Nervous Tissue Chapter 9

Structures of Nervous System Brain- Neurons enclosed in skull Spinal cord– connects to brain & enclosed in spinal cavity Nerves- bundles of neuronal axons Cranial emerge from brain; spinal nerves- emerge from spinal cord Ganglia- groups of cell bodies outside brain & spinal cord Enteric plexuses- networks in digestive tract Sensory receptors- monitor changes in internal or external environments

Figure 9.1

Function Sensory Receptors & afferent nerves Carry information into brain & spinal cord Integration- information processing Perception = awareness of sensory input Carry by short interneurons Motor activity- efferent nerves Signals to glands and muscles (effectors)

Organization Central Nervous System (CNS) Peripheral Nervous System (PNS) Subdivided: Somatic (SNS) & Autonomic (ANS) nervous systems Also INPUT-Afferent or Sensory division OUTPUT- Efferent or Motor division

Figure 9.2

Nervous System Neuron= nerve cell Specialized for signal carrying & information processing Neuroglia cells-support, nourish & protect neurons Neuroglia critical for homeostasis of interstitial fluid around neurons

Neuronal Structure Cell body- nucleus, cytoplasm with typical organelles Dendrites- highly branched input structures emerging from cell body Axon- conducts away from cell body toward another neuron or effector Emerges at cone-shaped axon hillock Axon terminals -at end of axon with synaptic bulbs

Figure 9.3

Myelination Axons covered with a myelin sheath Many layered lipid & protein creating insulations Increases speed of nerve conduction. Nodes of Ravier= gaps in the myelin Nodes are important for signal conduction Some diseases destroy myelin- E.g. multiple sclerosis & Tay-Sachs

Gray and White Matter White matter- primarily myelinated axons Gray matter- cell bodies, dendrites, unmyelinated axons, axon terminals & neuroglia spinal cord gray matter is centrally located

Other terms Gray matter in brain covers surface of cerebrum & cerebellum – cortex deep cluster of neuronal cell bodies = nucleus Bundle of white matter in CNS= Tract

Neuroglia ~ half the volume of CNS Cells smaller than neurons Can multiply and divide and fill in brain areas Do not conduct nerve impulses

Neuroglia-support Astrocytes- blood brain barrier Oligodendrocytes- myelin in CNS microglia - defense Ependymal cells- CSF production Schwann- PNS cell support Satellite cells- in PNS ganglia

Action Potentials Action potentials = nerve impulses Require a membrane potential electrical charge difference across cell membrane – like a Battery Ion Channels- allow ions to move by diffusion = current If no action potential then resting cell has resting membrane potential

Ion Channels Allow specific ions to diffuse across membrane Move from high concentration to low or toward area of opposite charge Leakage channels Gated channels- require trigger to open Voltage- Gated channels respond to a change in membrane potential

Resting Membrane Potential Leakage channels Cytosol high in K+ & interstitial fluid high in Na+ (sodium –potassium pumps) Leakage lets K+ through easily and Na+ poorly inside is negative relative to outside actual value depends on the relative leakage channel numbers

Figure 9.4

Action Potential (AP) Series of active events Channels actively open & close Some initial event is required to reach a voltage threshold (~ = - 55 mv) Stimulus = any event bringing membrane to threshold

Action Potential Then Depolarizing phase- Repolarizing phase- membrane potential rises and becomes positive Repolarizing phase- potential restored to resting value May overshoot =hyperpolarizing phase Then recovery to rest.

Active Events Stimulus to reach threshold Na+ channel opens=> Na+ ions enter=> positive potential=> Causes K+ channel opening => repolarization

All- or -None This sequence is always the same If threshold then the same size of changes occur no larger or smaller APs Stimulus must reach threshold to start After one AP there is a short period before next can be triggered= refractory period

Figure 9.5

Conduction of Nerve Impulses Each section triggers next locally Refractory period keeps it going the right direction unmyelinated fiber- continuous conduction With myelin- saltatory conduction Can only be triggered at Nodes of Ranvier Myelinated fibers faster & larger neurons faster

Figure 9.6a

Figure 9.6b

Synaptic Transmission Sequence of events at synapse Triggered by voltage change of the Action Potential Sending neuron = presynaptic Receiving neuron = postsynaptic Space between = synaptic cleft Neurotransmitter carries signal across cleft

Events at Synapse AP arrives at presynaptic end bulb=> Opens voltage gated Ca2+ channels=> Ca2+ flows into cell increased Ca2+ concentration => exocytosis of synaptic vesicles=> Neurotransmitter released into cleft Diffuse across and bind to receptors in postsynaptic cell membrane

Synaptic Transmission Binding at receptors Chemical trigger of ion channels May depolarize or hyperpolarize postsynaptic cell membrane If threshold reached at axon hillock then postsynaptic cell action potential results

Synaptic Transmission Finally the neurotransmitter must be removed from the cleft- Diffusion away Destroyed by enzymes in cleft Transport back into presynaptic cell Neuroglia destruction

Figure 9.7

Neurotransmitters AcetylCholine (Ach)- common in PNS Amino Acids- May be stimulatory or inhibitory Amino Acids- Glutamate, Aspartate, gamma aminobutyric acid (GABA), glycine Modified amino acids- Norepinephrine (NE), Dopamine (DA), serotonin Neuropeptides – endorphins Nitric oxide (NO)