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Module 2-Lecture 2: Nervous System

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Biological Sciences
Stephen Reid

1 Lecture 2.2-Nervous System Notes The Nervous Reflex Arc -there are various steps in the transmission of nervous signals 1. Pain stimulus detected by the receptor cell 2. Nervous signal initiated 3. Signal sent via afferent neuron to the central nervous system (brain and spinal cord) -afferent signal integrated by the CNS -in the CNS the signal is interpreted by the interneurons 4. Afferent neuron stimulates interneurons 5. Interneurons stimulate efferent neurons (outgoing) 6. Signal then goes to effector organs (nerves and glands) Example (in diagram): 1. Receptor neuron feels the hot touch of the stove (pain generated) 2. Afferent neurons take this developed signal and send it to the CNS-spinal cord in this case (through neuron to neuron or synaptic connections) 3. Stimulates the neuron that send signals to the biceps muscle and inhibits the neuron which sends signals to the triceps muscle 4. This causes the biceps muscle to contract and triceps muscle to relax 5. This allows for the hand to move away from pain Central Nervous System -consists of brain and spinal cord HINDBRAIN -the most primitive part is the hindbrain -contains medulla oblongata, pons, cerebellum Medulla: contains control centers for cardiovascular nad respiratory function Pons: important relay site and involved in involuntary control Cerebellum: involved in balance and motor coordination MIDBRAIN -involved with auditory and visual information -size decreases as veterbrates get bigger FOREBRAIN -contains the cerebral hemispheres (cerebrum and cerebral cortex) involved in consciousness -other structures are the thalamus, hypothalamus and pituitary Peripheral Nervous System -components not part of CNS AFFERENT NERVOUS SYSTEM -sends input to the CNS -it has sensory receptors EFFERENT NERVOUS SYSTEM -carries information from CNS to rest of the body -divided into 2 parts February 6, 2014 2 Lecture 2.2-Nervous System Notes SOMATIC NERVOUS SYSTEM -regulates skeletal muscle contraction AUTONOMIC NERVOUS SYSTEM -divided into 2 parts SYMPATHETIC NERVOUS SYSTEM -stimulates signals -all nerves originate from the spinal cord PARASYMPATHETIC NERVOUS SYSTEM -brings the body back to homeostasis -parasympathetic nerves originate from cranial nerves (optic nerve) or vagus nerve (some do originate from lower end of spinal cord) -the two systems have opposite effect on a particular organ -one system stimulates the other inhibits Sensory Receptors -come in many forms (they are basically a type of cell) -they detect multiple stimuli CHEMORECEPTORS -sense some form of chemical stimuli -this can be O2 levels, CO2 levels, pH, ions, peptides, sugars -sense of taste and smell due to these cells MECHANORECEPTORS -sense some form of physical distortion -increase in pressure -bending of hair follicle -stretching of lung -movement or muscle PHOTORECEPTORS -sense light (photons) 1. Receptor initially sensed by receptor protein or molecule on the cell membrane 2. Interaction of receptor protein and sensory stimulus activate signal transduction pathway in the cell -opening and closing ion channels or cell membrane or activation of intracellular secondary messenger pathway 3. Change in membrane potential of sensory receptor cell (the cell that receives the sensory stimulus) across the plasma membrane (usually becomes more positive) 4. Afferent nerve stimulated which stimulates sensory receptor cell 5. Nerve signal initiated and sent to the CNS Neurons and Nerves -nerve cell = neuron -nerve is a collection of axons from many different neurons (nerve cells) -cell bodies of all these neurons found in ganglia or nuclei February 6, 2014 3 Lecture 2.2-Nervous System Notes 1. Dendrite receives sensory information from another cell -pre-synaptic cell stimulates post-synaptic cel and receives information 2. Stimulation leads to small change in membrane potential within post-synaptic cell dendrites 3. Leads to stimulation of axon hillock (region of neuron cell) also called trigger zone or spike initiation zone 4. Action potential travels down axon 5. This leads to stimulation of another cell (neuron or effector organ) via synaptic transmission -branch of axon from pre-synpatic cell forms synapse with dendrite of post-synaptic cell -site of nervous (synaptic) transmission from one neuron to another neuron What is the function of the connective tissue in the diagram? Glial Cells and Myelin Formation -glial cells are support for neurons -3 types of glial cells 1. Microglia: removes waste 2. Astrocytes: help keep neurons in place and form blood-brain barrier (barrier to the movement of certain substances from the blood into the brain tissue) 3. Oligodendrocytes: within central nervous system 4. Schwann cells: within peripheral nervous system and they form myelin MYELIN -insulating sheath that covers axons -formed from oligodendrocyte and Schwann cells -plasma membrane of Schwann cells wraps around axon multiple times creating a myelin sheath -low permeability to ions -makes it a good electrical insulator -not all axon is covered in myelin NODES OF RANVIER -gaps in the myelin sheath -allows for rapid conduction of nerve impulses -this process is called salutatory conduction Generating and Transmitting a Nerve Signal 1. Electrical activity sent down the axon to the synapse -this is in order to send nervous signal to another neuron or an effector organ 2. Electrical activity generated in the axon hillock (action potential) a. Resting potential b. Depolarization phase (goes towards positive) c. Repolarization phase (goes towards negative) d. Undershoot (goes below resting potential) e. Threshold (returns back to resting potential after electrical signal is passed) 3. Action potential goes to the synapse (site at which a pre-synaptic neuron sends signal to post synaptic neuron or effector organ cell) 4. Synaptic Transmission: electrical signal from one neuron to the next accomplished by chemical or electrical signals February 6, 2014 4 Lecture 2.2-Nervous System Notes Resting Membrane Potential -plasma membrane of a cell has a electrical potential (electrical difference) across it -inside = negative -outside = positive -difference in the inside and outside is called the membrane potential -excitable cell: neuron or muscle cell, membrane potential changes depending on whether the cell is at rest or being excited -resting membrane potential: when cell is at
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