Nervous System PDF

Nervous system About this Chapter ◼ Organization of the nervous system ◼ Electrical signals in neurons ◼ Cell-to-cell communication in the nervous system  The Nervous System (NS) Functions Sensory function : gathering information Integrative functi...

Nervous system About this Chapter ◼ Organization of the nervous system ◼ Electrical signals in neurons ◼ Cell-to-cell communication in the nervous system  The Nervous System (NS) Functions Sensory function : gathering information Integrative function : processes & interprets sensory input, and decides if action is needed Motor function : response to integrated stimuli  effectors The NS is classified by structure & function Structural Classification Functional Classification The Nervous System Cells of the Nervous System Neurons – Functional Classification 1. Afferent – Conduct APs to the CNS 2. Interneurons – Conduct APs within the CNS 3. Efferent – Conduct APs to the PNS  Nervous Tissue: The Neurons Functional cells of the NS Specialized to transmit stimuli Consists of: cell body: nucleus & cell organelles cytoplasmic extensions from the cell body form Dendrites Axons The Nervous System Cells of the Nervous System Cells are grouped into two functional categories – Neurons Do all of the major functions on their own, are 1. Afferent 2. Interneurons 3. Efferent – Neuroglia Play a supporting role to the neurons Divided into CNS and PNS Neuroglia – CNS – PNS » Astrocytes » Neurolemmocytes » Oligodendrocytes (Schwann cells) » Microglia » Ependymal cells » Satellite cells Electrical Signals in Neurons Neurons are electrically excitable due to the voltage difference across their membrane. Electrical Signals in Neurons Neurons communicate with two types of electric signals: graded potentials that are used for short- distance communication only (i.e. local membrane changes). action potentials that can travel over both short and long distances within the body. (Nerve action potential is called nerve impulse)  As you touch the pen, a graded potential develops in the sensory receptors in the skin of the finger. The graded potential trigger an action potential which travel to the CNS.  Electrical Signals in Neurons The production of graded potentials and action potentials depends on: The resting membrane potential. (an electrical voltage difference across the plasma membrane) The presence of certain ion channels.  Types of Ion Channels Leakage (nongated) channels These channels are randomly alternate between open and closed positions (always open). Nerve cells have more K+ than Na+ leakage channels Membrane permeability to K + is higher which explains the resting membrane potential of -70mV in nerve tissue. Gated channels open and close in response to a stimulus results in neuron excitability  Gated Ion Channels 1. Voltage-gated channels respond to a direct change in the membrane potential. ◼ They participate in the generation and conduction of action potentials. 2. Ligand (chemical)-gated channels respond to a specific chemical stimulus. ◼ Chemical ligands include neurotransmitters, hormones and ions. ◼ Ligands may act directly (e.g. acetylcholine) or indirectly (e.g. hormones) 3. Mechanically gated ion channels respond to mechanical vibration (sound), pressure or stretching Terminology Associated with Changes in Membrane Potential Polarized: +ve outside and –ve inside at rest Depolarization: tracing moves upwards from rest (becomes more +ve, e.g. Na+ enters cell). Repolarization: tracing moves back to rest (returns to rest, K+ exits cell). Hyperpolarization: tracing moves downwards from rest (becomes more -ve, e.g. K+ exits cell). Graded Potentials  Action Potential ◼ Action potential (AP) is a sequence of electrochemical changes that increase membrane potential from resting value of about -70mV to a peak of about +30mV (depolarization), and then return it back to -70mV again (repolarization).  