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 Central nervous system physiology questions and answers by Dr Khaled Abulfadle

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مُساهمةموضوع: Central nervous system physiology questions and answers by Dr Khaled Abulfadle   السبت سبتمبر 25, 2010 6:56 pm

What are the Functional Zones of a Neuron

1. The cell body contains the nucleus and metabolic machinery for protein synthesis.
2. Dendrites are numerous~ usually short extensions that receive stimuli (input zones).
3. An axon is usually a single, rather long extension (conducting zone) that transmits impulses to other cells at its branched endings(output zones); signals actually arise in trigger zones.

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What are the factors maintaining resting membrane potential

I. The resting membrane potential is the result of three factors:
a. The concentrations of potassium, sodium ions, and other charged molecules are not the same on the two sides of the plasma membrane.
b. Channel proteins spanning the membrane influence the diffusion of specific types of ions.
c. Transport proteins spanning the membrane actively pump sodium and potassium ions.
2. There are more potassium ions inside and more sodium ions outside the resting neuron membrane.
a. Potassium ions have a tendency to leak out by facilitated diffusion through channel proteins.
b. Most of the sodium channels are “gated” and remain closed most of the time, keep¬ing the concentration outside high.
c. However, small amounts of sodium do leak in and must be pumped out (and potas¬sium pumped in) by the sodium-potassium pump.

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What are the chemical synapses

Chemical Synapses
A. A chemical synapse is a junction between a neuron and an adjacent cell, separated by a synaptic cleft into which a neurotransmitter substance is released.
1. The neuron that releases the neurotransmitter molecules into the cleft is called the presynaptic cell.
a. First, gated protein channels open to allow calcium ions to enter the neuron.
b. Calcium causes the vesicles to fuse with the membrane and release the transmitter substance into the cleft.
2. The neurotransmitter binds to receptors on the membrane of the postsynaptic cell.
a. Neurotransmitters may have excitatory effects if they drive a cell’s membrane to the threshold of an action potential.
b. Neurotransmitters may have inhibitory effects if they help drive the membrane away from threshold.
3. Acetylcholine is the transmitter at neuromuscular junctions.

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what do we mean by Neuromodulators

they are substances that enhance or reduce the effects of a neurotransmitter on target neurons.

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give examples of neurotransmitters in brain

I. Serotonin acts on brain cells to govern sleeping, sensory perception, temperature regulation, and emotional states.
2. Norepinephrine apparently affects brain regions concerned with emotions, dreaming, and awaking.
3. Dopamine is the specialty of neurons in brain regions dealing with emotions.
4. GABA is the most common inhibitory signal in the brain

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مُساهمةموضوع: رد: Central nervous system physiology questions and answers by Dr Khaled Abulfadle   السبت سبتمبر 25, 2010 7:40 pm

what do we mean by reflexes

they are simple, stereotyped movements made in response to sensory stimuli.

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what are the types of reflexes

a. monosynaptic: In the stretch reflex, receptors of sensory neurons (muscle spindles) transmit impulses to the spinal cord where direct synapses with motor neurons occur.
b. polysynaptic: In the withdrawal reflex, interneurons in the spinal cord can activate or suppress motor neurons as necessary for a coordinated response.

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what are the functional Divisions of the Nervous System

1. The central nervous system includes the brain and spinal cord.
a. The communication lines within the brain and spinal cord are called tracts; those in the white matter contain axons with glistening myelin sheaths and specialize in rapid transmission of impulses.
b. Gray matter consists of unmyelinated axons, dendrites, nerve cell bodies, and neuroglia cells, that protect and support neurons.
2. The peripheral nervous system includes all of the nerves carrying signals to and from the brain and spinal cord.

