Nervous System PDF

# Nervous System ## The Nervous System - General functions of the nervous system: - The nervous system is one of the major communication systems of the body and neurons help maintain the homeostasis of the body. - Organs of the nervous system: - **Central Nervous System (CNS)** - consists...

# Nervous System ## The Nervous System - General functions of the nervous system: - The nervous system is one of the major communication systems of the body and neurons help maintain the homeostasis of the body. - Organs of the nervous system: - **Central Nervous System (CNS)** - consists of brain and spinal cord (SC) - **Peripheral Nervous System (PNS)** - peripheral nerves connect the CNS to other body parts. The peripheral nervous system includes the cranial nerves and the spinal nerves. - The part of the nervous system that senses external environmental factors involves sensory receptors at the ends of the peripheral nerves. These receptors are specialized to gather information about what is going on around the body. They convert this information into signals (nerve impulses) which then are transmitted over the peripheral nerves (afferent, sensory nerves) to the CNS. - In the CNS, the nerve impulses are interpreted (integrated) and decisions are made as to what should be done. Signals are then sent from the CNS back down efferent (motor) neurons and cause the body to respond in some way. ## Nerve Tissue - Nerve tissue consists of **neurons** and **neuroglia cells**. - **Neurons** - react to physical and chemical changes in the environment, conduct nerve impulses to other neurons and to cells outside the nervous system (e.g. muscle, BVs, glands). - **Neuroglia cells** - supportive cells; provide structural support for the neurons, supply nutrition to the neurons, form scar tissue, form myelin, carry out phagocytosis. ## Neuron structure - cell membrane - mitochondria - lysosomes - Golgi apparatus - microtubules - neurofibrils - provide support for cell processes - Nissl bodies - similar to rough endoplasmic reticulum - inclusions - nucleus - nucleolus ## Dendrites - Usually many processes extend from the cell body. The dendrites are short and highly branched. - Dendrites receive impulses from other neurons. - Dendrites often have dendritic spines (thorn-like). ## Axons - Usually only one axon is present projecting off a cell body of a neuron. - The axon arises from the axonal hillock. - The axon conducts nerve impulses away from the cell body. - The axon may give off branches (collaterals). An axon may have many branches at its end. Each ending is called a presynaptic terminal (or synaptic terminal). The presynaptic terminal almost comes into contact with another cell, but a small space will remain between the cells. The space is the synaptic cleft. - An axon carries substances produced by the cell body down its length to the end of the axon by axonal transport. Ions, nutrients, and neurotransmitters may be carried by axonal transport. ## Neuroglia Cells - Six types of neuroglia: - **Oligodendrocytes** - form myelin in the CNS - **Schwann cells** - form myelin in the PNS - Astrocytes - Microglia - Ependymal cells - Satellite cells - **Schwann cells** - neuroglia cells that wrap themselves around some of the larger axons of the peripheral nerves. - A Schwann cell forms one segment of myelin around one axon. - A neurilemma surrounds the myelin sheath. The neurilemma is actually the cell membrane on the outside of the last (outer) wrap of the Schwann cell. - **Oligodendrocytes** - cells with hands that wrap around numerous axons of cells in the CNS - One oligodendrocyte produces many segments of myelin on multiple axons in the brain and SC. - Oligodendrocytes do not form neurilemmal sheaths. - **Myelination**: - Axons that have myelinated sheaths are myelinated (medullated) and those that do not have myelinated sheaths are unmyelinated. - Myelinated fibers are white and are responsible for the white matter in the brain and SC. - Unmyelinated nerve tissue is gray and forms the gray matter of the brain and SC. - Myelin begins to form during the 14th week of development but myelination continues into adolescence. - A newborn has very coarse movements because myelination of the neurons has not occurred yet. - Potty training is not successful until myelination of certain neurons has occurred. ## Classification of Neurons - **Classification By Structural Differences**: - **Unipolar neuron** - has a single process extending from the cell body. It divides into 2 branches (like a T). - One branch serves as a dendrite (connected to peripheral body parts), the other branch serves as an axon and enters the brain or SC. - The cell bodies of unipolar neurons are present in ganglia that are located outside the SC and brain (e.g. the dorsal root ganglia). - This type of neuron is found in the PNS, and is a sensory neuron. - **Bipolar neuron** - the cell body has two nerve processes extending outward - One