P1 OF 5 The nervous system controls/coordinates the activities of cells, tissues, & organs. The endocrine system also plays a role in control/coordination. The nervous system is more dominant. Its mechanisms are faster, more widespread, and more specific. It sends information via both electrical and chemical signals. The endocrine system relies solely on blood borne chemical messengers ( hormones ). The nervous system has 3 basic functions: 1. Sensory input it uses sensory receptors to monitor internal and external environments. 2. Integration it processes sensory information and determines the proper course of action. 3. Motor output the nervous system enacts a response by an organ/tissue/cell (i.e., an effector ) in response to a change in the internal or external environment. The nervous system has 2 main divisions: 1. Central nervous system the brain and the spinal cord. The primary function of the CNS is the integration and processing of information. 2. Peripheral nervous system the nervous tissue outside of the CNS (i.e., outside of the dorsal body cavity). Spinal nerves carry information between the tissues and the spinal cord while cranial nerves carry information between the tissues and the brain. The PNS is subdivided into the: 1. Sensory afferent division cells carrying information from sensory receptors to the CNS. 2. Motor efferent division cells carrying commands from the CNS to the effector organs. The motor efferent division of the PNS is further subdivided into the: 1. Somatic nervous system cells sending signals from the CNS to skeletal muscles. 2. Autonomic nervous system cells sending signals from the CNS to smooth muscle, cardiac muscle, and glands. The ANS is further subdivided into a sympathetic division (the fight or flight division) and a parasympathetic division (the rest and digest division). The primary cell of nervous tissue is the neuron. It is responsible for sensing information, integrating and processing, and issuing motor commands. Neurons are supported by cells known as glial cells. The functions of glial cells include: 1. Maintaining the neuron s surrounding chemical environment. 2. Facilitating nutrient transfer to neurons. 3. Destroying neuronal debris and microorganisms. 4. Lining the cavities of the brain and spinal cord. 5. Insulating the neuron fibers that transmit electrical signals. There are roughly 100 billions of neurons in the body and there is diversity of neuron shape and size. A typical neuron consists of a cell body (called the soma ) and one or more slender processes. The soma is the site of basic cellular functions. It contains the nucleus as well as the major organelles. The ER is quite well developed and is referred to as the Nissl body. The soma is the primary site of integration within the neuron. It is also part of the receptive region that receives signals from other neurons. Most somata are found within the protective environs of the skull and vertebral column (i.e., within the CNS). A group of somata in the CNS is referred to as a nucleus (note the new use of this word). A group of somata outside the CNS (i.e., in the PNS) is known as a ganglion.
P2 OF 5 Although neurons are capable of electrical signaling, the signals used between neurons or between neurons and effectors are chemical signals and many of them are manufactured by the abundant ER. The shape of the neuron is maintained by protein filaments known as neurofibrils. There are 2 types of neuronal processes that extend from the soma: dendrites and axons. Dendrites form the main region upon which a neuron is signaled by other neurons. Dendrites typically send electrical signals (amplitude based) towards the cell body. A neuron can have many dendrites. However, there is usually only one axon per neuron. The axon conducts electrical signals (frequency based) from the cell body towards the effector cells. The beginning of the axon is a conical region of the cell body known as the axon hillock. An axon may branch as it travels from the soma towards effector cells. Branches are known as axon collaterals. At its endpoint(s) the axon will branch enormously. The resulting processes are known as telodendria. Each telodendrion will end at a point adjacent to an effector cell. The endings of the telodendria are known as axon terminals (or synaptic knobs ). The junction between a telodendrion and the effector cell (or other neuron) is known as a synapse. Electrical impulses are conducted from the cell body to the axon terminals. In response to the frequency of these impulses, the axon terminals will releases chemical messengers ( neurotransmitters ) that will diffuse towards and affect the effector cells. The axon terminals contain membranous bags ( synaptic vesicles ) that are full of neurotransmitters. The electrical impulses are actually conducted by the plasma membrane of the axon (the axolemma ). Many axons are surrounded by an insulating material known as the myelin sheath. The myelin sheath increases the conduction speed of electrical impulses and protects the axons. Axons in the PNS are surrounded by a myelin sheath that is composed of Schwann cells ( neurolemmocytes ) that wrap around the axon. The outer plasma membrane of a Schwann cell is known as the neurilemma. The adjacent Schwann cells that are wrapping around an axon do not touch one another. The gaps between them are known as nodes of Ranvier. The # and arrangements of axon and dendrites allow neurons to be structurally classified in 3 ways: 1. Multipolar neurons have 3 or more processes (1 axon & the rest dendrites). (99% of neurons.) 2. Bipolar neurons have 2 processes, 1 axon and 1 dendrite that extend from opposite sides of the cell. Examples of bipolar neurons include olfactory neurons and retinal neurons. 3. Unipolar neurons have 1 short process that quickly divides into a peripheral process which extends to a sensory receptor and a central process that extends into the CNS. Unipolar neurons are weird in that the process is essentially an axon, however, it contains dendrite-like receptive area at the distal end of the peripheral process. Most sensory neurons are unipolar. Neurons are functionally classified in 3 ways as well: 1. Sensory neurons transmit electrical impulses from sensory receptors in the skin or internal organs towards or into the CNS. A.k.a. afferent neurons. 2. Motor neurons transmit electrical impulses from the CNS to the effector organs/cells. Typically multipolar. A.k.a. efferent neurons. 3. Interneurons btwn the sensory and motor neurons. Transmit information through the CNS and are the sites of integration. 99% of neurons are interneurons. A.k.a. association neurons.