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what are the parts of peripheral nervous system

Peripheral Nervous System
1. Somatic and Autonomic Subdivisions
a. The human peripheral system has two types of nerves based on location:
1) Spinal nerves (31 pairs) connect with the spinal cord and innervate most areas of the body.
2) Cranial nerves (12 pairs) connect vital organs directly to the brain.
b. Spinal and cranial nerves can also be classified on the basis of function:
1) The somatic nerves relay sensory information from receptors in the skin and muscles and motor commands to skeletal muscles (voluntary control).
2) The autonomic nerves sends signals to and from smooth muscles, cardiac muscle, and glands (involuntary control).
2. Sympathetic and Parasympathetic Nerves
a. Parasympathetic nerves tend to slow down body activity when the body is not under stress.
b. Sympathetic nerves increase overall body activity during times of stress, excitement, or danger; they also call on the hormone epinephrine to increase the “fight-flight” response.

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what are the functional divisions of the brain

Functional Divisions of the Brain
A. The body’s master control panel, the brain, is a continuation of the anterior end of the spinal cord, and is also protected by meninges and bones.
1. The forebrain, midbrain, and hindbrain form from three successive portions of the neural tube.
2. The most primitive of the tissue is the brain stem, which contains simple, basic reflex centers.
B. Hindbrain
1. The medulla oblongata has influence over respiration, blood circulation, motor response coordination, and sleep/wake responses.
2. The cerebellum acts as reflex center for maintaining posture and coordinating limbs.
3. The pons (“bridge”) possesses bands of axons that pass between brain centers.
C. Midbrain
1. The midbrain originally coordinated reflex responses to visual input; the tectum still integrates visual and auditory signals in vertebrates.
2. In mammals it is now mostly a pathway switching center.
D. Forebrain
1. The large olfactory lobes dominated early vertebrate forebrains.
2. The cerebrum integrates sensory input and selected motor responses.
3. The thalamus (below cerebrum) relays and coordinates sensory signals.
4. The hypothalamus monitors internal organs and influences responses to thirst, hunger, and sex.
E. The Reticular Formation
1. The reticular formation is an ancient mesh of interneurons that extends from the uppermost part of the spinal cord, through the brain stem, and into the cerebral cortex.
2. It serves as a pathway and activates centers in the cerebral cortex.
F. Brain Cavities and Canals
1. The brain and spinal cord are bathed with cerebrospinal fluid that exists within a system of cavities and canals.
2. The fluid cushions vital nervous tissue from sudden, jarring movements.
3. The blood-brain barrier operates at the plasma membranes of cells forming the capillaries that service the brain.
a. Tight junctions fuse the capillary cells together forcing substances to move through the cells to reach the brain.
b. Membrane transport proteins allow essential nutrients (glucose) to move through but bar wastes (urea) and certain toxins.

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what is memory

A. “Memory” is the storage and retrieval of information about previous experiences.
1. Association is the linkage of information into larger packages that can be sent to other brain regions for storage.
2. Information becomes stored in “memory traces”—chemical and structural changes in brain regions.
a. Short-term memory lasts from seconds to hours and is limited to seven to eight bits of information.
b. Long-term memory is more permanent and seems to be limitless.
3. Persons suffering from retrograde amnesia lose short-term memory, but long-term memory remains intact.
B. Information is moved into long-term storage with the cooperation of epinephrine, which increases a person’s state of arousal.

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what are the components of the sensory system

Each sensory system has three component parts:
1. Sensory receptors are the branched endings of sensory neurons or specialized cells adja¬cent to them that detect specific stimuli.
2. Nerve pathways lead to the brain.
3. Brain regions process the information into a sensation; later, perhaps, a perception (understanding) of the sensation will be made.

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what are the types of sensory receptors

Types of Sensory Receptors
1. Mechanoreceptors detect changes in pressure, position, or acceleration; they include receptors for touch, stretch, hearing, and equilibrium.
2. Thermoreceptors detect radiant energy, including infrared.
3. Pain receptors (nociceptors) detect tissue damage.
4. Chemoreceptors detect ions or molecules; they include olfactory and taste receptors.
5. Osmoreceptors detect changes in water volume (solute concentration) in the surrounding fluid.
6. Photoreceptors detect the energy of visible and ultraviolet light.

