Anatomy Chapter 12 Concepts (Nervous System: Nervous Tissue)
What is a glial cell?
- found within the CNS and PNS - smaller than neurons and mitotic - no electricity capability but assist neurons - physically protect and help nourish neurons, also providing structural support to synapse structure
What is a synapse?
...the specific location where a neuron is functionally connected to either a another neuron or an effector (muscle/ gland) ....2 types of synapses exist in the body: Chemical & Electrical
What are the 4 main functional neuron segments?
1. Receptive 2. Initial 3. Conductive 4. Transmissive
What composes Nervous Tissue?
2 Cell types compose Nervous Tissue: Neurons: ....excitable cells that initiate/ transmit electrical signals Glial Cells: ....non-excitable cells that primarily support and protect the neurons
Explain the functional organization of the Nervous system and how do they differ?
2 Functional Divisions: Sensory NS & Motor NS Sensory NS (Afferent) : detects (receives) stimuli and transmits information from receptors to the CNS A. Somatic Sensory: Sensory input consciously perceived from receptors (eyes, ears, and skin) B. Visceral Sensory: Sensory input that is not consciously perceived from receptors of blood vessels and internal organs (heart) Motor NS (Efferent) : initiates and transmits information from the CNS to effectors A. Somatic Motor: motor output that is consciously or voluntarily controlled; effector is skeletal muscle B. Autonomic Motor (visceral) : motor output that is not consciously or is involuntarily controlled; effectors are cardiac muscle, smooth muscle, glands
Explain how the RMP is established and maintained in neurons
A RMP is a consequence of the movement of ions across the plasma membrane through leak channels (K+/Na+)
Describe the conditions of a neuron at rest and define RMP
A neuron at rest has a negative RMP (-70mV) and all gated channels are closed, and concentration gradients exist for Na+, K+, and Cl- along the length of the neuron, and for Ca2+ at the synaptic knob The RMP is established and maintained by K+ leak channels, and Na+/K+ pumps
Distinguish between a pump and a channel
A pump moves substances up (against) a concentration gradient, a process requiring cellular energy. Channels provide a way for a substance to move down (with) its concentration gradient... neurons contain the following major types of channels: - Leak Channels: always open, allowing continuous diffusion of a specific type of ion from high to low concentration - Chemically Gated Channels: typically closed, open in response to binding of a neurotransmitter, when open they allow a specific type of ion to diffuse across membrane - Voltage Gated Channels: typically closed, open in response to changes in electrical charge (potential) across the membrane. Voltage- gated Na+ channels are unique because they have 2 gates- an activation/ inactivation gate and therefore exhibit 3 states
What is the "tug of war" that is occurring?
A single EPSP is incapable of causing the postsynaptic neuron to reach threshold, so, the occurence of both EPSP's and IPSP's rogether results in a tug of war....thus, numerous EPSP's must be generated in the receptive segment and arrive at the initial segment at the same time if the threshold is to be reached
What are the two different types of axon movement directions
Anterograde Transport: - movement of materials from the cell body to the synaptic knobs Retrograde Transport: - movement of materials from synaptic knobs to the cell body
Describe the structure and function of the 4 types of glial cells within the CNS
Astrocytes, Ependymal cell, microglia, oligodendrocytes 1. ASTROCYTES: - most abundant glial cell in the CNS - helps form Blood Brain Barrier (BBB) which controls movement of substances from exiting the blood and entering the nervous tissue in the brain...the BBB protects the brain from toxins - regulates interstitial fluid composition - provides structural support and organization to the CNS - assists with neuronal development occupy space of dying neurons 2. EPENDYMAL CELLS: (choroid plexus) - lines ventricles of brain and central canal of spinal cord - assists in production and circulation of cerebrospinal fluid (CSF) 3. MICROGLIA: - phagocytic cells that move through the CNS - protects the CNS by engulfing infectious agents and other potential harmful substances 4. OLIGODENDROCYTES: - myelinates and insulates CNS axons - allows faster actional potential propagation along axons in the CNS
Identify the structural components included in the CNS and those in the PNS
CNS: - Brain & Spinal Cord PNS: - Nerves & Ganglia
Describe the 3 basic anatomic features common to most neurons
Cell body: - (soma) the neurons control center, transmitting graded potentials to the axon Dendrites: - short, small, unmyelinated processes that branch off of the cell body - receive graded potentials toward the cell body...they receive input and transfer to cell body for processing Axons: - longer processes with longest tips being called synaptic knobs which contain synaptic vesicles containing neurotransmitter - initiate and propagate action potentials, triggering synaptic vesicles to release the neurotransmitter from the synaptic knobs
In Neurons...