P3 OF 5 Information is transmitted from one end of a neuron to the other via electrical signals. Neurons have a membrane potential. This means that there is an electrical difference btwn the cell interior and the adjacent extracellular fluid. When the neuron is not signaling, its membrane potential is the resting membrane potential. The voltage of the neuron s RMP is usually -70mV. B/c of the RMP, neurons are said to be polarized. This RMP exists because of sodium-potassium pumps and potassium leak channels. Sodium-potassium pumps are constantly breaking down ATP and using the energy from each to move 3 Na + from the inside of the cell to the outside and 2 K + from the outside of the cell to the inside. This sets up an imbalance of charge which results in the cell interior being more negative than the cell exterior. The NA/K pump maintains high extracellular levels of sodium and high intracellular levels of potassium. Potassium leak channels are integral proteins that allow potassium to flow from the cell interior to the exterior (i.e., down the K+ concentration gradient). This efflux of K+ continues until the tendency for K+ to move outward is balanced by the tendency of K+ to stay within in the cell due to the buildup of a negative voltage in the cell interior. The neuron RMP is also affected by the presence of sodium leak channels which allow a small amount of sodium to flow down its concentration gradient and enter the cell. Electrical signals travel from one end of a neuron to the other and are based upon altering the RMP. A change in the RMP that decreases the charge difference between the inside and the outside of the cell is a depolarization. A change in the RMP that increases the charge difference between the inside and the outside of the cell is known as a hyperpolarization. There are 2 basic means of electrical signaling within a neuron. They are graded potentials and action potentials. Both rely on altering the RMP. A graded potential is a local change in the RMP. Suppose a sensory neuron releases a neurotransmitter adjacent to the dendrite of an interneuron. The neurotransmitter binds to an integral protein in the dendrite s plasma membrane known as a receptor. The receptor is actually a chemically-gated ion channel. The binding of the NT causes the channel to open. Ions then move through the channel based on their electrochemical gradients. The main ion moving through the channel is Na+, which is entering the cell. Na+ entry decreases the local RMP and that particular region of the dendrite is depolarized. The entry of Na+ will push potassium ions at this locale away from the site of Na+ influx. Thus, neighboring regions of the dendrite membrane will become more positive and will have depolarized. This will continue; however the spread of depolarization will diminish with distance as most of the ions spreading away from the initial site will leave the cell via the abundant K+ leak channels in the PM. Note that the distance traveled by a graded potential depends on the initial amount of Na+ influx which in turn depends upon how many channels were opened. A depolarizing graded potential is referred to as an excitatory postsynaptic potential ( EPSP ). It s excitatory since a depolarization will move the axon hillock membrane potential closer to the threshold potential - the value at which an action potential is initiated. If the PSP is a local hyperpolarization (e.g., if chemically-gated Cl- channels were opened and Cl- entered the cell) it is referred to as an inhibitory postsynaptic potential ( IPSP ). It s inhibitory since a hyperpolarization will move a membrane farther
P4 OF 5 from the threshold potential. Normally a neuron will receive countless numbers of EPSPs and IPSPs. The net effect of them on the cell s membrane potential will determine whether the cell reaches threshold and if an action potential is fired. Such integration is known as summation. Summation comes in 2 flavors. Spatial summation occurs when the postsynaptic neuron integrates PSP s received at the same time from different presynaptic cells. Temporal summation occurs when the postsynaptic neuron integrates sequential PSP s from the same group of presynaptic neurons. Usually a combination of these summation events is occurring. Suppose a graded potential reaches the axon hillock and depolarizes the membrane there. If the graded potential is of sufficient size, it will depolarize the axon hillock membrane to a point known as threshold. Threshold is the membrane potential at which voltage-gated sodium channels open. V-gated Na+ channels are integral proteins in the axon plasma membrane that are normally closed, i.e. they bar Na+ from passing. However, at the threshold voltage, the channels change