Dendrites - receives information from other neuron cells - input receivers.

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1 The Nerve Tissue Neuron - the nerve cell Dendrites - receives information from other neuron cells - input receivers. Cell body - includes usual parts of the organelles of a cell (nucleus, mitochondria) except centrioles --> cannot divide. the cytoskeleton is composed of neurofilaments and neurofibrils. the proteins are synthesized in the cell body and have to be transported to the synaptic terminals. Axon - carries signal towards other cells. the axon hillock is where the signal is first generated. Myelin sheath - a fatty covering that protects the axon and increases conductivity. Synapses - the end of the neuron which transmits the message to other neurons via a chemical and electrical signal.

2 Classification of Neurons 1. Multipolar neurons: Most common in the CNS. 2. Unipolar neurons: The dendrites and axons are fused (continuous). Cell body lies off to one side. This comprises most of the sensory neurons in PNS. 3. Bipolar neurons: sensory neuron in retina, ear and olfactory system (smell). 4. Anaxonic neurons: Present in retina and CNS. 10 million sensory neurons (pick up any senses to send to brain). 20 billion interneurons (where the processing happens). 500,000 motor neurons (performing an action). Neuroglia - cells that support & protect neurons They make up have the volume of the nervous system. 1. Astrocytes: Gets its name because it looks like the starts (astro). Maintain blood-brain barrier (controlling basically what gets through to the brain). Provides structural support They regulate ion, nutrient and gas concentrations in interstitial fluid. 2. Oligodendrocytes: These are the myelin sheaths - wrap around/insulates the axon. One oligodendrocyte wraps around many neuron axons. 3. Microglia: Related to immune system s cells. They ingest/remove cellular debris and pathogens. 4. Ependymal cells: Are highly branched. Assists in producing, circulating and monitoring cerebrospinal fluid (CSF).

3 All those above were in the CNS, the PNS contains Satellite cells and Schwann cells. 1. Satellite cells: Surrounds neurons in ganglia. Regulate intercellular 2. Schwann cells: Insulate the axon - similar to oligodendrocytes, but the Schwann cells insulates neurons in the PNS. Myelination, and because its white --> white matter. If the axon is not fully wrapped around by myelin then its an called un-myelinated axon. White matter - regions with myelinated axons. Gray matter - regions with un-myelinated axons.

4 The areas between the Schwann cells are called Nodes. The Schwann cells themselves are known as Internodes. Neurons perform all communication, information processing and control functions of the nervous system. Neuroglia preserve physical and biochemical structure of neural tissue and are essential to survival and function of neurons. Transmembrane Potential This is due to the unequal distribution of charge across the membrane. When charged particles (Ions) are moving it generates and electrical signal. Inside the membrane is usually slightly more negatively charged. Resting potential - when the cell is not sending any information (-70 mv). Graded potential - when there is a temporary change caused by some type of stimulus. Action potential - electrical impulse produced by the graded potential - it will move across the axon to the synapses - there is a large graded potential. In order to get the neuron to fire/something to happen there needs to be a change in the resting membrane potential of -70 mv (make it more positive). If this happens there will be an action potential. On the plasma membrane of the neuron cell there are K+ and Na+ leakage channels in which Potassium and Sodium ions get leaked through (Sodium moving in and Potassium moving out), and to stop this from happening there is a Sodium Potassium exchange pump. In the sodium potassium pump, for every 3 Na+ there are 2 K+ ions moved. This is all to maintain the concentrated of the resting gradient of -70 mv. The pump is powered by ATP. Passive channels - the leaking channels which are always open. Active channels - the gated channels which open and close, usually closed during rest. The plasma membrane is selectively permeable (very picky).

5 The gated channels open and close based on the difference between the transmembrane potential. Sometimes the gated channels can open based on a physical distortion --> mechanical: When the channel opens and the Na+ move into the cell it becomes depolarized. When the Na+ move back out of the cell it becomes re-polarized --> back to the resting potential. When the K+ moves out of the cytoplasm and into the extracellular matrix it s known as hyperpolarisation. If the transmembrane potential gets high enough (-55 to -60 mv) then it has reached its threshold. This means that after it has reached this threshold the Na+ will keep coming in the cell, causing the transmembrane potential to increase rapidly. However at some point (maybe +30 mv) the channel will close and the sodium potassium pump will take over and reduce the transmembrane potential back down to the resting potential.

6 The action potential does not move along the axon by itself - it keeps regenerating adjacent axonal segments --> propagation. Saltatory propagation is the fastest way of information being sent across a neuron as the myelin sheaths keeps the current in the axon so hardly any of it decays as it s moving towards the next node: Strength of muscle contraction is encoded in the frequency of action potentials sent by the motor neurons.

7 Chemical Synapses 1. Action potentials arriving at the pre-synaptic terminal cause voltage-gated Calcium ion channels to open. 2. Calcium ions diffuse into the cell and cause synaptic vesicles to release acetylcholine, a neurotransmitter molecule. 3. Acetycholine molecules diffuse from the pre-synaptic terminal across the synaptic cleft (empty space) and bind to their receptor sites on the ligand-gated sodium ion channels. 4. This causes the ligand-gated sodium ion channels to open and sodium ions diffuse into the cell, making the cell membrane potential more positive. If the membrane potential reaches threshold level, an action potential will be produced. The post-synaptic cell can be neuron or other cell. Ligand = chemical. The more ACh is released the more channels respond and the larger the depolarization (Na+ moving into a cell). If the depolarization is great enough, an action potential will appear in the postsynaptic neuron.

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9 Postsynaptic Potentials 1. Excitatory postsynaptic potential (EPSP): Depolarization occurs --> Na+ flow in the cell --> shifts transmembrane potential closer to the threshold - facilitated. 2. Inhibitory postsynaptic potential (IPSP): Hyperpolarisation occurs --> opening chemically gated K+ channels. Shifts transmembrane potential further away from the threshold - inhibited. If theres more inhibition - a much larger stimulus is needed to reach the threshold. Temporal & Spatial Summation 1. Temporal Summation - before the EPSP can dissipate, another arrives. 2. Spacial Summation - involves multiple synapses activated simultaneously. The Autonomic Nervous System