Central and Peripheral Nervous System

Central and Peripheral Nervous System

• The nervous system comprises billions of cells, including neurons and glial cells.
• There are approximately 86 billion neurons in the brain and 100-150 billion in the entire nervous system.
• The nervous system collects sensory information and governs behaviors through motor control.
• The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
• The central nervous system (CNS) is composed of the brain and spinal cord.
• The peripheral nervous system (PNS) has nerves and sensory organs throughout the body; cranial nerves connect to the brain, and spinal nerves connect to the spine.
• Sensory neurons detect environmental changes and send information to the CNS.
• Interneurons are located in the CNS between sensory and motor neurons.
• Motor neurons control muscles and glands to control motor behavior. Neurons work together in circuits.

Parts of a Neuron

• Neurons, also known as brain cells or nerve cells, transmit information within the nervous system.
• Neurons have dendrites, axons, terminal buttons, and a soma (cell body).
• Dendrites receive chemical messages from other neurons.
• The synapse is the junction between the terminal buttons of the presynaptic neuron (sending cell) and the dendrites of the postsynaptic neuron (receiving cell).
• The axon is a long tube that transmits an electrochemical message known as an action potential.
• The myelin sheath insulates the axon, preventing neuron messages from interfering with each other.
• Axoplasmic transport transports substances between the soma (cell body) and the terminal buttons, and occurs via microtubules.
• Anterograde transport moves substances from the soma to the terminal button.
• Retrograde transport moves substances from the terminal button to the soma.
• Terminal buttons, located at the end of the axon, secrete chemicals called neurotransmitters into the synapse.
• Released neurotransmitters can have excitatory or inhibitory effects on the receiving cell.
• The soma (cell body) contains the nucleus and other essential machinery for neuronal function.
• The membrane is the cell border, composed of lipids and proteins acting as substance detectors, security guards, and transporters.
• The cytoskeleton provides structure to the cell using protein strands, and the space inside the membrane is filled with cytoplasm with organelles, like mitochondria and the nucleus.

Glial Cells

• Neurons need support for energy, nutrients, and protection, which glial cells provide.
• Glial cells physically hold neurons together and regulate the supply of chemicals and nutrients.
• They insulate neurons, improving the speed and efficiency of signal transmission, and remove dead neurons and debris.
• Four types of glial cells: astrocytes, oligodendrocytes, microglia, and Schwann cells.
• Astrocytes, oligodendrocytes, and microglia are located in the central nervous system (CNS).
• Schwann cells are located in the peripheral nervous system (PNS).
• Astrocytes regulate the chemical environment and provide nutrients to neurons, physically support neurons, and contain neurotransmitters, regulating their concentration at the synapse.
• Astrocytes use phagocytosis to remove damaged cells and debris.
• Oligodendrocytes create myelin, which insulates axons in the CNS, create myelin sheaths with small gaps called Nodes of Ranvier.
• Microglia are the immune cells of the CNS and use phagocytosis to remove dead cells and debris.
• Schwann cells produce myelin in the PNS, and each Schwann cell forms one segment of the myelin sheath.
• Multiple Sclerosis (MS) is a chronic illness that primarily affects the nervous system.
• The immune system attacks the Myelin Sheath, resulting in limited function.

Blood-Brain Barrier

• The blood-brain barrier regulates the passage of substances between the blood and the brain tissue.
• Unlike blood vessels elsewhere, the cells lining blood vessels in the brain are tightly packed, creating a selective barrier.
• The blood-brain barrier protects the brain from harmful substances and maintains a stable environment via controlling the entry and exit of molecules.
• Some molecules such as fat-soluble substances can pass through the barrier, while others, like glucose, require transport via specialized proteins.
• The area postrema, located in the hindbrain, is more permeable, detecting chemical threats and inducing nausea/vomiting to eliminate them.

The Withdrawal Reflex

• The withdrawal reflex illustrates how neurons work together in circuits: sensory neuron → interneuron → motor neuron.
• Touching something hot triggers the sensory neuron's dendrites to detect pain, sending an action potential down its axon.
• The sensory neuron's terminal buttons synapse with an interneuron's dendrites, releasing neurotransmitter.
• Excitation occurs when the neurotransmitter increases the likelihood of the postsynaptic neuron sending its own action potential.
• The interneuron then sends an action potential to the motor neuron, exciting it.
• The motor neuron's axon carries the action potential to a nerve, resulting in muscular contraction and withdrawal from the hot object.
• The withdrawal reflex can be overridden to prevent dropping something important using a second interneuron connected to a neuron from the brain which inhibits the motor neuron.

Neurons and Electrical Charge

• Neurons use electrical charge to transmit information.
• Membrane potential is the difference in electrical potential between the inside and the outside of a neuron.
• Resting potential is around -70mV; the inside of the neuron is negatively charged relative to the outside.
• Depolarization occurs when the inside of the neuron becomes less negative, increasing the likelihood of an action potential.
• Hyperpolarization occurs when the inside of the neuron becomes more negative, decreasing the likelihood of an action potential.
• The threshold of excitation is the level of depolarization required to trigger an action potential and is usually around -55mV.
• The action potential is a brief reversal of the membrane potential.