Charged particles are IONS, NOT like in a battery/ electricity (electrons) There is a difference in charge on either side of the plasma membrane (voltage) due to an unequal distribution of ions The plasma membrane phospholipid bilayer offers resistance b/c it generally does not allow the passage of ions Ion channels function to either increase or decrease resistance if they open/ close Current is generated when either positively charged ions or negatively charged ions diffuse across the plasma membrane through open channels
What is the difference between a Chemical synapse and an Electrical synapse?
Chemical Synapse: (most in the body) - composed of a presynaptic neuron (produces signal), and a postsynaptic neuron (signal receiver/target) - most commonly a synapse is with a dendrite of the post synaptic neuron - the synaptic knob of the presynaptic neuron doesn't quite make contact with the post synaptic neuron and are separated by the synaptic cleft - Transmission occurs when neurotransmitter molecules stored in the synaptic vesicles are released from the knob into the synaptic cleft - some of the neurotransmitters diffuses across the cleft and bind to receptors within the plasma membrane of hte postsynaptic neuron to initiate a graded potential Electrical Synapse: - much less common - the presynaptic and postsynaptic neurons are bound together physically (gap junctions exist) - no synaptic delay due to zero gap - located in limited regions in the brain and the eye
Describe and Action Potential (Conductive Segment)
Conductive segment = full length of the axon - main job of conductive segment- send that actional potential down the road Actional potential involves two processes : - Depolarization: gain of a positive charge within a neuron that occurs to such an extent to change the plasma membrane potential from negative to positive - Repolarization: return of polarity from positive back to negative the propagation of an action potential is called a NERVE SIGNAL, or nerve impulse
What are 2 primary factors that influence the velocity of action potential propagation
Diameter of axon: - axons with larger diameter...faster - less resistance to movement of ions within the larger axon, allowing axons to reach threshold more rapidly than smaller axons Myelination of the axon: - insulated axons allow propagation of an action potential to occur more rapidly
What are 5 features common to all Neurons
Excitability: in response to a stimulus Conductivity Secretion: neurons release neurotransmitters in response to conductive activity Extreme Longevity Amitotic: they do not replicate/ undergo mitotic activity
Distinguish between fast axonal transport and slow axonal transport, giving examples of the different substances moved by each
Fast Axonal Transport: - Both Anterograde and Retrograde movement Slow Axonal Transport: - only moved from cell body towards the synaptic knob (anterograde)
What is a postsynaptic potential?
GP's that occur in postsynaptic neurons are called postsynaptic potentials - PP's that result in the neuron becoming more positive are termed EXCITATORY POSTSYNAPTIC POTENTIALS (EPSPs) - PP's that result in the neuron becoming more negative are termed INHIBITORY POSTSYNAPTIC POTENTIALS (IPSPs) ...... both excitatory and inhibitory can be generated b/c a postsynaptic neuron can bind many neurotransmitter molecules at the same time
Compare Graded Potentials and Action Potentials (two types of electrical signals in neurons)
Graded Potentials: - occur in the receptive segment of a neuron (dendrite/ cell body) - due in part to the opening of chemically gated channels which open to allow passage of a specific type of ion across the membrane, resulting in a membrane potential either becoming more positive (depolarization) or negative (hyperpolization) than the resting membrane potential - the degree of change is dependent upon the magnitiude of the stimulus, therefore it is graded Action Potential: - generated within the Initial segment and propagated along the conductive segment of a neuron - action potential initiated when voltage gated channels open in response to a minimum voltage change (threshold value) - action potentials obey the all or none law
What is a graded potential?
