Divisions of the Nervous System

Describe the structure, function and location of sensory (afferent) neurons

Structure: Short axon which branches off in 2 directions (pseudounipolar)

Function: Receives external stimuli and converts them to electrical impulses

Location: Sensory organs and sensory ganglia in the PNS

Describe the structure, function and location of motor (efferent) neurons

Structure: Large cell body, highly branched dendrites and a single long axon (multipolar)

Function: Receives signals from the CNS and transmits them to effectors

Location: Cell bodies are in the CNS but axons reach into the PNS

Describe the structure, function and location of inter (association) neurons

Structure: Multiple short dendrites and short axons (multipolar)

Function: Pass signals from sensory neurons to motor neurons

Location: Exclusively in the CNS

Give 5 examples of sensory (afferent) neurons

• Photoreceptors (vision)

• Mechanoreceptors (touch and hearing)

• Thermoreceptors (temperature)

• Chemoreceptors (taste and smell)

• Nociceptors (pain)

Describe the stages from a stimulus to the generation of a nerve impulse

1. Stimulus detection: an environmental stimulus interacts with the dendrites/receptor cells of the sensory neuron

2. Receptor potential: the stimulus physically/chemically alters the cell membrane, causing ion channels to open. This changes the cell membrane voltage- creating a local electrical signal

3. Threshold reached: if the stimulus is strong enough, the receptor potential reaches the threshold

4. Action potential is generated: voltage-gated sodium channels open and changes the internal charge from negative to positive

5. Propagation and transmission: the action potential travels as a wave down the axon and triggers the release of neurotransmitters across the synapse

Describe the two types of motor (efferent) neurons

UMNs (upper motor neurons)

• Originate in the cerebral cortex and the brainstem

• Responsible for initiating voluntary movement 

• Do not directly contact the target muscles

• Key neurotransmitter: glutamate 

LMNs (lower motor neurons)

• Originate in the brainstem and the spinal cord

• Extend out of the CNS to directly innervate effector muscles and glands

• Key neurotransmitter: acetylcholine

Describe how neurons are classified based on their level of myelination

• Neurons are classified based on their nerve impulse velocity, using the Erlanger-Gasser classification system- which categorises fibres into types A, B and C

• The velocity of the impulse depends on the axon diameter and the presence of a myelin sheath 

• Group A: heavily myelinated

• Group B: moderately myelinated 

• Group C: unmyelinated (e.g in the tongue)

What are the 2 key mechanisms involved in myelination

• Saltatory conduction between nodes of ranvier 

• Highly concentrated ion channels at the nodes of ranvier

Describe the impact of myelination on nerve cells

• The myelin sheath drastically increases nerve impulse velocity, by forcing the electrical signal to ‘jump’ between gaps in the sheath- this is known as saltatory conduction

• This prevents the electrical signal from degrading, allowing impulses to travel much faster

Describe the types of neuroglia

• Schwann cells- wrap tightly around the axons of neurons, to form the myelin sheath, which acts as an electrical insulator. Schwann cells also release essential growth factors, which help repair and regenerate damaged axons

• Satellite cells- surround the cell bodies of neurons located in the peripheral ganglia. They serve as physical and nutritional support, and release factors which protect neurons from degeneration

Names:

1. Astrocytes: star shapes cells which provide physical and nutritional support to neurons

2. Oligodendrocytes: specialised cells which form the myelin sheath around axons

3. Microglial cells: resident immune cells of the CNS, monitoring for pathogens 

4. Ependymal cells: ciliated cells lining the CNS, which produce cerebrospinal fluid

What are NSAIDs? Include their target of action and an example

Non-steroidal anti-inflammatory drugs

Target of action: cyclooxygenase (COX)

Example: carprofen, meloxicam

What are corticosteroids? Include their target of action and an example

Steroid medications that mimic cortisol to suppress the immune system

Target of action: intracellular receptors 

Example: prednisolone

Why should you not combine NSAIDs and corticosteroids?

