Transmission of Nerve Impulses Honors Anatomy Physiology Electrical

Transmission of Nerve Impulses Honors Anatomy & Physiology

Electrical Signals in Neurons � neurons are electrically excitable � 2 types of electrical signals: 1. graded potentials � only for short distance communication 2. action potentials � allow communication over both short & long distances

Graded & Action Potentials � both depend on 2 basic features of plasma membrane in excitable cells: 1. existence of a resting membrane potential 2. presence of specific ion channels

Membrane Potentials � electrical voltage difference across the membrane � flow of ions thru ion channels => flow of current

Ion Channels � when open: allow specific ions to move across plasma membrane down their electrochemical gradient by diffusion ◦ ions move down their concentration gradient & (+) charged cations move toward (-) charged anions

Ion Channels � 4 1. 2. 3. 4. types: leakage channels voltage-gated channels ligand-gated channels mechanically gated channels

Leakage Ion Channels � channels randomly alternate (open/close) � typically are more K+ leakage channels than Na+ & K+ channels are leakier than Na+ ◦ (so membranes more permeable to K+ than Na+)

Voltage-Gated Channels � open in response to change in membrane potential � important in generation & conduction of action potentials

Ligand-Gated Channels � open/ close in response to a specific chemical stimulus � ligands include: ◦ neurotransmitters ◦ hormones ◦ particular ions � work in 1 of 2 ways: 1. directly: ligand molecule itself opens/closes gate 2. indirectly: ligand activates another molecule which in turn opens/closes gate

Mechanically Gated Channels � gates open/close in response to mechanical stimulation in form of: ◦ vibration (sound waves) ◦ pressure (touch) ◦ tissue stretching

Resting Membrane Potential � exists because there is small build-up of (-) charge just inside plasma membrane small build-up of (+) charge just outside membrane � separation of charge source of PE � greater the difference in charge the larger the membrane potential (voltage)

Resting Membrane Potential � neurons: membrane potentials range from � -90 mv to -40 mv � (-) indicates the inside of cell is (-) relative to the outside � cells with membrane potential are said to be polarized � most body cells are polarized with membrane potentials vary from +5 mv to -100 mv

Resting Potential Animation � http: //www. sumanasinc. com/webcontent/ani mations/content/electricalsignaling. html

Generation of Action Potentials (AP) � 2 1. ◦ 2. ◦ phases: (lasts ~ 1 msec) Depolarizing Phase (-) membrane becomes less (-) Repolarizing Phase normal membrane potential restored

AP Voltage-Gated Channels � 2 types of voltage-gated channels open & then close: ◦ present mainly in axon plasma membrane & axon terminals � 1 st to open: Na+ ◦ Na+ rushes into cell ◦ causes depolarization � then K+ channels open ◦ K+ flows out of cell ◦ produces repolarizing phase

Action Potential Characteristics 1. 2. “all-or-none” principle threshold: (~ -55 mv) must be reached for AP to occur

Action Potentials � http: //highered. mcgraw- hill. com/sites/0072495855/student_view 0/c hapter 14/animation__the_nerve_impulse. html

Refractory Period � period of time after an AP begins during which an excitable cell cannot generate another action potential

Propagation of Nerve Impulses � AP must travel from trigger zone axon terminal � this mode of travel called propagation or conduction ◦ uses (+) feedback ◦ when Na+ flows into cell it causes voltage-gated Na+ channels in adjacent segments to open

Neurotoxins � substances that produce their poisonous effects by acting on nervous system � Japanese pufferfish: produces very lethal neurotoxin TTX (tetrodotoxin) which blocks AP by inserting itself into voltage-gated Na+ channels so they cannot open

Local Anesthetics � drugs that block pain & other somatic sensations ◦ cold applied to area also produces anesthetic effect: axons propagate slower ◦ ice partially blocks axon propagation of pain � examples: ◦ Lidocaine, Novacaine � act by blocking the opening of voltage-gated Na+ channels: ◦ sensory nerve impulses cannot travel past obstructed region so pain signals never make it to the brain

Conduction � 2 1. 2. 3. 4. types: Continuous Conduction Saltatory Conduction http: //www. blackwellpublishing. com/matth ews/actionp. html http: //www. siumed. edu/~dking 2/ssb/saltc on. htm

