Types Of Human Tissues II PDF

Types of human tissues II Types of muscular tissue - smooth muscles - unstriated - involuntary (innervated by autonomic nervous system) - in the walls of visceral organs - small elongated cells which posess centrally located nucleus - cardiac muscle - striated...

Types of human tissues II Types of muscular tissue - smooth muscles - unstriated - involuntary (innervated by autonomic nervous system) - in the walls of visceral organs - small elongated cells which posess centrally located nucleus - cardiac muscle - striated - involuntary - in the heart - single nucleus, cells connected with special junctions – conducting electrical impulses - skeletal muscle - striated - voluntary (innervated by somatic nerves) - attached to the skeleton - multinucleated Skeletal muscle contraction -basic structure involved in contraction consists of myofilaments -thick filaments – myosin -thin filaments – actin and tropomyosin - filaments slide along each other during contraction / relaxation - sarcomere – functional unit of the striated muscles ATP = ADP+Pi ATP from - aerobic glycolysis - anaerobic glycolysis Types of muscle contraction. They differ in whether the muscle is allowed to shorten as it contracts or not. When a muscle contracts isotonically it generates a tension at least equal to to any forces oposing it (called loads) and so the muscle shortens. When a muscle contracts isometrically it creates tension but does not shorten because the load is greater than the force generated by the muscle.This occurs for example when you try to lift an object that’s too heavy for you to move, or when you stand still and your postural muscles hold your body upright. Isotonic = constant tension Isometric = constant lenght Nervous tissue - neuron (nerve cell) is the basic structure of the nervous tissue - each neuron consists of: - cell body - processes - long (axons) - short (dendrites) Types of neurons (morphological classification) - unipolar - bipolar - multipolar - Golgi type I (large with long axons) - Golgi type II (short axon) Neuroglia – neuron supporting cells 1) Astrocytes (fibrous, protoplasmatic) - insulation or barriers 2) Oligodendrocytes – myelin formation in central nervous system (CNS) 3) Microglia – residual macrophages 4) Ependyma – lining of the cavities of the brain 5) Schwann cells – myelin formation in peripheral nervous system (PNS) Nerve fibers 1) myelinated – surrounded by a myelin sheath which is formed by a supporting cell: - central (oligodendrocytes) - peripheral (Schwann cells) 2) nonmyelinated (smaller axons) Neuron reaction to injury - injury to nervous tissue elicits response by neurons and neuroglia - severe injury causes cell death - once a neuron is lost it can not be replaced because neurons are ’postmitotic cells’ - that is neurons are fully differentiated and no longer undergo cells division Neuron reaction to injury 1) chromatolysis (cell body and axon proximal to the site of injury) - disorganisation of rybosomes and cellular swelling 2) Wallerian degeneration (distal axon) - disintegration of axon and all the synaptic endings 3) Schwann cells exhibit mitotic activity and produce trophic substances 4) regeneration - many neurons can regenerate a new axon if the axon is lost through injury Signal conductivity in neurons - most animal cells have an electrical potential difference across their plasma membranes - the cytoplasm is usually electrically negative relative to extracellular fluid - the electrical potential difference across the plasma membrane in a resting cell is called the resting membrane potential - the resting membrane potential (50-100mV) plays a central role in the excitability of nerve and muscle cells - the resting potential is due to uneven distribution of ions between the inside and the outside of the cell membrane Action potential(AP) - maintenance of the membrane potential is a property of all living cells, excitability however is shown only by specialized cells - nerves and muscles - nerves and muscles respond to a stimulus by production of transient changes in the ion conductance and potential of their membranes - if the stimulus is sufficiently strong an action pottential is generated which in the case of the nerve is the signal that is transmitted along the nerve cell and in muscle leads to contraction Action potential(AP) An AP consists of the following events: - the stimulus reduces the resting membrane potential to a less negative value (depolarization) - when it reaches a critical voltage to a so called threshold potential, a Na+ channel becomes activated leading to fast Na+ influx into the cell - the Na + conductivity decreases, coupled with a slow rise in K + conductance (repolarization faze) Action potential(AP) - once the threshold potential is attained, the cells responds with all-or-none depolarization - for a brief period following the depolarization faze the nerve can not be excited even by the strongest stimulus – - this is the absolute refractory period and is followed by a relative refractory period (at the end of the repolarization faze) Propagation of the action potential in nerve fibers - 2 types of potential propagation: serial and salutatory serial - slow, in nerves devoid of myelin sheath, the conductance rate - about 1 m/s salutatory - much faster - in myelinated nerves the conductance rate up to 120 m/s - since the myelinated fibers are insulated like a cable, the depolarizing electrotonic discharge along the fiber can span a grater diatance, in this case the AP is transmitted „in jumps” (salutatory propagation) Synaptic potentials - the AP transmitted along the axon releases a transmitter substance from the terminal button - depending upon the type, this substance can bring about depolarization (excitation) or hyperpolarization (inhibition) of the postsynaptic membrane - excitatory transmitters: acetylocholin, substance P, glutamate - they evoke excitatory post synaptic potentials (EPSP) - single EPSP is usually insufficient to generate AP, but several simultaneous EPSPs are able to depolarize cells to the threshold potentials (spatial and temporal summation) Synaptic potentials - inhibitory transmitters (e.g. GABA, glycin) - cause hyperpolarization and lower the excitability of the cells, this is an inhibitory postsynaptic potential (IPSP) - EPSP and IPSP can occur at the same time in the same cells - the sum of all of the EPSPs and IPSPs determines whether or not the AP is transmitted postsynaptically