process serves as an axon, the other as a dendrite. - Bipolar neurons are found only in special sense organs, in the sensory areas of the eyes, nose, and ears. - Bipolar neurons work as sensory receptors. - **Multipolar neuron** - many (3 or more) nerve processes come off the cell body - One process is the axon, all others are dendrites. - Most neurons with cell bodies in the brain or SC are multipolar. - Motor neurons are multipolar neurons. - Multipolar neurons are the most common type of neuron. - **Classification By Functional Differences**: - **Sensory neurons (afferent neurons)** - carry impulses from the periphery into the brain or SC. - Sensory neurons either have specialized endings on the dendrites, called receptor endings, or they synapse with receptor cells. - Most sensory neurons are unipolar, some are bipolar. - Most have their cell bodies in ganglia. - **Motor neurons (efferent neurons)** - carry impulses from the brain or SC outward to muscles or glands. Motor neurons are multipolar, and located in the CNS. - **Interneurons (association neurons)** - lie in the SC or brain (CNS) and connect other neurons. They are multipolar and relay signals between different (sensory and motor) neurons. ## Nerves and Nerve Fibers - **Structure of a Nerve**: - A neuron is a nerve cell. - A nerve fiber is the axon of a neuron. - A nerve is a group of nerve fibers. - **Similar to muscle, nerves have layers of connective tissue**: - Epineurium - around entire nerve - Perineurium - around fascicle - Endoneurium - around individual axon - **Types of Nerves**: - Nerves coming to or from the SC and connecting to other body parts are **spinal nerves**. - Nerves coming to or from the brain and connecting to other body parts are the **cranial nerves**. - Nerves can either be sensory nerves (carry only afferent nerve fibers), motor nerves (carry only efferent nerve fibers), or mixed nerves (contain both sensory and motor nerve fibers). Most nerves are mixed nerves. ## Terminology - **somatic** - refers to the skeletal muscles and skin - **visceral** - refers to smooth muscles, glands, and internal organs ## Basic Principles of Electricity - Electrical charges of opposite signs have potential energy when separated. This potential energy is called voltage and is measured in either volts or millivolts. - Voltage is measured between 2 points and is called the potential difference or the potential between the 2 points. The greater the difference in the charge between the 2 points, the higher the voltage.. - In the body, electrical currents are usually generated by the flow of ions (charged particles) across cell membranes. Cells usually have a different number of positive and negative ions on the two sides of the cell membrane. This is charge separation, so there is a voltage across the membrane. ## Current - **Current** - the flow of electrical charge (charged particles) from 1 point to another. Current can do work. ## Resistance - **Resistance** - hindrance to the flow of charge, caused by substances that the current must pass through. - **Insulators** - have high electrical resistance - **Conductors** - have low resistance to current flow - **Resistance to current flow is provided by the plasma membrane.** - The plasma membrane has many channels. - The channels in the membrane may be **passive** or **leakage channels** that are always open, or they may be **active** or **gated channels**. The "gate" is usually a protein molecule that can change shape to open or close the channel. Gated channels only allow some substances to pass through under specific circumstances. Therefore gated channels provide resistance to current flow. - **Gated channels**: - **Chemically gated** - gates open when the appropriate NT (neurotransmitter) binds. - **Voltage-gated channels** - channels open and close in response to changes in the membrane potential, or voltage. - **Mechanically gated** - gates respond to stretch or deformation - Each type of channel is usually selective for the ion(s) it allows to pass. Potassium channels usually allow only K+ to pass, sodium channels allow Na+ to pass, etc. - When ions move passively from an area of higher concentration to an area of lower concentration, they are moving along **chemical gradients**. When they move toward an area of opposite electrical charge, they are moving along **electrical gradients**. - Combined, these gradients form the **electrochemical gradient**. - Ions that flow along electrochemical gradients form the basis for all electrical phenomena in neurons. ## Electrical Potentials - There are several electrical potentials associated with nerve cells: - Resting cell membrane potential - Graded or local potentials - Action potentials - **Resting Cell Membrane Potential (membrane potential)**: - A cell membrane is electrically charged or polarized. The inside is negatively charged with respect to the outside. The polarization is due to an unequal distribution of ions on the inside and outside of the cell membrane. - Because of the nature of the pores and channels, K+ passes through easiest, Na+ easy, Ca2+ not so easy. - Because K+ passes through easiest it is the major contributor to the membrane potential (polarization). - **When a cell is at rest (not conducting action potentials), the concentration of Nat is greater on the outside of the cell membrane, K+ is greater on the inside of the membrane. In the cytoplasm, many negatively charged ions (phosphate, sulfate, and protein) are large and cannot diffuse through the cell membrane.