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NERVOUS SYSTEM Nerve tissue of vertebrates consists of:- (a) Three types of cell:- (i) Neurons, many million, function transmit messages (impulses). (ii) Schwann cells, associate with neurons in PNS (iii) Neurological cells, found within CNS (b) Connective tissue and blood vessels. Neurons Described as unipolar, bipolar and multipolar, according to how many processes project from the cell body. Three types:- (i) Motor (ii) Sensory (iii) Connector (intermediate, relay or inter) 1 Motor (efferent) neurons Transmits impulses from CNS to effectors e.g. muscles. Cell body located CNS. Axon enters a peripheral nerve and terminates in a muscle. May be over a metre long. A peripheral nerve may contain several thousand axons. Axon enclosed within a fatty myelin sheath. At about 1mm intervals are constrictions called nodes of Ranvier. Function of sheath is protection, insulates axon and speeds up transmissions of impulses. Nodes allow exchange of materials between axoplasm and surrounding tissue. Some vertebrates have axons which are non-myelated, however majority are myelated. Both myelated and non myelated neurons associated with Schwann cells, which produces the myelin sheath in the case of myelinated neurons. Both types surrounded by a thin neurilemma which is part of the Schwann cell.
Mescaline and LSD produce their hallucinatory effect by interfering with nor-adrenaline. Reflexes A quick automatic response to a particular stimulus which do not require conscious control, e.g. knee jerk, blinking. Reflex arc - the pathway of a reflex impulse Minimum number of neurons is two, e.g. knee jerk, however usually three. Not as simple as they appear. Connector neurons also transmit impulses to brain which can override the reflex action, e.g. pick up a hot valuable object. Function Complete automation of all protective and avoiding reactions, also internal regulation mechanisms. Leaves higher centres of nervous system free to deal with more complex problems involved in coping successfully with the environment. These reflexes are not learned, i.e. unconditional reflexes. 1 Innate reflexes (born with) e.g., sucking reflex - baby will suck almost any object placed in its mouth. Vital reflex which activates expulsion of milk from mother's mammary glands during suckling. 2 Acquired reflexes - young infants acquire additional reflexes and later override them at certain stages of their growth and development, e.g. grasping reflex. Conditioned reflexes (learned reflexes). e.g. Pavlov experiment with dogs. (a) Primary stimulus: FOOD Response salivation - Innate reflex (b) Primary stimulus: FOOD secondary stimulus ringing bell Response salivation (c)
Impulses spread rapidly all over the muscle in a similar way as nerve impulses are transmitted. Causes contraction of the muscle. Using energy from ATP the bonds between actin and myosin break and reform near each Z line. The Z lines are thus pulled closer together as actin and myosin do not stretch. Bridges, seen connecting the thick and thin filaments. Bonds form between the bridges and the actin filament. On contraction the bridge swings through an arc, pulling the actin filament past the myosin filament. After is has completed its movement, each bridge detaches itself from the actin filament and re-attaches itself at another site further along. The cycle is repeated. Shortening of muscle thus brought about by the bridges going through a kind of ratchet mechanism. Just as the transmission of an action potential by a neuron is 'all or nothing' event, so are the contractions of the muscle cells they innervate. This means that an individual cell is either relaxed or fully contracted. However, muscles are capable of differing strengths of contraction. This is achieved by varying the number of muscle cells involved in the contraction, i.e. whereas as the muscle cells will be used in a strong contraction, only a few will be used in a weak one. ?? ?? ?? ?? Handout nervours system and muscles.doc
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