Flashcards in Neuro Physiology Deck (89):
The nervous system has 2 categories of cells
What is the major functional unit of the nervous system?
Neurons are composed of (7)
1. Dendrites- receive info
2. Cell body, soma, perikaryon- contains organelles
3. Axon- carries info
4. Axon hillock or trigger zone- axon origin, originates AP
5. Presynaptic terminal- end of axon, transmit info
6. Myelin sheath- enhances speed of info transfer
7. Node of Ranvier- gaps in the insulating myelin sheath
How do neurons communicate?
Where does myelin originate from?
Oligodendrocytes of CNS
Schwann cells of PNS
Schwann cell cytoplasm forms a ring inside and outside of the sheath
What does the myelin sheath facilitate and why?
Facilitates conduction because it is an electrical insulator - saltatory conduction
In saltatory conduction of the impulse, what jumps from node to node?
The action potential
Is depolarization faster or slower with myelin sheaths?
Conduction velocity in myelinated fibers is proportional to?
Diameter of the axon - Larger axons have longer internodes and faster conduction velocities
What are the 3 structures of neurons?
Multipolar, bipolar, unipolar
-Numerous cell processes
-1 axon, many dendrites
-2 processes: 1 axon, 1 dendrite
-Interneurons- interconnect neurons w/ brain and spinal cord
-Single stem process that separates into 2 processes: peripheral and central
Neurons classified according to function (3)
1. Motor or efferent (brain to target organs)
2. Sensory or afferent (receptors to brain)
3. Interneurons or associated neurons (connect motor and sensory)
Neuroglia or glial cells
-Smaller, no axons or dendrites
-Capacity to divide
-No action potentials
-Do not participate directly in synaptic interactions and electrical signaling
Function of glial cells
• Participate in the production of myelin sheaths of axons
• Modulate the growth of developing or damaged neurons
• Buffer extracellular concentrations of K and neurotransmitters
• Participate in the formation of contacts between neurons (synapses)
• Participate in certain immune responses of the nervous system
Where are glial cells found in the CNS?
Where are glial cells found in the PNS?
-Brain immune cells
-Release nitric oxide to prevent viral replication
-Protect brain against injury and infection
-Play a role in the developing brain
-Star shaped cells with numerous long processes
-50% of glial cell population in CNS
-Structural and metabolic support for neurons
-Release neurotrophic factors important for neuronal survival
-Mantain a neuron’s working environment- control neurotransmitter and ion levels
-Provide support to axons of neurons in the CNS
-Processes that extend to form myelin
-Cover the ventricles of the brain, central canal of the CNS, and choroid plexus in the 4th ventricle of the medulla oblongata
-Provide support to axons of the PNS
Brain and spinal cord
• The brain is surrounded by the skull
• The spinal cord is surrounded by a series of cervical, thoracic and lumbar vertebrae and ligaments
CNS subdivided into 7 regions
1. Spinal cord
2. Medulla oblongata
5. Midbrain or Mesencephalon
7. Telencephalon or cerebral hemispheres
What is the CNS protected by?
The meninges and CSF
3 Protective Layers of CNS
1. Pia mater- Innermost meningeal layer, lying next to the CNS
2. Arachnoid- Spiderweb appearence, traps CSF, subarachnoid space
3. Dura mater- Outermost meningeal layer, often fused with inner surface of the skull, thicker and protects CNS
Cerebrospinal Fluid (CSF)
-Clear, colorless fluid
-Produced in the ventricles of the brain
-Determinant of neuronal microenvironment
-Shock absorber for CNS
Where is CSF located?
• Central canal of the spinal cord
• Ventricular system of the brain
-Composed by all neural tissue outside CNS
-Deliver sensory info to CNS
-Carries motor commands to peripheral tissues and systems
Peripheral nerve fiber
-Axon and myelin sheath together
-Surrounded by endoneurium CT
-Bundle of nerve fibers
-Surrounded by perineurium CT
Nerve (cranial or spinal)
-Bundle of fascicles
-Surrounded by epineurium CT
Describe the nucleus in the cell body of the nerve.
What is the name of the Rough ER in a nerve cell body?
Nissl substance or nissl body
What do dendrites do?
Integrate the incoming info and determine whether action potentials will be produced by the neuron
In some neurons, the dendrites are covered with small membranous protrusions called?
-Each spine can synapse with different axons
How can neurons be transmitted to one another?
Chemical or electrical synapses
-Most common form of synapses - unidirectional transmission
-Presynaptic neuron secretes a neurotransmitter that will act on receptor proteins in postsynaptic neuron
-Can be excitatory or inhibitory
What does the action of the neurotransmitter in the postsynaptic membrane depend on?
