Download Neurophysiology

Neurophysiology
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Neurons
a)
They can remain optimally functional for a lifetime
b)
As neurons assume their function, they lose their ability to
divide
c)
High metabolic rate. Require abundant supply of oxygen
and glucose
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Neurons
Nerve cells
Body, axon and dendrites
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Neuron Cell Body
Contains a large nucleus
Dendrites and axons branch out from the body
Dendrites are the receptive or input regions
They are adapted to carry out this function due to their large
branching and surface area
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The Axon
Single axon arises from the cone-shaped region of the body
“axon hillock” (little hill)
A long axon is referred to as a nerve fibre
Axons are the conducing component of the neuron
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Myelination
The axon fibres are myelinated at regular intervals
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Myelination
[1]
Protects and electrically insulates fibres from one another
[2]
Increases the speed of transmission of nerve impulses
In the peripheral nervous system, myelin is synthesized by
“Schwann cells”
“Oligodendrocytes” form the central nervous system myelin
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Structural Classification Of Neurons
Grouped according to the number of processes extending from
their cell bodies
a)
Multipolar Neurons
Three or more processes
Numerous branching dendrites and one axon
Most common neuron type in the CNS
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B)
Bipolar Neurons
Two processes, an axon and a dendrite
Work as receptors
e.g. neurons in the retina of the eye and in the olfactory mucosa
C)
Unipolar Neurons
Singe process emerging from the cell body
Divides T-like into proximal and distal fibres
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Electrical Signals & Action Potentials
Nerve and muscle are excitable tissues
Propagation of the action potential is due to movement of
sodium and potassium ions across the membrane
At rest, the membrane potential is –70 mV “POLARIZATION”
During the excitation phase, sodium moves in
The membrane potential moves towards 0 mV and above
“DEPOLARIZATION”
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Electrical
Signals &
Action
Potentials
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Synaptic
Structure &
Function
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Synaptic Structure & Function
1)
Action potential propagates down the presynaptic neuron
2)
Synaptic vesicles in the synaptic knob release the
neurotransmitter (e.g. acetylcholine)
3)
Neurotransmitter travels across synaptic cleft
4)
Neurotransmitter binds to receptors on postsynaptic
neuron
5)
Action potential is propagated
6)
Neurotransmitter (e.g. acetylcholine) is degraded by an
enzyme (e.g. ACH esterase)
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The Nervous System
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Spinal Cord
Extends from brain stem
Descends through and is protected by the vertebral column
Paired spinal nerves emerge from spinal cord (cervical, thoracic,
lumbar, sacral and coccygeal)
Gross Anatomy Of Spinal Cord
Inner gray matter is butterfly shaped (unlike brain) and
surrounded by white matter
Gray matter = neuronal cell bodies and their dendrites
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White matter = “TRACTS”
Tracts “Bundles of nerve fibres”
Tracts wither begin or end within a particular region of the brain
[1]
Ascending Tracts
“Transmit to the brain, signals derived from the afferent input”
[2]
Descending Tracts
“Relay messages from the brain to the efferent neurons”
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Each half of the gray matter is divided into:
a)
Dorsal horn (cell bodies of afferent neurons)
b)
Lateral horn (Cell bodies of interneurons)
c)
Ventral horn (Cell bodies of efferent neurons)
Spinal nerves connect to both dorsal and ventral roots and
emerge from the spinal column
The collection of neuronal cell bodies outside the brain =
“Ganglion”
The 31 pairs of spinal nerves along with the 12 pairs of cranial
nerves constitute the peripheral nervous system
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Sensory Neuron Order
Afferent neurons interconnected in a certain sequence
First-order sensory neuron
“Afferent neuron with its peripheral receptor that FIRST detects
the stimulus
Second-order sensory neuron
“Synapse with first-order sensory neuron + located in spinal cord
or medulla”
Third-order sensory neuron
“In the thalamus”
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Neurophysiology
Spinal cord has two main functions:
1). SC connects a large part of the peripheral nervous
system to the brain.
2). SC acts as a minor coordinating centre responsible
for some simple reflexes (e.g withdrawal reflex).
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•31 pairs of spinal nerves arise along the spinal cord.
•These are "mixed" nerves because each contain both
sensory and motor axons.
Within the spinal column:
- all sensory axons pass into dorsal
root ganglion.
- all motor axons pass into ventral roots.
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•Spinal tracts are bundles of axons grouped together into columns that extend
length of the spinal cord
•A spinal tract consist of neuronal axons that have a similar destination and
function
•Part of a multineurone pathway that connect the brain to the rest of the body
•Each tract either:
- begins with a particular part of the brain
(Motor / descending tract)
- ends with a particular part of the brain
(Sensory /ascending tract)
•Tracts are named according to their origin and point of termination.
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Sensory (ascending) pathways and tracts
Conduct sensory impulses from the body to various parts of the
brain
Information obtained from:
- sensory receptors: (touch, pressure, pain, temp)
- proprioreceptors: monitor degree of stretch in
muscles, tendons and joints
Two main pathways:
- Posterior column – medial lemniscal
- Spinothalamic
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Sensory (Ascending) nerve tracts
•There are 2 main sources of sensation transmitted to the
brain via the SC
1.Skin:
- pain, heat, cold, and touch
- Nerve impulses are passed by 3 neurones to
sensory area in opposite hemisphere of cerebrum
where sensation and its location are perceived
- Crossing to other side, decussation, occurs
either at level of entry into spinal cord
(spinothalamic) or in the medulla (posterior
column – medial lemniscal).
