Chapter 12 Nervous Tissue Flashcards
Central nervous system (CNS)
consists of the brain and the spinal cord
General functions of the nervous system
1- collect information
2-processes and evaluates information
3-initiate response to information
Peripheral nervous system (PNS)
consists of the nerves and ganglia
Types of nerves
Structural classification:
-caranial nerves
-spinal nerves
Functional classification:
-sensory nerves
-motor nerves
-mixed nerves (both sensory and motor)
Sensory nervous system
detects stimuli and transmits information from receptors to the CNS
Motor nervous system
initiates and transmits information from the CNS to effectors
The 2 components of the sensory nervous system
somatic sensory and visceral sensory
Somatic sensory
sensory input from the receptors of the five senses and proprioceptors; senses we consciously perceive
Visceral sensory
sensory input from receptors of internal organs and blood vessels; senses we are unaware of
The 2 components of the motor nervous system
somatic motor and autonomic motor
Somatic motor
motor output to skeletal muscle; voluntary
Autonomic motor
motor output to cardiac muscle, smooth muscle, and glands; involuntary
What is a nerve composed of?
cable like bundle of axons, connective tissue layers, and blood vessels, and it is a component of the PNS
Epineurium
a thick layer of dense irregular connective tissue that encloses the nerve
Perineurium
a layer of dense irregular connective tissue that wraps each fascicle
Endoneurium
a delicate layer of areolar connective tissue that surrounds each axon
Know the structure of a nerve and ganglion
Are nerves vascularized?
yes
Neurotransmitters
are molecules stored in vesicles and when released, bind to an excitable cell to cause either an excitatory or inhibitory effect on these target cells
Know the structures in a typical neuron
General characteristics of a neuron
- excitability
- conductivity
- secretion
- extreme longevity
- amitotic
Ganglion
a cluster of neuron cell bodies within the PNS
Anterograde transport
the movement of materials from the cell body toward synaptic knobs
Retrograde transport
the movement of materials from synaptic knobs toward the cell body
Fast axonal transport
occurs at approximately 400 millimeters per day; involves movement along microtubules towards either direction (anterograde or retrograde)
Slow axonal transport
occurs at approximately 0.1 to 3 millimeters per day; results from the flow of axoplasm; only allows movement to the synaptic knob (anterograde)
Multipolar neurons
have many dendrites and a single axon that extends from the cell body; these are the most common types of neurons in the human body
Bipolar neuron
have two processes that extend from the cell body- one dendrite and one axon; the location of these neurons is relatively limited to humans
Unipolar neurons
have a single, short neuron process that emerges from the cell body and branches like a T
Anaxonic neurons
have only dendrites and no axons; they produce graded potentials, but not action potentials
Sensory neurons
responsible for conducting sensory input from both somatic sensory and visceral sensory receptors TOWARD the CNS; most are unipolar but some somatic sensory neurons are bipolar
Motor neurons
conducting motor output AWAY FROM the CNS to both somatic effectors and autonomic effectors; all motor neurons are multipolar
Interneurons
lie entirely within the CNS; receive, process, and store information and “decide” how the body responds to stimuli
Synapse
the specific location where a neuron is functionally connected to either another neuron or an effector
Chemical synapse
between two neurons; is composed of a presynaptic neuron and a postsynaptic neuron with a narrow fluid-filled gap
Presynaptic neuron
the signal producer
Postsynaptic neuron
the signal receiver
Synaptic cleft
an extremely narrow, fluid-filled gap between two neurons
Transmission
occurs between a presynaptic and postsynaptic neuron when neurotransmitter molecules stored in synaptic vesicles are released from the synaptic knob of a presynaptic neuron into the synaptic cleft
Synaptic delay
the time between the neurotransmitter release from the presynaptic cell, its diffusion across the synaptic cleft, and neurotransmitter binding to receptors in the postsynaptic neuron plasma membrane
Electrical synapse
composed of a presynaptic neuron and a postsynaptic neuron physically bound together
Glial cells
found both in the CNS and PNS; have mitotic ability; smaller than neurons; they do not transmit electrical signals, but they do assist neurons with their functions
4 types of glial cells found in the CNS
1- astrocytes
2- ependymal cells
3- microglia
4- oligodendrocytes
Astrocytes
exhibit a starlike shape due to projections from their surface; most abundant glial cells in the CNS
Astrocyte functions
- help form the blood-brain barrier (BBB)
- regulate interstitial fluid composition
- form structural support
- assist neuronal development