Effect of Axon Diameter � larger diameter axons propagate impulses faster than smaller diameter ones due to larger surface area � Classified: 1. A-fibers: (5 -20 μm) travel 12 -130 m/s ◦ 2. ◦ 3. ◦ used by somatic sensory neurons & motor neurons B-fibers: (2 -3 μm) travel 15 m/s found in sensory neurons going from viscera brain C-fibers: (. 5 -1. 5 μm) travel 0. 5 – 2 m/s all unmyelinated, found in some sensory for pain from skin and viscera & in autonomic motor fibers

Axon Diameter & Conduction Speed

Signal Transmission @ Synapses � presynaptic neuron: neuron sending the AP � postsynaptic neuron: neuron receiving the AP � synapse: space between the 2 � synapses 1. 2. can be: electrical chemical

Electrical Gap Junctions � common in visceral smooth muscle, cardiac muscle, & in developing embryo � 2 advantages to electrical synapses 1. faster communication ◦ 2. ◦ AP passes directly from presynaptic neuron postsynaptic neuron synchronization electrical synapses can coordinate the activity of a group of neurons or muscle fibers

Chemical Synapses � synaptic cleft: space between filled with interstitial fluid � in response to AP axon terminal of presynaptic neuron releases neurotransmitters that diffuse across the synaptic cleft bind to receptors in plasma membrane of postsynaptic neuron producing a postsynaptic potential � electrical signal (AP) chemical signal (neurotransmitters) electrical signal (AP)

Signal Transmission at a Chemical Synapse

Transmission of AP across Synapse � http: //www. mind. ilstu. edu/flash/synapse_1. s wf

Removal of Neurotransmitter � occurs 1. 2. 3. in 3 ways: diffusion enzymatic degradation uptake by cells

Neurotransmitters � 2 1. 2. classes: small-molecule neurotransmitters neuropeptides

Plasticity in Nervous System � nervous system exhibits plasticity: capability to change based on experience � @ level of individual neurons: ◦ sprouting new dendrites ◦ synthesis of new proteins ◦ changes in synaptic contacts with other neurons

Regeneration in Nervous System � limited capacity to replicate or repair itself � until recently: thought was little or no repair done to damage in CNS: have found neuroglial cells more capable of this than previously appreciated � PNS: as long as cell body intact & Schwann cell active new axon can be regenerated

Damage & Repair of a Neuron

Homeostatic Imbalances in the Nervous System � Multiple Sclerosis (MS) � progressive destruction of myelin sheaths of neurons in the CNS � autoimmune disorder � afflicts ~ 350, 000 in USA (estimated) with: ◦ >2 x female/male ◦ white > other races ◦ diagnosis difficult but most diagnosed ages 20 - 50

MS

MS Causes? � unknown but do see: � genetic susceptibility: having 1 st degree relative with it increases your chances several-fold � possible association to living farther away from equator (? ) so maybe something to do with vitamin D � possible association with history of herpes-6, Epstein Barr virus, chlamydia

Epilepsy � characterized by short, recurrent attacks of motor, sensory, or psychological malfunction � afflict ~1% of world’s population � http: //www. dailymotion. com/video/xaigxc_b rain-animation-of-epilepticseizu_creation#. Ua. Psb 0 B-8 TY

Causes of Epilepsy 1. ◦ 2. ◦ 3. ◦ 4. ◦ 5. 6. brain damage most frequently from birth trauma metabolic disturbances hypoglycemia, hypocalcemia, uremia, hypoxia infections encephalitis or meningitis toxins alcohol, tranquilizers, hallucinogens head injuries tumors & abcesses of the brain

Medical Terminology 1. ◦ ◦ 2. 3. ◦ ◦ ◦ Guillain-Barre Syndrome (GBS) acute demyelinating disorder in which macrophages strip axons of myelin in PNS may be response of immune system to bacterial infection Neuroblastoma most common tumor in infants immature neurons (neuroblasts) grow in abdomen or adrenals Neuropathy any disorder that affects the nervous system but particularly a disorder of a cranial or spinal nerve example: Bell’s palsy