Combining NSAIDs and corticosteroids is contraindicated, because it drastically increases the risk of severe, life-threatening GI ulceration, bleeding, and perforation

List 5 factors affecting drug mechanisms

• Age

• Gender

• Diet

• Environment

• Body weight

• Genetics

• Organ functions

• Neurological problems

• Drug-food interactions

List the advantages and disadvantages of oral medication administration

Advantages:

• Most convenient for owners

• Cost-effective

Disadvantages:

• Unreliable if vomiting/diarrhoea

• Palatability

List the advantages and disadvantages of topical medication administration

Advantages:

• Painless

• Good for localised skin/eye conditions

Disadvantages:

• Slow/erratic absorption

• May be licked/rubbed off

List the advantages and disadvantages of rectal medication administration

Advantages:

• Bypasses stomach acid

• Good for vomiting/unconscious animals

Disadvantages:

• Uncomfortable

• Faeces can reduce absorption

List the advantages and disadvantages of subcutaneous medication administration

Advantages:

• Simple to perform

• Quick, predictable absorption

Disadvantages:

• Reduced absorption in dehydrated/cold patients

• Granulomas may form

List the advantages and disadvantages of intramuscular medication administration

Advantages:

• Fast absorption

• Useful for patients who are gardening to restrain

Disadvantages:

• Painful for the patient

• Higher risk of hitting nerves/blood vessels and causing muscle damage

List the advantages and disadvantages of intravenous medication administration

Advantages:

• 100% bioavailability instantly

• Immediate therapeutic effect

Disadvantages:

• High risk of anaphylaxis and adverse reactions

List the advantages and disadvantages of intranasal medication administration

Advantages:

• Good for localised respiratory issues

• Avoids gut enzymes

Disadvantages:

• Variable absorption

• Can cause mucosal irritation

Define the term ‘resting membrane potential’

The electrical charge difference across a cell’s plasma membrane when it is not actively sending signals (typically around -70mV)

Define the term ‘action potential’

A  rapid, temporary spike in electrical voltage which travels along the cell membrane of a neuron/muscle cell

Describe the stages of neurotransmission along the axon, including a labelled graph

1. The voltage of the local membrane becomes less positive- resulting in the membrane reaching the ‘threshold’

2. Once the threshold is reached, voltage-gated sodium ion channels open, allowing sodium ions to flood into the cell: further depolarising the membrane

3. More voltage-gated sodium ion channels open- causing rapid depolarisation along the length of the membrane until the channels are inactivated and close

4. Voltage peaks at around +35mV- at which point, the membrane begins to repolarise

5. Repolarisation occurs due to the slow opening of potassium channels, and the outflow of potassium ions 

6. Potassium ion channels stay open for a time, so that the membrane is briefly hyperpolarised 

7. Resting membrane potential is restored as sodium ions leak in, and extracellular potassium ions are removed by astrocytes

Describe the mechanism of the sodium/potassium pump

• An active transport protein

• Uses ATP to constantly pump 3 sodium ions out of the cell, whilst pumping 2 potassium ions into the cell

• This creates a slight negative charge inside

Describe the mechanism of the potassium ion leak channels

• These are always open

• Because potassium ions are always being pumped into the cell, potassium naturally leaks out down the concentration gradient

• This establishes the slightly negative resting membrane potential

Describe the mechanism of the sodium ion voltage-gated channels

• Normally closed at rest, but open in response to a stimulus/voltage change

• Once open, sodium ions rush into the cell down the steep concentration gradient- depolarising the membrane and creating an action potential

Describe the mechanism of the potassium ion voltage-gated channels

• These open after the cell has depolarised

• They allow potassium ions to rapidly leave the cell

• This causes repolarisation (and brief hyperpolarisation)

What is propagation?

Describe what it is affected by

• This means the travel of an electrical signal down the axon of a neuron

• It is affected by myelination:

  • Instead of moving continuously down the axon, myelination allows the signal to jump between nodes of Ranvier 

  • The voltage-gated ion channels are dense at the nodes of Ranvier

  • Myelinated segments prevent ion leakage and conserve electrical charge

  • This increases speed and energy efficiency

Describe the action of lignocaine (lidocaine)

Temporarily halts nerve transmission in localised areas of the body, by blocking voltage-gated sodium ion channels to prevent the influx of sodium ions 

1. When applied/injected, a neural portion of the drug is allowed to pass through the cell membrane

2. Once inside the cell, the drug becomes charged by combining with hydrogen ions 

3. This charged molecule binds directly to the inner portion of the voltage-gated sodium ion channel

4. By binding to the channel, the drug locks it in an inactive state- physically blocking sodium ions from entering the cell

5. Because APs rely on sodium ion movement to travel, the nerve becomes temporarily ‘numb’

Describe the two types of sensory pathway

• Those which register conscious perception 

• Those which control unconscious coordination

Describe the action of the spinocerebellar pathway

• Convey unconscious proprioception from the limbs and trunk, directly into the cerebellum for motor refinement and posture