- alter synaptic activity
- occupy the space of dying neurons
The blood-brain barrier (BBB)
strictly controls the movement of substances from exiting the blood and entering the nervous tissue in the brain
Ependymal cells
ciliated simple cuboidal or simple columnar epithelial cells that line the internal cavities of the brain and the central canal of the spinal cord
Choroid plexus
helps produce cerebrospinal fluid, a clear liquid that bathes the external surfaces of the CNS and fills its internal cavities
Microglia
typically small cells that have slender branches extending from the main portion of the cell; they represent the smallest portion of CNS glial cells; classified as phagocytotic cells of the immune system
Oligodendrocytes
are large cells with a bulbous body and slender cytoplasmic extensions or processes; insulate axons within the CNS to form a myelin sheath through a process called myelination
2 types of glial cells in the PNS
1- satellite cells
2- neurolemmocytes
Satellite cells
are flattened cells arranged around neuronal cell bodies in a ganglion; physically separating cell bodies from their surrounding interstitial fluid; they electrically insulate the cell body and regulate the continuous exchange of nutrients and waste products between neuron cell bodies and their environment
Neurolemmocytes
elongated and flattened cells wrap around and insulate axons within the PNS to form a myelin sheath through myelination
Myelination
the process by which part of an axon is wrapped with myelin
Myelin
the insulating covering around the axon that consists of repeating concentric layers of plasma membrane of glial cells
Neurilemma
the periphery of the neurolemmocyte contains the cytoplasm and nucleus
Neurofibril nodes (nodes of Ranvier)
gaps in between the neurolemmocytes
Unmyelinated axons
no myelin covers them; in the PNS are also associated with neurolemmocytes, which help to protect and support the axon
Axon regeneration process
Pumps
maintain specific concentration gradients by moving substances against a concentration gradient, a process that requires cellular energy
Channels
provide the means for a substance to move with a concentration gradient
Major types of channels
- leak channels
- chemically-gated channels
- voltage-gated channels
Leak channels
always open, allowing continuous diffusion of a specific type of ion from a region of high concentration to a region of low concentration
EXAMPLES:
- Na+ leak channels
- K+ leak channels
Chemically gated channels
normally closed; they temporarily open in response to the binding of a neurotransmitter; when open, they allow a specific type of ion to diffuse across the plasma membrane
EXAMPLES:
- Chemically gated K+ channels
- Chemically gated Cl- channels
Voltage-gated channels
normally closed, but they temporarily open in response to changes in electrical charge across the plasma membrane; when open, they allow a specific type of ion to diffuse across the membrane
EXAMPLES:
- Voltage-gated Na+ channels
- Voltage-gated K+ channels
- Voltage-gated Ca2+ channels
Voltage-gated Na+ gates
1- activation gate
2- inactivation gate
The 3 stages of voltage-gated Na+ channels
1- resting state
2- activation state
3- inactivation state
Stages of voltage-gated Na+ channels
Modality-gated channel
normally closed, but open in response to specific type of sensory stimulus
Know the distribution of pumps and channels in the plasma membrane of a neuron
Electrical energy
the movement of charged particles, and that all usable forms of energy are available to do work
Voltage
amount of difference in electrical charge
Current
movement of charged particles across plasma membrane through open channels
Resistance
opposition to movement of charged particles
Ohm’s Law
current= voltage/resistance
Know neurons and Ohm’s law
Neuron at rest
Neurons at rest characteristics
-ion concentration gradients exist for K+, Na+, and Cl- across the plasma membrane along the entire neuron
- a Ca2+ concentration gradient exists at the synaptic knob
- gated channels are CLOSED
- there is an electrical charge difference across the plasma membrane
Resting Membrane Potential (RMP)
the neuron is at rest, the membrane potential is at RMP; -70 mV
Role of Na+/K+ pumps
push 3 Na+ out and pulls K+ in; maintains the concentration gradients for these ions
Graded potentials
are relatively small, short-lived changes in the resting membrane potential that are caused by the movement of small amounts of ions across the plasma membrane
Characteristics of graded potentials
- established in the receptive segment by the opening of chemically gated channels
- local currents associated with the graded potentials are short-lived because the flow or current of ions along the plasma membrane experiences resistance
- they vary in both the degree of change and the direction of change of the RMP
Know the events in each neuron segment
Postsynaptic potentials
graded potentials that occur in postsynaptic neurons
Excitatory postsynaptic potentials (EPSP)