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2.The tendons, muscles and joints
(a).
(b).
- proprioceptors stimulated by stretch
- maintenance of posture and balance, and
position of body (in conjunction with impulses
from eyes/ears)
- nerve impulses have 2 destinations;
3 neurone system by which the impulses
reach sensory area of the opposite
hemisphere of cerebrum (posterior columnmedial lemniscal pathway)
2 neurone system by which nerve impulses
reach cerebellar hemisphere on same side
(spinocerebellar)
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Descending (motor) pathways and tracts
Impulses from brain to spinal cord. Divided into 2 groups:
- Pyramidal/Corticospinal tracts
-major motor pathways
- concerned with skilled precise voluntary
movement
- pathway extends from cerebral cortex to
spinal cord, then to muscles
- Pyramidal tracts decussate in medulla
- Others (extrapyramidal)
-Subconscious control, muscle
coordination, muscle tone, posture and
balance
-Pathways follow complex circuits that
involve motor cortex, basal ganglia,
cerebellum
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Corticospinal motor pathways
•Consists of 2 sets of neurones:
•Upper Motor Neurones (UMN)
- Begin in cortex and extend to spinal cord /
cranial nerves
•Lower Motor Neurones (LMN)
- begin in spinal cord and extend to skeletal
muscles
•Interneurones link UMN with LMN
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Lesions/damage to pathways/tracts
Any localised damage to spinal cord or spinal roots
will attribute to some form of functional loss.
- Paralysis:
(loss of motor
function)
- Parasthesias:
(loss of senses)
The effects of disease or injury upon the CNS and
periphery depend on the:
- severity of the damage
- type of neurones involved
- position of neurones involved
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• Normal muscle function requires intact connections
along motor pathway.
• Chain of nerve cells that runs from the brain through the
spinal cord out to the muscle is called the motor
pathway.
• Damage at any point reduces brain's ability to control
muscle's movements.
• Reduced efficiency causes weakness (paresis).
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• Complete loss of communication prevents any willed
movement.
• Lack of control is called paralysis.
• Paralysis may affect an individual muscle, but usually
affects an entire body region.
• Distribution of weakness an important clue to location of
the nerve damage that is causing the paralysis.
• Words describing the distribution of paralysis use the
suffix "-plegia," from the Greek word for "stroke.“
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The types of paralysis are classified by region:
Monoplegia:
affecting only one limb
Diplegia:
affecting the same body region on
both sides of the body (both arms, for
example, or both sides of the face)
Hemiplegia:
affecting one side of the body
Paraplegia:
affecting both legs and the trunk
Quadriplegia:
affecting all four limbs and the trunk.
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•The nerve damage that causes paralysis may be in the:
- brain or spinal cord (CNS)
- nerves outside the spinal cord (PNS).
•The most common causes of damage to the brain are:
- Stroke
- Tumour
- Trauma (caused by a fall or a blow)
- Multiple sclerosis (destruction of Myelin
sheath))
- Cerebral palsy (defect or injury to the brain that
occurs at or shortly after birth)
- Metabolic disorder (interferes with body's ability
to maintain itself).
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•Damage to spinal cord is most often caused by trauma,
(fall/car crash). Other conditions that may damage nerves
within or immediately adjacent to spine include:
- Tumour
- Herniated disk (also called a ruptured or slipped
disk)
- Spondylosis (a disease that causes stiffness in
the joints of the spine)
- Rheumatoid arthritis of the spine
- Neurodegenerative disease (a disease that
damages nerve cells)
- Multiple sclerosis.
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• Paralysis originating in the brain may sometimes be
flaccid, that is, the affected muscles may be loose,
weak, flabby, and without normal reflexes.
• More frequently it is spastic, that is, the affected
muscles are rigid and the reflexes accentuated.
• Paralysis originating in a motor nerve (UMN) of the
spinal cord is always spastic
• Paralysis originating in peripheral nerves (LMN) is
always flaccid.
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Cerebrovascular Accident (Stroke)
•CVAs are:
bleeds into the brain
obstruction of blood supply to brain
•CVAs often affect Motor cortex and its major pathways.
•These tracts cross in medulla therefore:
- left hemiplegia
(stroke on right side of brain)
- right hemiplegia
(stroke on left side of brain)
•Small bleeds close to brain surface may result in weakness on one side
(hemiparesis)
- good chance of recovery
•Larger/deeper bleeds may cause profound paralysis
- may result in permanent damage
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Pupillary Reflex
• Clinical test for brain stem function
• Shine bright light into patient’s eye
• Normal response: pupils constrict in response to light
stimulus
• Reflex via autonomic nervous system
• Sensory input of bright light- to brain via optic nerve (II) –
parasympathetic impulses out via oculomotor nerve (III) –
circular muscles of eye constrict
• Pupil observation important when considering head injury
care
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Plantar (sole) reflex
• Tests integrity of spinal cord from L4-S2
• Determines functionality of corticospinal tracts
• Normal response is a downward flexion (curling) of toes
• If corticospinal tract damaged, normal plantar’s reflex
replaced by Babinski’s sign
• Toes fan backwards
Normal
Abnormal
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