In the first segment (RECEPTIVE) the significant event that occurs is the establishment of the graded potential...... GP's are small, short lived changes in the resting membrane potential that are caused by movement of small amounts of ion across the plasma membrane - GP's vary in both the degree of change and the direction of change... - the degree of change is dependent on the the magnitude of the stimulus.... larger stimulus opening more chemically gated channels
Name and describe the 4 STRUCTURAL categories of neurons (based on 3 of neuron processes emanating from cell body)
Multipolar Neurons: - multiple processes extend directly from cell body - typically many dendrites and one single axon; MOST COMMON type - all motor neurons on multipolar neurons Bipolar Neurons: - two processes extend directly from the cell body; one dendrite and one axon - relatively uncommon Unipolar Neurons: - single short process extends from the cell and looks like a T as a result of the fusion of two processes into one long axon - most sensory neurons Anaxonic Neurons: - processes ONLY dendrites - interneurons of the CNS
Define Myelination
Myelination - the process by which part of an axon is wrapped with myelin - myelin is the insulating covering around the axon that is a repeated layers of plasma membrane composed of glial cells - high lipid content = insulation
What is the structure of a Nerve?
Nerve: - cable like bundle of parallel axons that are components of the peripheral nervous system Epineurium- encloses the entire nerve Perineurium- wraps fasicles (bundles of axons) Endoneurium- wraps an individual axon in a myelinated neuron
What are the 3 FUNCTIONAL categories of neurons and where each is primarily located
Neurons are classified functionally according to the direction the action potential travels relative to the CNS... Sensory Neurons: (afferent neurons) - neurons of the sensory system, responsible for conducting sensory input from both somatic and visceral sensory receptors to the CNS - most are Unipolar Motor Neurons: (efferent neurons) - neurons of the motor system, responsible for conducting motor output from the CNS to both somatic effectors and visceral effectors - all are multipolar Interneurons: (association neurons) - lie entirely within the CNS - they receive, process, and store information and "decide" how the body responds to stimuli - they facilitate communication between sensory and motor neurons - MOST prevalent neuron (99%)
Summary of Postsynaptic Potentials in the Receptive Segment
Neurotransmiter released from presynaptic neurons corsses the synaptic cleft and initiates a graded (postsynaptic) potential. ....binding of a neurotransmitter causes either: a) an excitatory postsynaptic potential (EPSP) b) an inhibitory postsynaptic potential (IPSP) ...depending on the neurotransmitter released and the specific type of receptor to which it binds.
What are the 3 general functions of the nervous system?
Primary communication and control system.... electrical activity transmitted along nervous system cells called Neurons to accomplish: 1. Collect information (receptors): use of receptors that monitor stimuli 2. Process and evaluate information: after processing sensory input, the brain and spinal cord determine what response, if any, is required 3. Initiate response to information: brain/ spinal cord initiate response as motor output via nerves to effectors
Describe 3 specific states of a voltage gated Na+ channel
Resting state: (closed) Activation : (open) Inactivation: (closed)
Describe the structure and function of the 2 types of glial cells within the PNS
Satellite cells, neurolemmocytes 1. SATELLITE CELLS: - electrically insulate PNS cell bodies - regulates nutrient and waste exchange for cell bodies in ganglia 2. NEUROLEMMOCYTES: - myelinates and insulates PNS axons - allows for faster action potential propagation along an axon in the PNS
What are the 2 types of summation that can occur in the Initial Segment?