postsynaptic potentials that result in the neuron becoming more positive
Inhibitory postsynaptic potentials (IPSP)
postsynaptic potentials resulting in the neuron becoming more negative
Generation of an EPSP
Generation of an IPSP
Summation
the changes in the membrane potential associated with these graded postsynaptic potentials are “added” in the initial segment to determine if an action potential is initiated
Threshold membrane potential
the minimum voltage change
Spatial summation
multiple locations on the cell’s receptive regions receive neurotransmitter simultaneously and generate postsynaptic potentials
Temporal summation
a single presynaptic neuron repeatedly releases neurotransmitter and produces multiple EPSPs within a very short period of time
All-or-none law
if threshold is reached, action potential generated and propagated down the axon without any loss in intensity; if threshold not reached, voltage-gated channels stay closed, no action potential
Action potential
involves depolarization and repolarization
Depolarization
gain of positive charge as Na+ enters through voltage gated Na+ channels
Repolarization
return to negative potential as K+ exits through voltage gated K+ channels
Nerve signal
propagation of of an action potential
Generation of an Action Potential: Depolarization and its Activation
Generation of an Action Potential: Repolarization and its Propagation
Refractory period
period of time after start of action potential when it is impossible or difficult to fire another action potential
Absolute refractory period
no stimulus can initiate another action potential
Relative refractory period
another action potential is possible but the minimum stimulus strength is now greater; some K+ channels are still open and cell is slightly hyperpolarized and further from threshold
Continuous conduction
occurs in unmyelinated axons and involves the sequential opening of voltage-gated Na+ channels and voltage-gated K+ channels located within the axon plasma membrane along the entire length of the axon
Saltatory conduction
occurs in myelinated axons; action potential only occurs ate neurofibril nodes ; much faster than continuous conduction and myelinated cells use less ATP to maintain resting membrane potential
Propagation of an action potential (continuous and saltatory conduction)
Steps involved when a nerve signal reaches the transmissive segment
Know the difference between a graded potential and an action potential
Shown on table 12.2
Velocity of action potential propagation is influenced by what two factors
1- diameter of the axon
2- myelination of the axon
Nerve fiber
an axon and its myelin sheath
Neurotransmitters
small, organic molecules that are:
- synthesized by neurons and stored within vesicles in synaptic knobs
- released from the vesicles when action potential triggers calcium entry into the synaptic knob
- bind to a specific receptor in a target cell
- trigger a physiologic response in the target cell
4 categories of neurotransmitters based on chemical structure
1- acetylcholine (ACh)
2- biogenic amines
3- amino acids
4- neuropeptides
Acetylcholine
structure differs substantially from other transmitters
Biogenic amines
an amino acid is slightly modified to synthesize the transmitter
Catecholamines (dopamine)- are made from tyrosine
Indoleamines (serotonin)- are made from histidine or tryptophan
Amino acids
include common transmitters glutamate, glycine, GABA
Neuropeptides
chains of amino acids including endorphins, substance P, enkephalin, somatostatin
Neurotransmitters are also classified by function:
1) excitatory- transmitters cause EPSPs
2) inhibitory- transmitters cause IPSPs
Neurotransmitters are also classified by action:
1) direct- transmitters bind to receptors that are chemically gated channels
2) indirect- transmitters bind to receptors that involve G-proteins and second messengers
Acetylcholine release, removal from synaptic cleft, and action
Acetylcholinesterase
an enzyme that resides in the synaptic cleft
Neuromodulators
chemicals that alter responses of local neurons; modify neurotransmitters
Facilitation
modulation that causes greater response in postsynaptic neurons; may increase the amount of neurotransmitter in cleft or number of postsynaptic receptors
Inhibitation
modulation that causes weaker response; may decrease the amount of neurotransmitter in cleft or number of postsynaptic receptors
Nitric oxide
might be a neurotransmitter or a modulator; a short-lived, nonpolar gas; made and released by postsynaptic neurons in the brain where it is believed to strengthen memory by affecting presynaptic cells; effects in the PNS include blood vessel dilation
Endocannabinoids
influence same receptors; small, nonpolar molecules; made and released by postsynaptic neurons; have effects on presynaptic neurotransmitter release; influence memory and appetite
Subthreshold
any change in voltage below the threshold value is not sufficient to open voltage-gated channels
