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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

- 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;(a). 3 neurone system by which the impulses reach sensory area of the opposite hemisphere of cerebrum (posterior column- medial lemniscal pathway)

(b). 2 neurone system by which nerve impulses reach cerebellar hemisphere on same side (spinocerebellar)

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Descending (motor) pathways and tractsImpulses 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 brainobstruction 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