Spatial & Temporal Summation Spatial Summation: - when multiple presynaptic neurons release neurotransmitter at various locations onto the receptive segment generating EPSPs, IPSPs, or both in the postsynaptic neuron Temporal Summation: - when a single presynaptic neuron repeatedly releases neurotransmitter to produce multiple EPSPs in the postsynaptic neuron at the same location repeatedly within a very short period of time Think of: - imagine throwing rocks into a pool, the threshold is represented by water splashing over the side of one end of the pool (postsynaptic neuron) and you're standing at the other end of the pool (presynaptic neuron) - you throw 1 rock into the pool creating a small ripple (an EPSP) water doesn't splash out of the other end (threshold not reached) - Spatial Summation is demonstrated by several people throwing rocks into different areas of the pool.... the collective "ripples" added together cause water to splash over the end of the pool (threshold reached) - Temporal summation is demonstrated if you throw rocks into the same area of the pool repeatedly and quickly, and the collective ripples are added together to cause water to splash over, threshold reached.
Explain propagation of an action potential in both unmyelinated and myelinated axons
Specifically how an action potential is propagated along the axon is dependent upon whether the axon is myelinated or unmyelinated in unmyelinated axons, Continuous Conduction occurs where voltage gated Na+ and K+ channels located within the axon plasma open sequentially along the entire length of the axon in myelinated axons, Saltatory Conduction occurs, where action potentials don;t occur in regions that are myelinated- they are instead propagated at neurofibril nodes myelination allows action potential propagation to be much quicker and also require less energy to send
Explain how nerves are classified structurally and functionally
Structural Classification comes from the CNS component from which the nerve extends... Cranial Nerves (brain), and Spinal Nerves (spinal cord) Functional Classification based on the functional type of neuron (sensor or motor neuron) a nerve contains.... Sensory nerve - contain sensory neurons that relay info to the CNS Mixed nerve-contain both sensory and motor neurons ... ...most named nerves are Mixed nerves
What is the degree of change in the RMP dependent upon?
The degree of change in the Resting Membrane Potential is dependent upon the amount of neurotransmitter bound per unit of time - as more is released by the presynaptic neurons, more channels open in the receptive segment of the postsynaptic neuron, and there is a greater change in the membrane potential
What happens when a propagated action potential reaches the transmissive segment
The transmissive segment is the synaptic knob where the neurotransmitter is released from synaptic vesicles Prior to the arrival of the action potential, Ca2+ pumps in the plasmam membrane of a synaptic knob establish a calcium concetration gradient by pumping Ca2+ out to the interstitial fluid ......so there is more calcium outside o thee knob than inside of it - when the nerve signal reaces the knob at the end of the aon, the voltage change associated with depolarization triggers the opening of coltage gated Ca2+ channels .....calcium ions move down their gradient from the IF into the knob, neurotransmitter is then released out of the knob 1. Nerve signal reaches the knob 2. Voltage gated Ca2+ channels open and Ca2+ enters the knob and binds to proteins associated with synaptic vesicles 3. Synaptic vesicles fuse with the knob plasma membrane and neurotransmitter is exocytosed 4. Neurotransmitter diffuses across cleft and attaches to receptors on a muscle or to receptors of a neuron or gland
What is voltage, current, and resistance
Voltage: measure of the amount of difference in electrical charge between two areas and represents potential energy Current: movement of charged particles across the barrier that seperates this charge difference - the greater the movement of charged particles, the greater the current Resistance: the opposition to the movement of charged particles...the barrier between the charged areas
When are the voltage gated channels opened?
When the minimum voltage change (threshold membrane potential) is reached, the voltage gated channels are stimulated to open and initiate the generation of an action potential that will be propagated along the axon
What is the all or none law?
applies to action potentials propagated along the plasma membrane of neurons - if threshold is reached, an action potential is initiated and propagated along the axon without decreasing in intensity - if threshold is not reached (sub threshold), it is not initiated
What is a refractory period?
the brief time period after an action potential has been initiated during which an axon is either incapable of gernerating another action potential or a greater than normal amount of stimulation is required to generate another action potential ...it's the time that the plasma membrane needs to recover Has 2 phases: Absolute & Relative Refractory Period Absolute: time after an action potential onset when no amount of stimulus can initiate a 2nd action potential - voltage gated NA+ channels are opened Relative: immediately after absolute period, where a 2nd action potential could occur if stronger than the original
What is Summation in the "Initial Segment"
the process whereby the changes in the membrane potential associated with the graded postsynaptic potentials are "added"