their shape and open. Na+ then rushes in and depolarizes that area of membrane. This depolarization will cause adjacent areas to depolarize and their v-gated Na+ channels will open. Sodium will then rush in at this point and depolarize this new area of membrane which will cause more adjacent v-gated Na+ channels to open. This continues down the length of the axon. The sequential opening of v-gated Na+ channels allows the wave of depolarization to move from the axon hillock to the axon terminals. However the v-gated Na+ channels only remain open for 1ms. Then they will close again. Thus the depolarization phase is brief. But it is long enough to allow for the influx of enough sodium so that adjacent v-gated Na channels will be stimulated. By the time a v-gated sodium channel has closed, a neighboring voltage-gated potassium channel will have opened. This results in the efflux of K+ from the cell. The movement of K+ from the cell will make the interior of the cell more negative and thus move the membrane potential back towards its initial resting value. The egress of K+ is thus responsible for the repolarization of the membrane. The repolarization wave will travel down the axon from the hillock to the terminals shortly after the depolarization wave. This allows the axon membrane to return to the resting membrane potential so that it may conduct another electrical signal. The wave of depolarization and repolarization travelling down the axon is an action potential. Note that the v-gated K+ channels stay open a bit too long. This causes the membrane potential to become more negative than the normal RMP). This is known as hyperpolarization. The action potential is self-propagating (since it causes the continual opening of adjacent v-gated sodium channels). It is also unidirectional. It travels from the axon hillock towards the axon terminals. This is because there exists a brief period following the closure of a v-gated sodium channel where it cannot reopen. Thus the depolarization wave can only open v-gated channels once and cannot reopen them and travel back towards the hillock. This period of v-gated sodium channel inactivity is known as the refractory period. The action potential is an all or none event. It either happens completely or not at all. If threshold is reached, the action potential will occur. If threshold is not reached, no action potential will take place. Thus no information can be encoded by the size of an action potential. Instead information is encoded by the frequency of action potentials. The speed of action potential propagation depends on whether the axon is myelinated or unmyelinated.
P5 OF 5 In unmyelinated axons, the wave of depolarization travels sequentially down the axon as every bit of membrane is depolarized in turn. This type of propagation is known as continuous conduction. In myelinated axons, the depolarization only occurs at those areas of the axolemma that are not covered by the myelin sheath (nodes of Ranvier). This type of propagation is known as saltatory conduction. Information is sent from one end of a neuron to the other via electrical signals primarily action potentials. But how is a signal sent from one neuron to another or from one neuron to an effector cell? Recall that an axon terminal will abut either the dendrites or soma of another neuron or an effector cell. This junction is known as a synapse. The space between the 2 cells is the synaptic cleft. The cell that is the sender of information is the presynaptic cell. The cell receiving the signal is the postsynaptic cell. Imagine an action potential traveling down an axon. The wave of depolarization begins at the axon hillock and travels to the axon terminals. When the membrane of the axon terminal is depolarized, v-gated calcium channels will open. This results in the influx of calcium into the interior of the axon terminal. Recall that axon terminals are full of synaptic vesicles carrying neurotransmitters. The influx of calcium causes the exocytosis of synaptic vesicles releasing NTs into the synaptic cleft. The NTs will then diffuse to the postsynaptic cell membrane and bind to receptors there. The resulting action will depend on the nature of the postsynaptic cell: If it is a muscle cell it will contract; A gland cell will secrete something; And another neuron will generate a graded potential. The postsynaptic cell is influenced by the presynaptic cell for as long as the presynaptic cell releases NTs. Neurotransmitters do not remain in the synaptic cleft for long. They may be degraded by enzymes in the synaptic cleft. They may be taken up by the presynaptic axon terminal or by glial cells. Or they may simply diffuse away from the synapse into the surrounding extracellular fluid.