Action Potential ◼ Chemical or mechanical stimulus caused a graded potential to reach at least (-55mV or threshold) ◼ If graded potential reaches threshold AP occurs (All or none principle). ◼ Voltage-gated Na+ channels open & Na+ rushes into cell (Depolarization). ◼ only a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably(up to +30mV). ◼ Positive feedback process. ◼ Voltage-gated K+ channels open but slowly & K+ rushes outside cell (Repolarization) The Action Potential: Summarized Potentials in Electrical Signaling The Process Action Potential Formation Membrane is at rest -70mV ECF Chemically gated Na+ channel ICF Potentials in Electrical Signaling Action Potentials – The process – Excitatory stimulus (mechanical, electrical, chemical) applied & activates corresponding Na+ gated channel ECF Chemically gated Na+ channel ICF Potentials in Electrical Signaling Action Potentials – The process – Na+ enters in causing slight depolarization Possibly to threshold ECF Chemically gated Na+ channel ICF Potentials in Electrical Signaling Action Potentials – The process – The Rising phase – If threshold is reached All of the voltage gated Na+ channels will open, increasing membrane permeability some 6000 fold! Causing further depolarization of the membrane to +30 mV ECF V-gated Na+ Channels V-gated Na+ Channels Chemically gated V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Na+ channel ICF Potentials in Electrical Signaling Action Potentials – The process – The falling phase next, slow voltage gated K+ channels open K+ flows down its concentration gradient… Membrane potential falls ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Channels Channels Channels Channels ICF Potentials in Electrical Signaling Action Potentials – The process – In the meantime… – The voltage gated Na+ channels have closed (both gates) – Membrane potential continues to fall as K+ continues its outward flow ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Channels Channels Channels Channels ICF Potentials in Electrical Signaling Action Potentials – The process – Next the slow voltage gated K+ channels start to close – There is additional K+ that diffuses through during the closing, causing membrane potential to hyperpolarize slightly ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Channels Channels Channels Channels ICF  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF ATP  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF ATP ADP  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF ◼ Why does the AP only travel in ONE direction (towards the synapse)? ◼ Because the voltage gated Na+ and K+ channels undergo a refractory period  Refractory Period of Action Potential ◼ Period of time during which neuron can not generate another action potential. ◼ Absolute refractory period ◼ even very strong stimulus will not begin another AP ◼ inactivated Na+ channels must return to the resting state before they can be reopened ◼ Relative refractory period ◼ a suprathreshold stimulus will be able to start an AP ◼ K+ channels are still open, but Na+ inactivation channels have return to resting state. Propagation of Action Potential  Signal Transmission at Synapses Electrical Chemical Chemical Synapses Structure of neurotransmitter receptors 1- Ionotropic receptor  Ionotropic receptor Ionotropic receptors form an ion channel pore. When an ionotropic receptor is activated, it opens a channel that allows ions such as Na+, K+, or Cl- to flow 2- Metabotropic receptor  Excitatory & Inhibitory Potentials ◼ The effect of a neurotransmitter can be either excitatory or inhibitory ◼ EPSP ◼ it results from the opening of ligand-gated Na+ channels ◼ the postsynaptic cell is more likely to reach threshold to give AP ◼ IPSP ◼ it results from the opening of ligand-gated Cl- or K+ channels ◼ it causes the postsynaptic cell to become more negative or hyperpolarized  Removal of Neurotransmitter ◼ Diffusion ◼ move down concentration gradient ◼ Enzymatic degradation ◼ acetylcholinesterase ◼ Uptake by neurons or glia cells ◼ neurotransmitter transporter Summation Spatial Summation ◼ Summation of effects of neurotransmitters released from several presynaptic end bulbs onto one neuron Temporal Summation ◼ Summation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron  Neurotransmitters Small-Molecule Neurotransmitters ◼ Acetylcholine (ACh) ◼ released by many PNS neurons & some CNS ◼ excitatory on NMJ but inhibitory at others ◼ inactivated by acetylcholinesterase ◼ Amino Acids ◼ Glutamate is excitatory neurons in the brain ◼ inactivated by reuptake. ◼ GABA is inhibitory neurotransmitter for 1/3 of all brain synapses  Small-Molecule Neurotransmitters ◼ Biogenic Amines ◼ modified amino acids (decarboxylation) catecholamine ◼ norepinephrine ◼ epinephrine ◼ dopamine ◼ serotonin  Small-Molecule Neurotransmitters ◼ ATP and other purines (ADP, AMP & adenosine) ◼ excitatory in both CNS & PNS ◼ released with other neurotransmitters (ACh & NE) ◼ Gases (nitric oxide or NO) ◼ formed from amino acid arginine by NO synthase  Neuropeptides ◼ 3-40 amino acids linked by peptide bonds ◼ Substance P -- enhances our perception of pain ◼ Pain relief ◼ enkephalins -- pain-relieving effect by blocking the release of substance P