How Nerve Signals Maintain Homeostasis Chapter 9 The Nervous System Intro • Every single stimulus (light, touch, feeling of muscles moving, pressure, blinking, etc.) involves your nervous system. • Coordinated motions (which do not seems to be coordinated) are controlled mostly by the nervous system. • nervous system + endocrine system control the actions of the body. The Nervous System • Elaborate communication system – Contains 100 billion nerve cells in the brain alone. – Humans: memory, learning, language, etc. Vertebrate Nervous System • 2 main divisions: – Central nervous system (CNS) • Brain + spinal cord coordinating centre – Incoming and outgoing info. – Peripheral nervous system (PNS) • Info between organs of body and CNS. – Two peripheral divisions: » Somatic • Skeletal muscles, bones, and skin • Sensory somatic PNS: relay info about environment to CNS. • Motor somatic PNS: initiate appropriate responds. » Autonomic • Motor nerves which control internal organs of body. • Sympathetic • Parasympathetic Organization of the Nervous System Nerve Cells • Glial cells – Nonconducting cells structural support and metabolism of nerve cells. • Neurons – Functional units of nervous system (conduct electricity) • Sensory neurons – “afferent neurons” – Sense and relay info (stimuli) from environment to CNS » (ex// photoreceptors, chemoreceptors, thermoreceptors, etc.) » Ganglia: clusters of sensory receptors. • Interneurons – Integrate and interpret sensory and motor neurons (found mostly in the CNS) • Motor neurons – “efferent neurons” – Relay info to effectors: muscles, organs, and glands (can produce a response) Anatomy of a Nerve Cell (Neuron) • All neurons contain: – Dendrites • Receive info from other nerve cells • Conduct info towards the neuron’s cell body – Soma: neuronal cell body (containing nucleus) – Axon • Extension of cytoplasm • Carries nerve impulse towards other neurons or effectors. • Creates an “attachment” to other neurons. • Super super thin (100 could fit inside single human hair!) Myelin Sheath • Insulation – Myelin sheaths are formed by special glial cells called Schwann cells – Myelin sheaths prevents the loss of ions from axon. • (important when you understand how these cells work!) • Made of ‘fatty protein’ Nodes of Ranvier • Between sections of myelin sheath • Nerve impulses jump from one node to another speed up movement of nerve impulses. – Nerve impulses move much faster along myelinated nerve fibres than nonmyelinated ones. (Nerve impulses move much faster along smaller diameter axons) Neurilemmas • Thin membrane which surrounds the axon in peripheral nervous system – Promotes regeneration of damaged axons – Severed neurons can be rejoined. – (feeling gradually returns to your finger following a paper cut) White and Grey Matter • White matter – Nerve cells within the brain that contain myelinated fibres and neurilemma • Myelinated axons white in appearance • Grey matter – Nerve cells within the brain and spinal cord referred to as grey matter: lack myelin sheath and neurilemma do not regenerate after injury • Damage to grey matter is usually permanent. Categories of Neurons • Sensory Neurons: AFFERENT neurons: relay info from environment CNS – Located in clusters called ganglia • Photoreceptors (eyes) • Chemoreceptors (tongue and nose) • Thermoreceptors (skin and hypothalamus) • Interneurons – Interpret sensory info and send info to outgoing motor neurons (mostly in CNS) • Motor neurons: EFFERENT neurons: relay info from interneurons effectors (produce a response) – Muscles – organs, – glands Organization of Nerves • Most nerves are part of many neurons/axons held together in a ‘bundle.’ Neural Circuits • Reflex – Involuntary and often unconscious • Example: touching a hot stove – How long does it take you to remove your hand? – Nerve impulse is carried to the spinal chord motor neuron (skipping brain) sensation felt after removing hand. Reflex arc • Simplest nerve pathway. • Occurs without brain coordination • Five essential components: – Receptor – Sensory neuron – Interneuron (spinal chord) – Motor neuron – Effector Seatwork/Homework • Page 417 1) # 1-6, 8. 2) Draw/ Label a general neuron: dendrite(s), soma, nucleus, axon, myelin sheath, axon terminals, direction of impulse. 3) Read page 415 article. Make brief notes.