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The Nervous System

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The Nervous system: I. Hard-Core facts: ? An organism's reaction to stimuli is called a response. ? The ability to respond to a stimulus is called sensitivity. ? Invertebrates have a nervous system, but not a well developed brain. Vertebrates have a nervous system as well as a well developed brain and sp. Sense organs. ? Sense organs: o Help organisms to adjust rapidly to any changes in their environment. o Enable the various parts of the body to co-ordinate with one another quickly. ? In mammals, many activities are not controlled consciously. These are called involuntary actions. ? All nerve impulses are similar. What makes them seem different is tat the impulses are sent to different parts of the brain. ? Co-ordination by chemicals is brought about by the endocrine system. ? Glands and muscles are called effectors because they go into action whenever they receive nerve impulses or hormones. ? Nerve impulses from the sense organs to the CNS is called sensory impulse. ? Those from CNS to effectors are called motor impulses. ? The nerves which connect the body to the CNS make up the PNS. ? Nerve fiber is a long filament of cytoplasm which is surrounded by an insulating sheath. ? The cell bodies of the neurones are mostly located in the brain and the spinal cord and it is the nerve fibres which run in the nerves. ? Ca2+ and Na+ ions also help in the transmission of nerve impulses. ...read more.


? Elec Impulse cannot travel thru the gap. At synapse, they secrete a chemical called neurotransmitter. ? When these chemicals touch the dendrite if the otger neurone, it produces the same elec signal. VI. NERVES: ? A nerve is a bundle of nerve fibres or neurones enclosed in a sheath of connective tissue. ? They can emerge from brain or spinal cord: ? They may contain: o Sensory nerve fibres only. Conduct imp from sense organs. o Motor only. Conduct imp to effectors. o Mixed fibres. Spinal nerves have both motor and sensory. VII. Grey Matter and White matter ? Grey Matter is the inner part of the spinal cord. It has the cell bodies of neurones. ? White matter has the nerve fibres. Outside part of spinal cord. VIII. Spinal Cord: ? Passes thru vertebral column which protects it. ? Spinal nerves emerge at intervals along the length of the spinal cord. ? 31 pairs of spinal nerves in humans. ? A narrow, central canal runs thru the center of the spinal cord. ? This canal contains fluid that brings nutrients to the spinal cord. ? This fluid is called cerebrospinal fluid. The structure of spinal nerves: ? Dorsal Root: It joins the dorsal part of the spinal cord. It contains only sensory neurons. ? Dorsal Root Ganglion: The cell bodies of the sensory neurones are clustered together in a small swelling called the dorsal root ganglion. ? Grey matter: The axons of the sensory neurones end in the gray matter of the spinal cord whereas their dendrones are located in the dorsal root and spinal nerve, ? ...read more.


3) Spinal Cord: nerve impulses are transmitted first across a synapse to the relay neurone, and then across another synapse to the motor neurone. At the same time, nerve impulses are transmitted to the brain. 4) Motor Neurones: nerve impulses leave the spinal cord along the motor neurones to the effector. 5) Effector muscle: The effector is your biceps muscle which ten contracts. This causes the hand to withdraw suddenly. Because the reflex action happens all so fast , the brain receives nerve impulses only after the effectors muscle re4ceives them. That is when you feel the pain of the hot object. Spinal reflexes and cranial reflexes: The spinal cord and the brain are the reflex centers. XI. Voluntary Actions: * Starts in the brain, * The brain sends motor impulses down the spinal cord in the nerve fibres. * These make synapses with motor fibres which enter spinal nerves and make connections with sets of effector muscles. * One of the main functions of the brain is to co-ordinate these actions so tat they happen at the right time and place and in the correct sequence. Reflex Stimulus Response Action Coughing(c) Particles making contract with respiratory track Violent contraction of diaphragm and intercostal muscles Prevents lungs from getting damaged; also prevents blockage of respiratory track Pupil Reflex(c) Bright light falling on retina Contraction of circular muscles of iris Protecting retina Knee jerk(Is) Stretching of tendon Contraction of muscles of upper thigh so that legs straighten Leg can support body's weight during walking. Swallowing(c) Food particles make contact with back of throat. Contraction of muscles of epiglottis so that entrance to trachea is closed. Prevents chocking ...read more.

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