Neuron Resting Membrane Potential

• A neuron's resting membrane potential is negative, with the inside of the neuron more negatively charged than the outside.
• This difference is due to varying concentrations of ions inside and outside the neuron: organic anions (negative, inside), potassium ions K+ (positive, mostly inside), chloride ions Cl- (negative, mostly outside), and sodium ions Na+ (positive, mostly outside).
• Ion concentrations are determined by electrostatic pressure and diffusion.
• Electrostatic pressure states ions of the same charge repel each other, while ions of different charges attract.
• Diffusion is when ions move from areas of high concentration to low concentration.
• Organic anions are located only inside the cell and cannot pass through, and potassium is pressured outwards by diffusion but inward by electrostatic pressure.
• Chloride ions are pressured inwards by diffusion but outward by electrostatic pressure.
• Both diffusion and electrostatic pressure push sodium ions into the neuron.
• The sodium-potassium pump consists of proteins that actively push sodium ions out of the cell while transporting potassium ions in, maintaining the high concentration of sodium in the extracellular fluid, consuming a lot of energy.

Voltage-Gated Ion Channels

• Voltage-gated ion channels open or close depending on the cell's charge, regulating ion flow.
• When an ion channel is closed, ions cannot move in or out; when open, ions can move freely.
• The number of open channels determines the membrane's permeability to that ion.
• The opening and closing of sodium (Na+) and potassium (K+) voltage-gated ion channels is what causes the action potential to occur.
• At the threshold of excitation, voltage-gated Na+ ion channels open, causing rapid depolarization.
• After depolarization, voltage-gated K+ ion channels open; since the neuron is positively charged, electrostatic pressure and diffusion cause K+ ions to leave the neuron.
• Na+ channels become refractory. The K+ channels remain open, and K+ ions continue to exit, repolarizing the cell.
• As the K+ channels eventually close, Na+ channels reset, ending the refractory period, and the cell becomes slightly hyperpolarized before returning to its resting state.
• The sodium-potassium pump helps restore balance.
• An action potential is caused by brief increases in permeability to sodium ions followed by potassium ions.

Conduction of the Action Potential

• Changes in ion concentration alter the membrane potential of a neuron, helping explain the action potential.
• An action potential is first generated in a section of the neuron called the axon hillock, which has many voltage-gated sodium ion channels, which are important for depolarization.
• Voltage of the action potential remains the same as it travels down the axon.
• Once a neuron reaches the threshold of excitement, the action potential cannot be stopped and remains the same strength.
• The rate law states that to indicate differences in intensity, the rate at which action potentials are generated changes.
• Myelin protects axons, but doesn’t form a single long tube around the axon, instead it is broken up into segments with small gaps in between called the nodes of Ranvier.
• The Nodes of Ranvier allow for saltatory conduction of an action potential which is speedy and economical.

Neuron Communication

• Neurons communicate with each other via synaptic transmission.
• The presynaptic cell sends the message, and the postsynaptic cell receives the message.
• Terminal buttons release neurotransmitters into the synapse, and dendrites pick up the neurotransmitters.
• Neurotransmitters are detected by receptor proteins located in the postsynaptic cell's membrane.
• Neurotransmitter binding causes a postsynaptic potential (PSP), which are brief changes in the charge of the receiving cell.
• Changes in charge can be depolarization (more positive) or hyperpolarization (more negative).
• PSPs can cause the postsynaptic neuron to reach the threshold of excitation, leading to an action potential.
• Axodendritic synapse is between the terminal buttons and the dendritic membrane; axosomatic is between the soma and the terminal button, and axoaxonic is between two terminal buttons. The synaptic cleft is the fluid-filled space between the pre- and postsynaptic membranes.
• When an action potential reaches the terminal button, synaptic vesicles release neurotransmitter into the synapse.

Neurotransmitter-Dependent Ion Channels

• Neurotransmitter molecules attaching to binding sites on postsynaptic receptors cause neurotransmitter-dependent ion channels to open.
• Anions are negatively charged ions and cations are positively charged ions that move in and out of a cell through ion channels.
• These channels only open when neurotransmitter molecules bind to a site on a postsynaptic receptor, causing postsynaptic potentials.
• Ionotropic receptors open ion channels directly, while metabotropic receptors open ion channels indirectly.
• Whether a postsynaptic potential is excitatory or inhibitory depends on the type of neurotransmitter-dependent ion channel that is opened following neurotransmitter binding.
• Sodium (Na+) influx leads to excitatory postsynaptic potentials (EPSPs).
• Potassium (K+) efflux leads to inhibitory postsynaptic potentials (IPSPs).
• Chloride (Cl-) influx results in either no change or inhibitory postsynaptic potentials (IPSPs)
• Calcium (Ca2+) influx leads to excitatory postsynaptic potentials (EPSPs) and other cellular effects.

Autoreceptors and Neuromodulators

• Postsynaptic potentials (PSPs) are changes in charge in a neuron, however receptors are also located on the terminal buttons.
• Postsynaptic receptors are on the receiving cell, while autoreceptors are on the sending cell.
• Synapses are formed between the terminal buttons of two neurons at axoaxonic synapses.
• The autroreceptors respond to neurotransmitters released by the presynaptic neuron itself.
• The function of autoreceptors is to regulate neurotransmitter production
• Neuromodulators diffuse across extracellular fluid and influence the activity of several neurons at once, relating to behaviours such as vigilance.
• Hormones are released by endocrine glands and distributed through the bloodstream, influencing neuronal activity via neurons with special receptors, which in turn modulate activity to shape behaviour.