They traverse this space, called the synaptic cleft, by simple diffusion. Neurotransmitters leavethe presynaptic neuron and diffuse toward the postsynaptic neuron, where they settle on receptors and initiate a reaction. Neurons can synapse with: 1. Neurons 2. Muscle 3. Glands Synapse’s Neurons communicate with one another at junctions called synapses. At a synapse, one neuron sends a message to a target neuron—another cell. Most synapses are chemical; these synapses communicate using chemical messengers. Other synapses are electrical; in these synapses, ions flow directly between cells. At a chemical synapse, an action potential triggers the presynaptic neuron to release neurotransmitters. These molecules bind to receptors on the postsynaptic cell and make it more or less likely to fire an action potential. Chemical Synapses At the end of a neuron’s axon is an enlarged region of the axon known as the axon terminal. It is separated from the next cell by a small gap known as the synaptic cleft. When an action potential reaches the axon terminal, it opens voltage-gated calcium ion channels. When an action potential, or nerve impulse, arrives at the axon terminal, it activates voltage-gated calcium channels in the cell membrane. Calcium which is present at a much higher concentration outside the neuron than inside, rushes into the cell. The Calcium allows synaptic vesicles to fuse with the axon terminal membrane, releasing neurotransmitter into the synaptic cleft. Chemical Synapses Electrical Synapses Formed when 2 neurons are connected by small holes called gap junctions. The gap junctions allow electric current to pass from one neuron to the other, so that an action potential in one cell is passed directly on to the other cell through the synapse. Electrical synapses are faster and more reliable than chemical synapses. Electrical synapses are found throughout the nervous system, yet are less common than chemical synapses. Electrical Synapses At electrical synapses, unlike chemical synapses, there is a direct physical connection between the presynaptic neuron and the postsynaptic neuron. This connection takes the form of a channel called a gap junction, which allows current— ions—to flow directly from one cell into another. Resting Membrane Potential (RMP) The original membrane potential of a resting neuron is a. 70 mV. b. 90 mV. c. 0 mV. d. dependent on neuron location. Resting Membrane Potential (RMP) The first ion to enter the neuron at the beginning of an action potential is a. calcium. b. potassium. c. sodium. d. ATP. Resting Membrane Potential (RMP) The period of time immediately after an action potential, during which the neuron cannot send a second action potential is the a. refractory period. b. depolarisation c. dead zone. d. sodium/potassium ATPase period. Task