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what are the types of receptors near the body surface

Receptors Near the Body Surface
1. Free nerve endings are simply branched endings of sensory neurons in the skin that function as mechanoreceptors, thermoreceptors, and pain receptors.
2. Encapsulated receptors are of several types:
a. Meissner corpuscles adapt slowly to vibrations of low frequencies.
b. The bulb of Krause is a thermoreceptor that is sensitive to temperatures below 20 degrees C.
c. Ruffini endings are sensitive to steady touching and pressure, and to temperatures above 45 degrees C.
d. Pacinian corpuscles are located both in the dermis and near joints; they are able to detect rapid pressure changes associated with touch and vibrations.

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what is phantom pain

Phantom pain is the sensation that amputees may feel from a limb that is no longer a part of the body.

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مُساهمةموضوع: رد: Central nervous system physiology questions and answers by Dr Khaled Abulfadle   السبت سبتمبر 25, 2010 8:08 pm

Classification of Receptors
• By modality:
– chemoreceptors, thermoreceptors, nociceptors, mechanoreceptors and photoreceptors
• By origin of stimuli
– interoceptors = detect internal stimuli
– proprioceptors = sense position & movements of body
– exteroceptors = sense stimuli external to body
• By distribution
– general (somesthetic) sense --- widely distributed
– special senses --- limited to head

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Unencapsulated Nerve Endings
• Found as receptors for the general senses
• Dendrites not wrapped in connective tissue
• Free nerve endings include
– warm, cold & pain
• Tactile discs are associated with cells at base epidermis
• Hair receptors monitor the movement of hairs

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Encapsulated Nerve Endings
• Dendrites wrapped by glial cells or connective tissue
– tactile (meissner) corpuscles
• light touch & texture
– krause end bulb
• tactile corpuscles in mucous membranes
– lamellated (pacinian) corpuscles
• deep pressure, stretch, tickle & vibration
– ruffini corpuscles
• heavy touch, pressure, joint movements & skin stretching

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• Endogenous opiods (enkephalins, endorphins 7 dynorphins)
– produced by CNS and other organs under stress
– found especially in dorsal horn of spinal cord, explaining the spinal gating of pain
– act as neuromodulators blocking the transmission of pain

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• Spinal gating stops pain signals at dorsal horn
– descending analgesic fibers from reticular formation travel down reticulospinal tract to dorsal horn
• secrete inhibitory substances that block pain fibers from secreting substance P
• pain signals never ascend
– dorsal horn fibers inhibited by input from mechanoreceptors (rubbing a sore arm reduces pain)

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مُساهمةموضوع: رد: Central nervous system physiology questions and answers by Dr Khaled Abulfadle   الإثنين سبتمبر 27, 2010 9:39 am

what do we mean by a reflex?

it is a a rapid, “wired" motor response to a specific stimulus; involuntary, subconscious
. it may be:

1. somatic reflex - skeletal muscle
2. visceral (autonomic) reflex – smooth muscle, cardiac muscle, visceral
organs, glands

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what are the components of a typical reflex arc?

Components of a Typical Reflex Arc

1. Receptor - site where stimulus is

a. mechanoreceptors - sensitive to touch, pressure, vibration and stretch

i. Meissner's corpuscles - receptors for discriminative touch, below skin epidermis

ii. Merkel's discs - light touch receptors in deep layers of epidermis

iii. Pacinian corpuscles - respond to skin pressure/vibration, deep in the dermis

iv. Ruffini's corpuscles - respond to stretch of dense connective tissues

v. Krause's End Bulbs - respond to pressure and touch, in mucous membranes

vi. muscle spindles - respond to stretch in "intrafusal fibers" of skeletal muscle

vii. Golgi tendon organs - respond to stretch in tendon, close to muscle insertion point

viii. joint kinesthetic receptors - monitor position of synovial joints (position)

b. thermoreceptors - respond to
temperature changes

c. photoreceptors - respond to light
(the retina)

d. chemoreceptors - respond to
different molecules in the blood,
kidney, tongue, nose, etc.

e. nociceptors - respond to harmful
stimuli (painful); heat, cold,
pressure, chemical

2. Sensory Neuron - transmits the signal from receptors (AFFERENT nerve fiber) to the CNS

3. Integration center - point where sensory and motor neurons integrate information to respond

a. monosynaptic pathway - sensory neurons synapse directly with the motor neuron

b. polysynaptic pathway - one or more "interneurons" between sensory and motor neurons

4. Motor Neuron - transmits motor signal from CNS to the effector organ via EFFERENT nerve

5. The Effector - the muscle or organ that responds to signal of the motor neuron

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give short notes on the classic stretch reflex?