• Dorsal and cuneocerebellar tracts: simple 2-neuron pathways

• Ventral and rostral spinocerebellar tracts: serve as a feedback loop regarding spinal motor activity

Describe the function of the cuneate and gracile tracts

• Manage conscious proprioception and discriminative touch 

• Uses a 3-neuron chain: first-order neurons enter the spinal cord and ascend without synapsing

• Fasciculus gracilis: transmits sensory information from the pelvic limbs and caudal trunk

• Fasciculus cuneatus: transmits sensory information from the thoracic limbs and cranial trunk

• Decussation: both fasciculi synapse in their respective nuclei in the medulla oblongata

What is proprioception? Give a brief overview of how it works and the components involved

• Proprioception is the subconscious ability to sense the body’s position, movement and balance in space without having to look

• Sensory receptors constantly detect stretching, contracting and pressure 

• These receptors constantly send signals to the CNS- allowing the brain to calculate the exact orientation of the limbs 

How is the white matter of the spinal cord divided?

The white matter of the spinal cord is divided into 3 bilaterally symmetrical pairs of columns (funiculi)

1. Dorsal funiculus (contains primarily ascending sensory tracts)

2. Lateral funiculus (contains a mix of both ascending and descending tracts)

3. Ventral funiculus (contains primarily descending motor tracts)

Describe the action and components of the ascending (sensory) tracts and descending (motor) tracts

Ascending (sensory) tracts

• Relay afferent information up to the brainstem, cerebellum and cerebral cortex- typically following a sequence of three neurons 

• Spinothalamic tract

• Spinocerebellar tract 

• Spinocervicothalamic tract 

Descending (motor) tracts

• Initiate voluntary movement, regulate muscle tone and maintain posture

• Corticospinal tract (part of the pyramidal system)

• Vestibulospinal, reticulospinal, rubrospinal tracts (part of the extrapyramidal system)

How are spinal cord pathways organised?

Spinal cord pathways are organised into bilaterally symmetrical tracts within the white matter

These tracts are primarily divided into ascending pathways (sensory) and descending pathways (motor)

What is the function of the thalamus?

The thalamus filters raw sensory inputs, and controls what information reaches conscious awareness

Describe the following fibre types, including their conduction times:

• A-alpha (type I) myelinated

• A-beta (type II) myelinated

• A-delta (type III) thinly myelinated

• C (type IV) non myelinated

A-alpha myelinated

• 80-120m/s

• Can be type Ia (sense rate of change of muscle length) or type Ib (golgi tendon organs, sense muscle tension)

A-beta (type II) myelinated 

• 33-75m/s

• Muscle spindles and skin sensation

A-delta (type III) thinly myelinated

• 3-30m/s

• Free endings, pain, cold 

C (type IV) non myelinated

• 0.5-2 m/s

• Pain, warmth

Describe the:

• First order neuron

• Second order neuron

• Third order neuron

Involved in pain and temperature

First order neuron

• Cell bodies reside in the dorsal root ganglion 

• They receive stimuli via free nerve endings (nociceptors and thermoreceptors) and enter the spinal cord dorsal horn

Second order neuron

• Synapses in the dorsal horn

• Axons decussate 

• They ascend via the lateral spinothalamic tract, in the ventrolateral funiculus of the spinal cord

Third order neuron

• Axons terminate in the thalamus, which then projects to the somatosensory cortex for conscious localisation and perception

Describe the:

• First order neuron

• Second order neuron

• Third order neuron

Involved in crude touch and pressure

First order neuron 

• Cell bodies are in the DRG

Second order neuron

• Synapses in the dorsal horn 

• Ascend contralaterally via the anterior spinothalamic tract in the ventral funiculus

Third order neuron

• Relays from the thalamus to the somatosensory cortex

Describe the:

• First order neuron

• Second order neuron

• Third order neuron

Involved in fine touch and conscious proprioception

First order neuron 

• Cell bodies are in the DRG

• Receptors detect fine touch, vibration and joint position 

Second order neuron

• Synapses in the medulla oblongata 

• The axons decussate at the medulla, and ascend via the medial lemniscus to the thalamus

Third order neuron

• Projects from the thalamus to the somatosensory cortex

Describe the:

• First order neuron

• Second order neuron

• Third order neuron

Involved in facial sensation

First order neuron 

• Housed in the sensory ganglion

Second order neuron

• Reside in the sensory nuclei of the trigeminal nerve 

• Axons cross the midline and ascend to the thalamus 

Third order neuron

• Project to the face region of the somatosensory cortex

Describe the composition of the vagus nerve

It is made up of 80% sensory fibres and 20% motor fibres

Describe the origin and exit of the vagus nerve in the CNS

Origin

• 10th cranial nerve (CN X)

• Between CN IX and XI

• Exits as a series of rootlets from the lateral surface of the medulla oblongata

Exit

• From the cranial cavity- the jugular foramen with CNs IX and XI

• The vagus nerve also has fibres which are destined to be in the recurrent laryngeal nerve

Describe the distribution and branches of the vagus nerve in the head and neck

Below the base of the skull, the vagus nerves gives off:

• Auricular branch (to external acoustic meatus : sensory)

• A pharyngeal branch (to pharynx : sensory and motor to pharyngeal muscles)

• The cranial laryngeal nerve (to larynx : sensory as far as the glottis, motor to cricothyroid m.)

Continuing caudally, the vagus nerve:

• Joints the sympathetic trunk at the cranial cervical ganglion

• This forms the vagosympathetic trunk

Describe the vagosympathetic trunk and the fibres that run within it

The vagosympathetic trunk is a structure that contains several components

Sympathetic fibres running cranially: to the head/neck

Parasympathetic fibres running cranially: from receptors and viscera 

Parasympathetic fibres running caudally: to the heart and viscera

Describe the distribution and branches of the vagus nerve in the thorax

The vagus nerve:

• Enters the thorax at the thoracic inlet

• Separates from the sympathetic tract at the middle cervical ganglion

• It does not synapse at the middle cervical ganglion, but just keeps on going

In the thorax, the vagus gives off:

• Recurrent laryngeal nerve (axons from CN XI)

• Cardiac branches: to cardiac plexus supplying the heart

• Bronchial branches: to the pulmonary plexus supplying lungs

In the thorax:

• The vagus on each side divides caudal to the heart, into a dorsal and ventral branch 

• May become a fine plexus 

• Dorsal branches unite on the oesophagus, to form the dorsal vagal trunk

• Ventral branches unite to form the ventral vagal trunk

• Dorsal and ventral vagal trunks pass through the oesophagus hiatus of the diaphragm 

Describe the distribution of the recurrent laryngeal nerve

Fibres leave the vagal trunk in the thorax

Right RLN hooks around the right subclavian artery

Left RLN hooks around the aortic arch (longer route)

Describe the distribution and branches of the vagus nerve in the abdomen

Dorsal and ventral vagal trunk:

• Provide preganglionic parasympathetics to abdominal plexuses

• Supply preganglionic parasympathetics to the gut through to the transverse colon

Dorsal vagal trunk: fibres to coeliac and cranial mesenteric plexuses

Ventral vagal trunk: fibres to gastric and hepatic plexuses

Describe the distribution and branches of the vagus nerve in the abdomen of ruminants

• The ruminant stomach is classically described as having dorsal and ventral vagal supplies, but there is crossover- so resecting one trunk does not denervate a section of the rumen

• Dorsal vagal trunk: rumen (ruminal motor function)

• Ventral vagal trunk:

  • Reticulum, omasum, abomasum

  • Gastric plexus, branches to duodenum, pancreas, liver, hepatic plexus

Describe Horner’s Syndrome

Damage to the sympathetic nerves within the vagosympathetic trunk (brain and spinal cord damage can also be a cause)

Usually one-sided

Loss of supply to:

• Smooth muscle in the periorbital tunic

• Smooth muscle of iris dilators

Results in:

• Enophthalmos: globe retracts into orbit, due to tone in retractor bulbi mm., allowing the 3rd eyelid to move into view

• Miosis: constricted pupil

Describe vagal indigestion

Occurs in ruminants, mainly cattle

Usually due to traumatic reticuloperitonitis (wire/hardware disease)

This is a chronic disease

Syndrome of ruminoreticular distension

Four types (I-IV)
Not acute bloat- it is chronic paralysis with poor motility

Surgery may be required, depending on the cause

Describe where these parts of the brain develop from:

• Forebrain

• Midbrain

• Hindbrain

As the embryo develops, vesicles are formed- which form the basis of the brain and spinal cord

The forebrain develops from the telencephalon and diencephalon

The midbrain develops from the mesencephalon

The hindbrain develops from the metencephalon and myelencephalon

Which structures make up:

• The forebrain

• The hindbrain

• The brainstem

• Within the forebrain: cerebrum (made up of cerebral cortex), thalamus, hypothalamus, pituitary gland

• Within the hindbrain: pons, medulla oblongata, cerebellum 

• The brainstem is made up of the medulla oblongata, pons and the midbrain

Draw a diagram of the developed brain and the areas of:

• The forebrain

• The midbrain

• The hindbrain

Describe the lobes which make up the cerebral cortex. Include a labelled diagram

• Frontal lobe: at the front of the brain- involved in executive functioning, formulating plans and strategies

• Parietal lobe: involved in somatosensation, receiving sensory information from around the body and the environment

• Temporal lobe: incorporates the primary auditory cortex, which receives auditory information

• Occipital lobe: receives visual information

Why do mice and sheep have significantly larger olfactory bulbs and rhiencephalons?

Mice and sheep have significantly larger olfactory bulbs and rhinencephalon, because they are macrosmatic animals (highly developed sense of smell)

In sheep, what is the rhiencephalon associated with?

In sheep, the rhinencephalon (nose brain) is associated with olfaction and emotion

Draw a complete labelled diagram of the brain

Describe the function of the cerebral hemisphere

• The left brain is primarily responsible for logical, sequential and analytical tasks (language, logic, maths, detail)

• The right brain specialises in holistic, spatial and intuitive processing (creativity, emotion, spatial awareness)

• Both sides control the opposite half of the body, and communicate constantly via the corpus callosum

Describe the function of the basal ganglia

• Regulate voluntary movement by filtering out unwanted actions, and selecting the right ones: motor control, cognition, habit formation, motivation

Describe the function of the hippocampus

• Memory consolidation, spatial navigation, emotion and context- working closely with the limbic system

Describe the function of the corpus callosum

• Integrating and transferring sensory/motor/cognitive signals- allowing both sides of the brain to communicate and coordinate essential daily functions

Describe the function of the limbic system

• Made up of different structures within the brain

• Regulating emotion, behaviour, memory, and motivation

Describe the function of the thalamus

• Major relay station for sensory information

• Integration of motor information from the cerebellum and basal ganglia, before passing on to the motor regions of the cortex

Describe the function of the medulla oblongata

• Regulating heart rate, breathing, blood pressure and essential reflex actions

Describe the function of the pons

• Relaying sensory-motor signals, generating REM sleep, regulating breathing patterns and controlling facial movements

Describe the function of the cerebellum

• Integration and control of posture/movement

Describe the function of the cerebral cortex

• Planning of movement, perception and learning

What is meant by the term ‘rhythm‘?

A function which oscillates or cycles at a regular frequency 

• Biological rhythms are the overt, measurable activities generated by some internal oscillator (or clock)

What is meant by the term ‘chronobiology‘?

A field of science that examines periodic (cyclic) phenomena in living organisms, and their adaptation to external (environmental) rhythms

What is meant by the term ‘circadian‘?

A daily rhythmical change in behaviour or in a physiological proces

What is meant by the term ‘infradian‘?

A rhythm with a period longer than the period of a circadian rhythm 

What is meant by the term ‘ultradian‘?

A rhythm with a period shorter than the period of a circadian rhythm (e.g REM cycle in sleep)

What is meant by the term ‘zeitgeber‘?

Any external cue that entrains the internal timekeeping system of organisms

Describe the function of biological rhythms

Biological clocks are internal physical systems that enable organisms to live in harmony with the rhythms of nature (e.g cycles of day and night or the seasons). This is known as external synchronisation 

Circadian rhythms also maintain temporal organisation of endogenous processes. This is known as internal synchronisation 

The strongest zeitgeber (for both plants and animals) is light. Other zeitgebers include temperature, social interaction, pharmacological manipulation and eating/drinking patterns

Describe the biological clocks in:

• The retina

• The SCN

• The pineal gland

Retina

• A photo pigment (melanopsin) present in ganglion cells in the retina, whose axons transmit information to the SCN, the thalamus and the olivary pretectal nucleus 

Suprachiasmatic nucleus (SCN)

• A nucleus situated atop the optic chiasm

• It contains a biological clock, that is responsible for organising many of the body’s circadian rhythms

Pineal gland

• A gland attached to the dorsal tectum

• It produces melatonin in response to signals from the SCN, and plays a role in circadian and seasonal rhythms

What are the inputs/outputs of the SCN in regards to biological rhythms?