** - **K+ diffuses freely from the inside to the outside of the cell (through leakage pores), less Na+ comes in (because of permeability of pores). The net effect is that more positively charged ions leave a cell than enter it, so the cell membrane is more negative on the inside (therefore is negatively charged), and more positive on the outside.** - **Na+ - K+ pump** - actively pumps Nat outward, and K+ back in. Tries to return the leaking K+ back into the cell and the leaking Na+ back out. - **Potential difference (or potential)** - the difference in electrical charge between two points, measured in volts. - In a resting nerve cell, the **resting cell membrane potential** is -70 mVolts. The cell is polarized. The charge on the inside of the cell, positive or negative, is what determines the sign of the potential. Since the cell is more negative on the inside than on the outside, the cell has a negative potential (-70 mV). ## Membrane Potentials That Act as Signals - A change in membrane potential (at rest) is used by neurons and muscle cells as signals for receiving, integrating, and sending information. - A change in membrane potential can be produced by: - Anything that changes membrane permeability to an ion - Anything that changes the ion concentrations on the two sides of the membrane - Two types of signals are formed by changes in membrane potential: - **Graded potential** - signal over short distances (e.g. at dendrites and across cell body) - **Action potential** - signal over long distances (e.g. down the axon) ## Graded Potentials (Local Potentials) - Nerve cells are excitable, they can respond to their surroundings. Incoming stimuli usually affect the resting membrane potential in a particular local region of the cell membrane (e.g. usually on the dendrites, sometimes on cell body or axon). - **Incoming signals (from another nerve or sensory receptor) change the resting membrane potential. If the resting potential becomes less negative (goes from -70mV to -30mV), moves toward 0, the membrane is depolarizing. The nerve has been stimulated.** - **If incoming signals cause the resting membrane potential to become more negative (eg. goes from -70 mV to -90mV), the membrane is hyperpolarizing. The nerve has been inhibited.** - **Repolarization** - membrane potential that was previously depolarized returns toward normal resting potential. ## Action Potentials (APs) - **When the threshold potential is reached (and only when the threshold potential is reached), the portion of the cell membrane that is being stimulated undergoes certain changes.** - The area of the cell membrane being stimulated has a change in permeability. - Channels selective for Na+ ions open and Na+ rushes into the cell (remember the cell is negatively charged on the inside, so it attracts the positively charged Na+). - When Na+ rushes into the cell, the inside of the cell becomes more positively charged, so it depolarizes. - Then channels open that allow K+ to pass out of the cell. Now inside of cell becomes more negatively charged again, the membrane is now repolarized and can be stimulated again. - **This rapid change of the cell membrane charge is an action potential (AP).** - **Hyperpolarize**- - Ion concentrations don't change significantly. - **Na+ - K+ pump**. Actively pumps Na+ out of cell, K+ into cell. Reestablishes the original concentrations of the ions and the original resting potential. - **When graded potentials reach the threshold potential of the nerve cell an AP is first generated in the axon hillock.** - **When an AP occurs at one point (eg. at the axon hillock), it causes an electric current to spread out to adjacent portions of the membrane. The current then stimulates the membrane to its threshold level, which then triggers other APs. This wave of APs traveling down the nerve fiber is a nerve impulse.** - **After a nerve impulse passes over the fiber, another impulse cannot be stimulated for a brief period of time. This brief period of time in which an AP cannot be activated is called the refractory period.