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Unit 3: Control Systems of the Human Body . Dr. Achilly. Part 1: Nervous Tissue. “Concepts” chapter 14. Nervous System--overview. One of the smallest, but most complex body systems. Made of: Brain Cranial nerves—12 pairs emerge from base of brain.
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Unit 3: Control Systems of the Human Body Dr. Achilly
Part 1: Nervous Tissue “Concepts” chapter 14
Nervous System--overview • One of the smallest, but most complex body systems. • Made of: • Brain • Cranial nerves—12 pairs emerge from base of brain. • Spinal cord—connects to the brain thru foramen magnum in skull & is encircled by vertebrae.
Nervous System--overview • Spinal nerves—emerge from spinal cord & serve specific regions of body. • Ganglia—masses of nerve tissue (mainly neuron cell bodies) outside brain or spinal cord. • Enteric plexuses—networks of neurons that regulate the digestive system. • Sensory receptors—ends of sensory neurons that monitor internal or external environmental changes.
Nervous System--overview • Nervous system has 3 basic fxns: • Sensory—detects internal & external stimuli. Carries this info to brain & spinal cord. • Integrative—the analyzing, storing & responding to sensory info. • Motor—carry out appropriate response like myo contraction or gland secretion. Info is carried from brain or spinal cord to effectors.
Nervous System--overview • The two main divisions of the nervous system are: • Central nervous system (CNS)—consisting of brain and spinal cord • Peripheral nervous system (PNS)—all the nervous tissue outside of CNS
Nervous System--overview • PNS can be further divided: • Somatic nervous system—carries sensory fibers from head, body wall, limbs, special senses, etc. Also carries motor neurons to skeletal myos.
Nervous System--overview • Autonomic nervous system—carries sensory neurons from most of the organs and motor neurons to smooth & cardiac myo and glands. • The ANS can be further divided into the sympathetic division which handles “flight or fight” responses and the parasympathetic division for “rest and digest” responses.
Nervous Tissue • The functional unit of the nervous system is the neuron. • It has electrical excitability & can propagate an electrical signal called an action potential. • Various sizes, but all contain similar parts.
Nervous Tissue • Cell body—contains nucleus, cytoplasm, all other cellular organelles. • Dendrites—these are the receiving fibers of the neuron. Usually many of them. • Axon—propagates action potential away from cell body toward another neuron. Usually singular. Place where the axon joins the cell body is an important area called axon hillock.
Nervous Tissue • The end of each axon contains many fine projections called axon terminals. • From here a neuron can communicate with another thru the synapse (the gap between neurons). • The axon terminal contains many membrane-enclosed sacs called synaptic vesicles. They store many types of neurotransmitters which are chemicals that help the electrical impulse cross the synaptic gap btwn neurons.
Nervous Tissue • Many axons are surrounded by a lipid & protein covering called a myelin sheath. • The sheath electrically insulates an axon & speeds conduction. Also aides in regeneration of injured neurons. • In the PNS the myelin is produced by a support cell called a Schwann cell. • Gaps in Schwann cells are called nodes of Ranvier.
Nervous Tissue • In CNS it’s the oligodendrocytes that myelinate the axons. • Little re-growth after injury. • Amount of myelin increases from birth to maturity.
Nervous Tissue • The areas of the nervous system that have myelinated axons appear white (white matter). • Areas of neuronal cell bodies, dendrites & unmyelinated axons appear gray (gray matter).
Nervous Tissue • In addition to neurons, about ½ the nervous system consists of support cells called neuroglia. • These cells do not propagate electrical impulses. • Can divide & “fill in” areas of injury.
Nervous Tissue • Neuroglia of CNS • Astrocytes—give structural support, wrap around capillaries of brain to form blood-brain barrier • Oligodendrocytes—form myelin • Microglia—fxn as phagocytes to remove cellular debris • Ependymal cells—produce & circulate cerebral spinal fluid which nourishes the brain and spinal cord.
Nervous Tissue • Neuroglia of PNS • Schwann cells—form myelin • Satellite cells—surround neuron cell bodies. Regulate exchange of materials btwn them & interstitial fluid (the fluid found surrounding all cells).
Electrical Signals • Production of nerve impulses depends on two features of the plasma membrane. • Membrane potential—the separation of ions across the membrane leading to an electrical voltage difference. • Specific ion channels.
Electrical Signals • When ion channels are open, ions will move down their concentration gradient thru the channels & according to charge (+ towards -, and vice versa) • The opening or closing of ion channels is due to presence of “gates.” • Four types of channels:
Electrical Signals • Leakage channels randomly alternate btwn opened & closed. • Usually there are more K+ channels than Na+. Also K+ ones are “leakier.” Result is higher membrane permeability to K+. • Voltage gated channels open in response to change in membrane potential (voltage).
Electrical Signals • Ligand-gated channels open/close in response to specific molecules that bind to the channels. • E.g. neurotransmitter binding to it
Electrical Signals • Mechanically gated channel opens/closes in response to mechanical stimulation: • Vibration • Pressure • Stretch
Electrical Signals • Resting membrane potential • Exists b/c of build up of (-) ions just inside the neuron cell membrane & (+) ions outside. • Separation of charges is a form of potential energy. About -70mV in a typical cell. • Dominant cation inside is K+, many anions (phosphates, amino acids) also. K+ can leak out, anions can’t.
Electrical Signals • Negative ions inside cell work to attract K+ back in. • Eventually an equal # of K+ ions enter & leave. • Na+ leaks inward a little. • Na+/K+ pump maintains charge difference.
At rest, the inside of a neuron's membrane has a negative charge. As the figure shows, a Na+ / K+ pump in the cell membrane pumps sodium out of the cell and potassium into it. However, because the cell membrane is a bit leakier to potassium than it is to sodium, more potassium ions leak out of the cell. As a result, the inside of the membrane builds up a net negative charge relative to the outside.
Electrical Signals • Graded potentials • Arises when a stimulus causes a ligand or mechanically gated channel to open or close. • Depending on the type of ion channel opened, the membrane can become more negative (hyperpolarized) or more positive (depolarized).
Electrical Signals • These signals are “graded” b/c they vary in size depending on the strength of the stimulus. • Large stimulus more gates open • Ion flow is localized, so it’s only useful for communication over short distances. • Usually present in dendrites.
Electrical Signals • Action Potentials • Has depolarization & repolarization phases • All or nothing response once threshold is reached. • Before an AP begins the membrane is at its resting potential. • The only movement of ions is thru leakage gates.
Electrical Signals • At rest both Na+ & K+ gated channels are closed; membrane is at -70mV resting potential.
Electrical Signals • A stimulus opens some Na+ channels. The # of channels opened depends on the strength of the stimulus. If enough channels are opened, the inside of the neuron becomes slightly positively charged b/c of all the Na+ flowing in.
Electrical Signals • Depolarization—so many Na+ channels are open that the inside of cell becomes very positive. Positive feedback is involved here. In other words, the more positive the inside becomes, the more Na+ gates that open and so on.
Electrical Signals • Repolarization—finally K+ gates open & K+ rushes out. At the same time Na+ gates close.
Electrical Signals • Undershoot—so much K+ leaves the cell that it becomes more negative than the original resting potential. K+ gates close. This phase is also called a refractory period. Another AP cannot occur in this portion of the membrane until Na/K pumps can restore the original ion concentration gradient & resting potential.
Electrical Signals • In order to relay information the AP must travel all the way down the axon. Called propagation. • When one segment of the membrane depolarizes the flood of Na+ causes gated channels in the next section to open, etc. • Called continuous conduction.