Human Physiology - Structure and Function of the Nervous System I lyrics

Nerve Cell Structure Neurons are the basic functional units of the nervous system. They have a number of different structures which distinguish them from any other cell in the body: 1. Dendrites 2. Cell Body 3. Axon 4. Myelin Sheath 5. Nodes of Ranvier 6. Synaptic End-Bulb Neuroglial Cells Although they do not directly participate in electrical signaling, glial cells play an important supporting role in the nervous system. They outnumber neurons three to one. There are three types of glial cells in the CNS 1. Astrocytes 2. Oligodendrocytes 3. Microglial Cells The Neural Circuit A nerve impulse – or electrical signal – is created when stimulus is substantial enough to change the electrical potential of the cell membrane. The membrane potential of a cellular structure is determined by the in-flow and out-flow of sodium and pota**ium. Although this electrical charge is extremely localized on the cell membrane, it is propagated along the length of the neuron so long as the initial stimulus is greater than the threshold potential. The threshold potential is partly determined by the extent to which the membrane is polarized. If a membrane becomes too polarized, this will inhibit impulse propagation. When depolarization occurs, however, the cell is said to be “excited”. Action Potential 1. Resting membrane potential 2. Depolarizing stimulus 3. Membrane depolarized to threshold 4. Rapid Na+ entry 5. Na+ channels close; K+ channels open 6. K+ moves into extracellular fluid 7. Hyperpolarization occurs 8. K+ channels close 9. Cell returns to resting membrane potential Propagation Potential Two factors determine the velocity of impulse propagation: 1. Myelination 2. Axon diameter Nervous System Communication Synapse Neurons communicate with each other across a narrow gap called the synaptic cleft. A nerve impulse, which begins as an electrical signal in the presynaptic neuron, will be translated chemically across the synaptic cleft by a neurotransmitter, but will be translated back into an electrical signal once it reaches the postsynaptic neuron. Neurotransmitters 1. Rapid-acting a. Acetylcholine b. Norepinephrine 2. Slow-acting (neuropeptide) Postsynaptic Response As previously mentioned, once the neurotransmitter binds to receptors on the postsynaptic neuron, the message becomes electrical once again. This can result in either an excitatory postsynaptic potential or an inhibitory postsynaptic potential. In the axon hillock, the sum total of all incoming EPSPs and IPSPs is tallied and this will determine whether an action potential is triggered. They can be summed in two ways, temporally or spatially. Central Nervous System With a better understanding of how neurons transmit impulses, we can now look at the Central Nervous System, occupied by the brain and spinal cord. Brain The human brain is incredibly complex, but for our purposes, we will divide it into 4 major regions: 1. Cerebrum a. Primary Motor Cortex b. Basal Ganglia c. Primary Sensory Cortex i. Frontal lobe: general intellect and motor control ii. Temporal lobe: auditory input and interpretation iii. Parietal lobe: general sensory input and interpretation iv. Occipital lobe: visual input and interpretation v. Insular lobe: diverse functions usually linked to emotion and self-perception 2. Diencephalon a. Thalamus b. Hypothalamus 3. Cerebellum 4. Brain Stem a. reticular formation Spinal Cord The spinal cord is continuous with the medulla oblongata of the lower brain stem. It is specially designed to allow two-way conduction of nerve impulses, with sensory (afferent) fibers carrying signals from sensory receptors to the brain, while motor (efferent) fibers originate in the brain and transmits action potentials to end organs. Peripheral Nervous System Though we began with the structure and function of nerve cells, it's important to take a closer look at the PNS. In all, it contains 43 pairs of nerves: 31 spinal and 12 cranial. There are also special subdivisions of the PNS. Sensory Division Sensory neurons are vital to maintaining equilibrium in an environment that is in constant flux. There are five primary types of receptors, all of which are important to sport and athletic mobility: 1. Mechanoreceptors 2. Thermoreceptors 3. Nociceptors 4. Photoreceptors 5. Chemoreceptors Motor Division Once sensory information has been processed by the CNS, a response is calibrated and corresponding instructions are sent to the relevant organs of the body, via motor neurons. The motor division is often divided into: 1. Autonomic Division a. Sympathetic Nervous System b. Parasympathetic Nervous System 2. Somatic Division Reflex Pathways Regardless of where the sensory impulse ends up, it may elicit a motor response as soon as a stimulus is received. There are places where this response may originate: 1. Spinal cord 2. Lower brain 3. Motor cortex As one would expect, the lower the origin of the response within the CNS, the simpler the response will be. What's interesting about reflex pathways is that they also a**ist with sensory-motor integration under certain conditions. This is where the idea of “muscle memory” comes from, though, this shouldn't be confused with motor reflexes exactly. There are two very specific reflex activities that pertain to muscle function: 1. Muscle spindles 2. Golgi tendon organ Motor Response At this point, we have yet to discuss exactly what happens once a motor neuron impulse reaches skeletal muscle. It may be easiest to break it down into component parts: 1. Alpha motor neuron 2. Neuromuscular junction 3. Motor-end plate 4. T-tubules Glossary acetylcholine adrenergic afferent nerves axon hillock axon terminal central nervous system (CNS) cholinergic depolarization efferent nerves end branches excitatory postsynaptic potential (EPSP) Golgi tendon organ graded potential hyperpolarization inhibitory postsynaptic potential (IPSP) motor nerves motor reflex muscle spindle myelin sheath nerve impulse neuromuscular junction neuron neurotransmitter norepinephrine peripheral nervous system (PNS) resting membrane potential (RMP) saltatory conduction sensory nerves sensory-motor integration sodium-pota**ium pump synapse threshold