** - **Absolute refractory period** - cannot be stimulated. Na+ permeability is changing. - **Relative refractory period** - only a very high intensity stimulus can cause an AP. - Refractory periods limit the rate at which nerve impulses can be conducted, and help insure that the APs travel in one direction (down the axon, not up the axon). ## Nerve Impulse Conduction - **Nerve impulse conduction** - transmitting the nerve impulse down the axon. - **Nerve impulse conduction is an all-or-none response. If a nerve fiber responds at all (reaches threshold), it responds completely. When the threshold is reached, an AP is generated. All APs are of the same strength, regardless of strength of stimuli.** - **If each AP is the same (in each neuron), how does the neuron encode the strength of a stimulus? Strength of stimulus is coded by the frequency of APS.** - **Myelin on the nerve axon serves as an insulator. Myelin is formed by Schwann cells and oligodendrocytes. The space between Schwann cells is the Node of Ranvier.** - **APs are generated only at the Nodes of Ranvier in a myelinated axon. The axon is permeable to Na+ and K+ only at the nodes.** - **Saltatory conduction (in myelinated axon) - nerve impulse jumps from node to node.** - **Unmyelinated neuron - conducts an impulse over its entire surface.** - The speed of impulse conduction is related to the diameter of the nerve fiber. The larger the fiber, the faster the impulse. - **Various ions affect impulse conduction in nerve fibers**: - **Ca2+** - needed to close the Na+ channels in the nerve cell membrane. - IF LOW- Na+ channels may stay open, so impulses may be repeatedly transmitted. Causes muscle spasms, tetany. (May happen during pregnancy, or after diarrhea). - IF HIGH - APs cannot occur (resting potential too low because Na+ channels stuck closed) so muscles become sluggish, paralyzed. ## The Synapse - Presynaptic neuron - Postsynaptic neuron - synaptic cleft - telodendria ## Synaptic Transmission - Axons have synaptic terminals (synaptic knobs, synaptic boutons) on their ends. The synaptic terminals have synaptic vesicles containing neurotransmitters (NTs). The NT substances cause either excitation or inhibition of the neurons. - The neurotransmitters are synthesized in the cell body, transported to the synaptic terminal, and stored in vesicles. When an AP travels down the axon to the synaptic terminal, it increases the permeability to Ca++ ions. Ca++ moves into the cell and causes the synaptic vesicles to fuse to the membrane of the synaptic terminal and release the NT into the synaptic cleft. The NT then diffuses across the cleft and binds with receptors on the membrane of the postsynaptic neuron. The amount of NT released is proportional to the amount of Ca++ in the cell. If enough NT is released (graded responses), the postsynaptic membrane reaches threshold level and a nerve impulse is triggered in the axon hillock. This impulse is then conducted down to the axon terminal and the whole process starts over again. ## Neurotransmitter Substances - Some neurons release only one type of NT. Others may release several different NTs. - Neurotransmitters include: - **monoamines** - - Acetylcholine (Ach) = Adrenaline - Epinephrine (Epi) = Noradrenaline - Norepinephrine (NE) - Dopamine - Seratonin - **amino acids** - - Glycine - GABA - glutamate - **peptides** - - enkephalins - relieve pain, opiate - endorphins - relieve pain, opiate - Substance P - causes pain - somatostatin - cholecystokinin (CCK) - **After the synaptic vesicles release their NT, they break away from the membrane and get new NT, recycle.** - After being released into the synaptic cleft, some NTs are rapidly decomposed by enzymes in the cleft, others are taken back into the neuron or neuroglial cells. - Acetylcholine - decomposed by acetylcholinesterase (in membranes of synapse) - Epinephrine, norepinephrine - decomposed by monoamine oxidase (MO) found in mitochondria. - The removal of the NT from the cleft is necessary to prevent continuous stimulation of the postsynaptic neuron. ## Neuromodulators - **Neuromodulator** - substance that changes the neuron's response to a NT, or blocks the release of a NT. - **Enkephalins** - believed to play a role in pain relief. Are synthesized during painful stress, bind to opiate receptors in the brain (like morphine). - **Endorphins** - found in pituitary gland, pain-relieving - **Substance P** - functions as a NT or neuromodulator for pain sensation. ## Synaptic potentials - The local potentials at synapses are **synaptic potentials**. They are graded and can be either hyperpolarizing or depolarizing. - **NTs can either cause excitation or inhibition of the postsynaptic neuron. Some NTs cause some channels to close, other NTs cause some channels to open.** - If a neurotransmitter causes the Na+ channels to open in the postsynaptic membrane, Na+ moves into the cell and causes depolarization. This is an **excitatory postsynaptic potential (EPSP)**. - If a NT causes K+ to diffuse out, membrane becomes hyperpolarized, so AP not likely to occur, is **inhibitory postsynaptic potential (IPSP)**. - Many neurons are interacting with each neuron in the brain and SC, so the integrated sum of all the inputs will determine whether an AP is generated at the axon hillock of the postsynaptic neuron.