Opened by excitatory neurotransmitters
Opened by inhibitory neurotransmitters
Neurotransmitter receptor that acts directly on ion-gated channels
Neurotransmitter receptor that acts through 2nd messengers, GPCR, cAMP pathway
• The cytoplasm of adjacent cells are directly connected by clusters of ion channels, gap junctions
• Bidirectional transmission
• Coexist and interact with chemical synapses
The potential difference across the membrane of resting neurons is about?
What 3 things determine membrane potential?
• The movement of ions toward a dynamic equilibrium
• Differential permeability of the membrane to diffusion of ions
• Na K pump
Where does ATP for the pump come from?
Intracellular metabolism of glucose and O2
What are graded potentials?
Brief local changes in postsynaptic membranes are generated
T/F: The amplitude of the graded potential is directly proportional to the intensity of the stimulus applied at synaptic sites.
• Each synaptic site generate graded potentials
• They travel to reach the axon hillock or trigger zone
Where are graded potentials integrated to generate action potentials?
In order to initiate an action potential, graded potentials must reach a level called?
Threshold potential or threshold voltage
The minimum voltage change to trigger an action potential, -55mV
When will the action potential be triggered in graded potential?
Once the sum of graded potentials exceeds the threshold
What happens if the depolarization does not reach the threshold?
No action potential will occur
How do graded potentials modulate the postsynaptic neuron?
By shifting the resting membrane potential, toward or away from the threshold potential
Shift the membrane potential toward more positive
Excitatory Postsynaptic Potentials (EPSP)
Depolarizing graded potentials
Drives the membrane potential toward the threshold
Shift membrane potential toward more negative
Inhibitory Postsynaptic Potentials (IPSP)
Hyperpolarizing graded potentials
What are the 2 modes of summation?
Spatial summation and temporal summation
-Graded potentials induced by different synapses summate in the postsynaptic neuron
-Simultaneous summation of inhibitory (IPSP) and excitatory (EPSP) graded potentials also occur
Succesive discharges from a single presynaptic terminal summate in the post synaptic neuron
What are the 3 phases of the action potential?
Absolute refractory period
-The time needed for the voltage-gated sodium channels to revert from the inactivated state to the resting state
-AP initiation is completely blocked
Relative refractory period
-Voltage-gated potassium channels are closed
-AP is inhibited, not blocked
-A strong stimuli can shift the membrane potential to the threshold and generate an AP
Describe saltatory conduction
-AP occurs only at Nodes of Ranvier
-Nodes of Ranvier are rich in ion channels
-APs jump from node to node
-Na influx depolarizes the membrane
How are the neuraxis and meninges drained?
By veins and sinuses
The ventricular system comprises 4 expanded regions forming ventricles, interconnecting foramen and tubes. What are they?
-Lateral ventricles: longitudinally in each hemisphere and connected with the 3rd ventricle
-Third ventricle: surrounding the interthalamic adhesion and connected with 4th ventricle
-Fourth ventricle: connected with the subarachnoid space
T/F: Only some of the CSF is formed at the choroid plexus of the ventricles.
False, most of the CSF
What is the choroid plexus?
A cauliflower-like growth of capillaries covered by a thin layer of ependymal cells (glial cell)
In the choroid plexus, what type of junctions join the ependymal cells?
What regulates the passage of ions and molecules across the ependymal cell barrier?
Membrane transporters and selective channels
List some functions of CSF.
• Cushion the brain, protect it against blows to the head
• Mantain a consistent extracellular microenviroment for the neurons and glia
• Effective waste control system that can remove potentially harmfull cellular metabolites
• Act as a brain distribution system for some peptide hormones and growth factors
Most of the CSF is absorbed from the subarachnoid space into the venous sinuses through what?
-Small finger like projections of the arachnoid membrane that poke through the dura mater in the lumen of venous sinuses
T/F: Absorption of CSF tends to be pressure dependent and bidirectional.
What 2 barriers are located both at the choroid plexus and at the tissue capillary membranes in most areas of the brain?
Interaction between what is important for the maintenance of the BBB?
Endothelial cells and astrocytes
Which glucose transporter is used in the BBB?
Primary or 1st order neurons
The primary afferent neuron that receive the signal and send information to CNS
Secondary or 2nd order neurons
Conduct impulses from spinal cord or brainstem to thalamus (cross over to opposite side before reaching thalamus)
Tertiary or 3rd order neurons
Conducts impulses from thalamus to primary somatosensory cortex
-Originate from cutaneous areas, muscles and joints
-Respond to mechanical, chemical, thermal stimuli
-Produce sensation of touch, pressure vibration, pain and warm/ cold
-Originate from internal structures of the body
-Some are consciously detectable
-Stretching of stomach and bladder
What are the 3 types of sensory receptors classified by their structure?
1. Simple receptors
2. Complex receptors
3. Special senses receptors
• No special modification – free nerve endings
• Not myelinated
• Most common type
• Ensheated by connective tissue capsule
• Encapsulated portions of axon are not myelinated