Spinal Nerves, Membrane Potentials and Action Potentials Flashcards
Spinal Nerves- how many? where do they come from?
Thirty-one pairs of mixed nerves arise from the spinal cord and supply all parts of the body except the head
cervical nerves
8 cervical (C1-C8)
thoracic nerves
12 thoracic (T1-T12) intercostal nerves lumbar enlargement
lumbar nerves
5 Lumbar (L1-L5)
sacral nerves
5 Sacral (S1-S5)
coccygeal
1 Coccygeal (C0)
Brachial Plexus
Formed by C5-C8 and T1 (C4 and T2 may also contribute to this plexus) It gives rise to the nerves that innervate the upper limb
There are four major branches of the brachial plexus
Roots – five ventral rami (C5-T1) Trunks – upper, middle, and lower, which form divisions Divisions – anterior and posterior serve the front and back of the limb Cords – lateral, medial, and posterior fiber bundles Branches
nerve plexuses…where are they found?
All ventral rami except T2-T12 form interlacing nerve networks called plexuses Plexuses are found in the cervical, brachial, lumbar, and sacral regions Each resulting branch of a plexus contains fibers from several spinal nerves Fibers travel to the periphery via several different routes
what happens to damage to one spinal segment?
Damage to one spinal segment cannot completely paralyze a muscle
Brachial plexus
Second most common postop peripheral neuropathy
Axillary
– innervates the deltoid and teres minor
Musculocutaneous
sends fibers to coracobrachialis, biceps brachii and brachialis. Flexion at elbow
Median
branches to most of the flexor muscles of forearm and opponens pollicis Pronation of forearm Flexion of wrist Opposition of thumb Flexion of lateral three fingers
Ulnar
supplies the flexor carpi ulnaris and half of the flexor digitorum profundus Flexion of wrist Adduction of fingers Flexion of medial two fingers **Ulnar nerve- post op compression from the hard operating table**
Radial
Supplies ALL extensors of arm and forearm muscles. Extension at elbow Supination of forearm Extension of wrist and fingers **Radial fracture of the humerus- this nerve can be snapped**
Intercostobrachial nerve
Can be blocked to reduce pain from tourniquet inflation during IV regional neural anesthesia
neuron
Basic building block of CNS ~ 100 billions Integration and transmission of nerve impulses
Schwann cells
produce myelin sheath which wraps around axon, provides insulation
Node of Ranvier
gaps in myelin sheath
Saltatory conduction:
Node to node jumping of depolarization.
where are neurotransmitters synthesized.
Neurotransmitters are synthesized in cell body and transported to synaptic knobs All or none law
Multiple Sclerosis
Demyelination results slow or blocked conduction
Bipolar Neuron
has short axon process from arising from one side of the cell body, and a short dendritic process arising from the opposite side
Special senses neurons
Bipolar neurons- those found in the eyes, ears, and nose are bipolar neurons
Unipolar Neuron:
has single large extension from its cell body. They are found in lower invertebrate ,never in humans.
Pseudounipolar neuron:
Present in dorsal root ganglia
Multipolar Neuron:
comprise one axon and multiple dendritic process. They are the MOST COMMON type in brain and spinal cord.
dorsal root ganglia
Drg cell body lies
Depolarization
Makes the cell membrane potential less negative due to movement of positively charged sodium ions (Na+) into the cell. excitability
Repolarization
Change after depolarization, that returns the membrane potential back to resting potential. Repolarization results from the movement of positively charged potassium ions (K+) out of the cells.
Hyperpolarization
Makes the membrane potential more negative due to movement of negatively charged chloride ions (Cl-) into the cell. excitability
Inward current
Is the flow of positive charge into the cell. Inward current depolarizes the membrane potential.
Outward current
Is the flow of positive charge out of the cell. Outward current hyperpolarizes the membrane potential.
Action potential
Is a property of excitable cells (nerve & muscle) that consists of a rapid depolarization, or upstroke, followed by repolarization of the membrane potential. Action potential have stereotypical size and shape, are propagating and are all-or-none
Threshold
Is the membrane potential at which the action potential is inevitable. At threshold potential, net inward current becomes larger that net outward current. The resulting depolarization becomes self-sustaining and gives rise to upstroke of action potential. If net inward current is less than net outward current, no action potential will occur (i.e. all- or- none response)
Resting membrane potential
is the measured potential difference across the cell membrane in mV (-70 to - 90mV) At rest , the nerve membrane is far more permeable to K+ than to Na+ Leaky K+ channels are responsible for resting membrane potential. The Na+/K+ pump maintains resting membrane potential ( 3Na+ out and 2K+ in)
Action Potential “Nerve Impulse”
is a property of excitable cells (nerve, muscles) that consist of a rapid depolarization (interior becomes less negative) or upstroke , followed by repolarization of membrane potential.
Upstroke of the Action potential (depolarization)
Inward Na+ movement Lidocaine block these voltage sensitive Na+ channels and abolish action potential
Downstroke of the Action potential (repolarization)
Outward K+ movement Inward current (flow of Na+ into the cell) depolarizes the membrane potential , while outward flow of K+ hyperpolarize the membrane potential. Both ions flow by simple diffusion.
Properties of AP Speed? other properties?
Constant size and shape Propagation/ spread ( 60 m/sec ) Myelinated vs. non-myelinated fibers All-or-none Law (no percentage) Threshold is the membrane potential at which the AP is inevitable
Refractory periods
Absolute refractory period: is a period during which another action potential cannot be elicited, no matter how large the stimulus. Due to closure of inactivation gates of Na+
Relative refractory period:
is a period during which an action potential can be elicited only if a larger than usual stimulus is provided. Refractory period protects the cell from over-excitation. It allows a recovery period between the action potentials.
Spread of Depolarization
Opening of Na+ channels generates local current circuit that depolarizes adjacent membrane, opening more Na+ channels…
myelination signal transmission
schwann cells surround the nerve axon forming a myelin sheath Sheath is interrupted every 1-3 mm : node of Ranvier ( Na+/K+ channels)
Action potential occurs where?
AP only occur at the nodes (area rich in Na+/K+ channels) Increased velocity Energy conservation