Nerve and Synaps. The elementary functions of neuron. Neurons all have same basic structure, a cell body with a number of dendrites and one long axon.

Transcription

1 Nerve and Synaps 1 Neuron structure Neurons all have same basic structure, a cell body with a number of dendrites and one long axon. Dr Kayvan Yaghoobi 2 The elementary functions of neuron (1) Receive the excitations or inhibitions induced by internal or external stimulations. (2) Analyze and integrate the information from every organs. Neuron (3) Generate or carry the demands regulating the activities of the effectors. (4) Some neurons have neuroendocrine function. 2 Types of neurons

2 Neurons have 4 important zones Soma and dendrites receive the information,generate and integrate the local potential changes. Initial segment - action potentials are generated. Axon process - transmits the impulses to the nerve endings. Nerve endings - release the synaptic transmitters. The Generation of an Action Potential Figure The Generation of an Action Potential Signal transmission in neurons NaK-ATPase pumps 3Na out for 2 K pumped in. الکتروژنیک حفظ حجم سلول Graded potentials vs Action Potential

3 Divisions of the nervous system سیستم اعصاب مرکزي 2 6 سیستم اعصاب محیطی 7 اجرا کننده ها 1) عضلات (صاف مخطط و عضله قلبی) 2) غدد دکتر کیوان یعقوبی 13 1 جزوه مقدمات مغز - گیرنده ها ) سوماتیک احشایی و اختصاصی) 14 Non-excitable cells of the nervous system سیستم اعصاب مرکزی سیستم اعصاب محیطی Schwann cells cont. In unmyelinated nerves, each Schwann cell can associate with several axons. These axons become embedded in the Schwann cell, which provides structural support and nutrients. جزوه مقدمات مغز Myelination I Myelination II In the central nervous system, myelin is formed by the oligodendrocytes. One oligodendrocyte can contribute to the myelin sheath of several axons. In the peripheral nervous system, myelin is formed by Schwann cells. Each Schwann cell associates with only one axon, when forming a myelinated internode

4 Axoplasmic transport Anterograde axoplasmic transport: soma terminals - Rapid transport : 410mm/d, organelles with membrane, neurotransmitters( neuropeptide), mitochondria and enzymes - Slow transport: 1-12(0.5-10)mm/d. microtubule and microfilament, Retrograde axoplasmic transport : soma terminals 205mm/d. NGF, virus and toxin,etc. by endocytosis Types of nerve fibres Fibre type Aα (I) Function motor α fibres spindle afferents (Ia) tendon organs (Ib) Axon diameter mm / Myelin - Conduction velocity, m/s 9-18/ Aβ (II) touch and pressure 5-12/ Aγ Aδ(III) motor to muscle spindles pain, pressure, temperature 3-6/ / B preganglionic 3/ Physiologic classifications and functions of nerve fibers. C (IV) pain, touch, heat 1/ Synaptic transmission Signal integration

5 Signal integration cont. Section 2 General interactions between neurons Classifications of s Chemical synapses Electrical synapses Major types of s A: axo-somatic synapse B axo-dendritic synapse C axo-axonic synapse Typical s (chemical synapses) The small gap or space between the axon terminals of one neuron and the dendrites or cell body of the next neuron is called the. 27 Process of Typical Synaptic Transmission 1. An arriving action potential depolarizes the presynaptic membrane. 2. Calcium ions enter the cytoplasma of the synaptic knob. 3. Neurotransmitters release. 4. Neurotransmitters diffuse to and bind to the receptors on postsynaptic membrane. 5. Receptors on the postsynaptic membrane are activated, producing a postsynaptic potential.= receptive potential= local response= GP 6. Neurotransmitters are broken down. 28 Electrical Activities of Postsynaptic Neurons (Postsynaptic Potential) Forms of the postsynaptic potential Excitatory postsynaptic potential (EPSP) Inhibitory postsynaptic potential (IPSP)

6 Postsynaptic Potentials When a neuron responds to the neurotransmitter postsynaptically, it allows ions to move across its membrane. The movement of ions changes the membrane potential of the postsynaptic neuron. It is called the postsynaptic potential. Inhibitory postsynaptic potential - Excitatory postsynaptic potential 31 EPSP Excitatory transmitters Synaptic cleft bind to receptors the postsynaptic membrane's permeability to Na, Ca 2 enter the postsynaptic neuron produce a depolarizing potential IPSP Inhibitatory transmitters Synaptic cleft bind to receptors the postsynaptic membrane s permeability to Cl - or K Cl - enter the postsynaptic neuron generate a hyperpolarizing potential. They can also produced by closure of Na or Ca 2 channels - 33 The EPSP is produced by depolarization of the postsynaptic membrane. During this potential, the excitability of the neuron to other stimuli is increased, and this potential is called the EPSP. The IPSP is produced by hyperpolarization of the postsynaptic membrane. During this potential, the excitability of the neuron to other stimuli is decreased, and this the potential is called the IPSP EPSP: Types of postsynaptic potentials excitatory postsynaptic potential can help lead to the production of an action potential causes a depolarization IPSP: inhibitory postsynaptic potential can help to prevent the production of an action potential causes a hyperpolarization Features of post-synaptic potential Post-synaptic body works as a device of integration. Algebraic sum of EPSP and IPSP generated simultaneously. summation

7 Inactivation of Neurotransmitters 1. Be reuptaken by presynaptic membrane or by Glial cells (Serotonin,NE) 2. Diffusion (Neuropeptide) 3. Enzymatic degradation (ACh ) Electrical synaptic transmission --- gap junction Gap junctions allow charges to flow from on cell to the next. These ions may depolarize the adjacent cell to threshold. An AP is generated Synaptic inhibition It can be divided into postsynaptic inhibition and presynaptic inhibition according to the location. بر اساس محل به دو نوع مهار پیش سیناپسی و پس سیناپسی تقسیم می شوند Presynaptic inhibition In presynaptic inhibition, there is no change in the postsynapstic membrane, but a reduction in the release of transmitter at the presynaptic terminal of the excitatory synapses. ھیچگونھ تغییری در غشای پس سیناپسی ایجاد نمي شود اما آزادسازی نورو ترانسمیترھا در پایانھ پیش سیناپسی سیناپس تحریکی کاھش مي یابد. Presynaptic inhibition is induced by an activation of axo-axonic synapse (combined with axo-somatic synapse) Postsynaptic inhibition The inhibitory interneuron releases the inhibitory neurotransmitter which induces the postsynaptic membrane hyperpolarizing. Afferent collateral inhibition: The branches of axon synapse the inhibitory interneuron, which connect with other excitatory neurons. (reciprocal inhibition) Recurrent inhibition: The inhibitory interneuron is connected in such a way that they act back on the excited cell itself. Afferent collateral inhibition - Presynaptic inhibition - مهار شاخه جانبی آوران ) مهار متقابل یا دوجانبه) شاخه هاي آکسونی با اینتر نورون مهاري که با دیگر نورونهاي تحریکی در ارتباط است سیناپس می کنند

8 Recurrent inhibition مهار برگشتی: اینتر نورون مهاري به گونه اي عمل میکند که بر روي خود سلول تحریک شده اثر می گذارد Axo-axonic Section 3 neuronal circuit Divergence: responsible for motor and sensory systems. Convergence: responsible for the interpretation of the sensory stimuli. Chain circuit: facilitate or inhibit the signals. Recurrent circuit: prolong or shorten the signal activities. Recurrent circuit prolong or shorten Chain circuit facilitate or inhibit divergence and convergence