The Classic Stretch Reflex

1. Anatomy of Skeletal Muscle Stretch Reflex

a. intrafusal muscle fiber - special muscle fiber that responds to stretching; center is non-contractile; ends have contractile property

i. type Ia sensory fiber - wraps around CENTER of intrafusal fiber; respond to rate and degree of stretching of the muscle

ii. type II sensory fiber - wrap around ends of intrafusal fiber; respond only to degree of stretch of the muscle [records "resting length" of the intrafusal fiber]

iii. gamma motor fiber - synapses on contractile end of intrafusal fiber; maintains level of sensitivity of this stretch receptor, depending on the length of the muscle at any given time [sets the "resting length" of intrafusal fiber]

b. extrafusal fiber - normal contractile muscle fibers of a skeletal muscle

i. alpha motor fiber - synapses on normal extrafusal fibers to allow a response to a stretched muscle

2. Mechanism of Skeletal Muscle Stretch Reflex

a. Excitation of Muscle Spindle (Stretching)

i. external stretch - external force (weight) on muscle or contraction of antagonistic muscle (eg. quadriceps - hamstrings)

ii. internal stretch - activated gamma motor neurons cause contraction of intrafusal fiber, stretching midsection of spindle

b. Type Ia and II Sensory Fiber Activation

i. AFFERENT (sensory) fiber synapses directly on alpha motor neuron, causing contraction of extrafusal muscle fibers of the stretched muscle

ii. Reciprocal Inhibition - AFFERENT (sensory fiber synapses on interneurons, which then INHIBIT motor neurons to the antagonistic muscle (eg. quadriceps -hamstrings)

iii. Relay of information to brain (cerebellum) also occurs via "dorsal white columns" of the spinal cord

c. Regulation of Muscle Spindle Length

i. The length of the muscle spindle organ, and therefore its sensitivity to stretch, is regulated by activation/inhibition of the gamma motor neuron reflex arc.

3. Example: The Patellar Tendon (Knee-Jerk) Reflex

a. reflex hammer hits patellar tendon, stretching the quadriceps muscle

b. intrafusal fibers of muscle spindle are stretched, type Ia and II sensory (afferent) fibers are activated

c. alpha motor neuron to quadriceps activated, resulting in quadriceps contraction

d. alpha motor neuron to hamstrings inhibited (via interneuron), resulting in hamstring relaxation

e. gamma motor neuron to end of muscle spindle regulates sensitivity to stretch

4. Clinical Significance of Patellar Tendon Reflex

a. test integrity of sensory neurons, motor neurons, and the spinal cord

b. hypoactive - when damage to peripheral nerves, dorsal root ganglion (sensory cell bodies), or ventral horn of spinal cord (motor cell bodies)

c. hyperactive - when inhibitory influence from brain (via corticospinal tracts) is reduced; e.g. polio and stroke patients

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what do we mean by: Flexor (Withdrawal) Reflex - Crossed Extensor Reflex -Plantar Reflex - Abdominal reflexes

The Flexor (Withdrawal) Reflex - allows the withdrawal of a limb from a painful stimulus

1. ipsilateral, polysynaptic reflex - this occurs on the same side of the body as the stimulus, and involves at least one interneuron in the pathway

The Crossed Extensor Reflex - allows one limb to act in opposite way as other limb

1. Lifting foot from a hot coal, other leg must react to support the weight of the body

2. contralateral reflex - stimulus on one side of the body affects action of opposite (contra) side

Plantar Reflex - gently stroking the lateral aspect of the bottom of the foot (test L4 – S2)

a. normal adult - the toes will flex (curl down)

b. Babinski sign - in infant (up to 6 months) and in adults with damaged corticospinal tract, toes will fan out

2. Abdominal reflexes - gently stroke skin on lateral aspects of the abdomen (test T8 – T12)

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Central nervous system physiology questions and answers by Dr Khaled Abulfadle
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