Input

• Light

Output

• Sleep/wakefulness

• Feeding and energy expenditure 

• Glucose homeostasis

• Hormonal regulation

• Body temperature regulation

• Blood pressure and respiratory changes

What are the inputs/outputs of the peripheral clocks in regards to biological rhythms?

Input

• Feeding/fasting

• Signals from SCN

Output

• Lipogenesis

• Insulin secretion

• Glucose clearance

• Food absorption

• Fat accumulation

What are the consequences if the biological rhythm is disrupted?

Sleep: poor sleep, depression, bipolar disorder, Alzheimers

Metabolism: obesity, type II diabetes, fatty liver disease

Microbiota: increased body weight 

Cardiovascular: atherosclerosis, cardiovascular risk 

Aging: life span, age-related markers

Cancer: enhanced cellular proliferation

Draw a labelled diagram of a synapse

Describe the stages of neurotransmission across a synapse

Step 1 (Release)

• When an AP reaches the end of the presynaptic terminal, it triggers the opening of calcium channels

• The influx of calcium causes tiny sacs (called synaptic vesicles) to fuse with the cell membrane

• The neurotransmitters inside are released into the synaptic cleft through exocytosis

Step 2 (Diffusion)

• The neurotransmitters float across the synaptic cleft

• They move down their concentration gradient

Step 3 (Binding)

• On the postsynaptic neuron, the neurotransmitters lock into specialised receptor proteins

• This binding opens ion channels in the receiving cell- allowing charged particles to flow in or out- which dictates whether the signal is passed on or halted

Step 4 (Activation)

• To prevent continuous stimulation, the neurotransmitters must be cleared from the cleft

• This is done via reuptake, enzymatic degradation or simple diffusion away from the synapse

Give the ionotropic and metabotropic receptor for:

• Acetylcholine

• Glutamate

• GABA

• Serotonin (5-HT)

Acetylcholine

• Ionotropic: nicotinic receptors

• Metabotropic: muscarinic receptors

Glutamate

• Ionotropic: AMPA, NMDA, Kainate

• Metabotropic: mGlu(1) to mGlu(8)

GABA

• Ionotropic: GABA(A)

• Metabotropic: GABA(B)

Serotonin (5-HT)

• Ionotropic: 5-HT(3)

• Metabotropic: 5-HT(1a) to 5-HT(7)

Describe the deactivation methods in a synapse

Reuptake

• The recycling process by which a neurotransmitter is actively pumped back into the presynaptic neuron (or neighbouring glial cells) to terminate the signal 

• Specialised transporter proteins bind to the neurotransmitter and drive it back across the cell membrane, using an electrochemical gradient

• Once inside the presynaptic cell, the neurotransmitter is either immediately repackaged into synaptic vesicles for future reuse, or metabolised by enzymes within the cell

Enzymatic deactivation

• Enzyme deactivation terminates neurotransmitter signals by destroying the messenger in the synaptic cleft, using specialised catalytic proteins

• An enzyme binds to the specific neurotransmitter, and breaks its molecular bonds (meaning it can no longer connect to postsynaptic receptors)

• e.g acetylcholine: acetylcholinesterase breaks acetylcholine into choline and acetate

Describe the difference between ionotropic and metabotropic receptors

Ionotropic receptors

• These receptors combine a receptor site and an ion channel into a single protein complex

• When a neurotransmitter binds, the receptor changes shape: opening a pore that allows specific ions to flow across the membrane

• These mediate fast synaptic transmission 

Metabotropic receptors

• These receptors do not contain an ion channel, and are instead coupled to intracellular G-proteins

• When a neurotransmitter binds, the receptor activates the G-protein, initiating a cascade of biochemical reactions 

• These mediate slow synaptic transmission

Describe the blood supply to the brain

Blood supply to the brain is supplied by 5 main arteries:

1. Rostral cerebral arteries- supply the medial aspect of the cerebral hemispheres

2. Middle cerebral arteries- supply the lateral and ventrolateral aspects of the cerebral hemispheres

3. Caudal cerebral arteries- supply the occipital lobes

4. Rostral cerebellar arteries- supply the rostral aspects of the cerebellum

5. Caudal cerebellar arteries- supply the caudal and lateral aspects of the cerebellum

Draw a labelled diagram of the Arterial circle and the arteries that branch off/to it

What is the rete mirabile?

The rete mirabile is a network of vessels located within the cavernous sinus. This is postulated to cool the blood, or reduce fluctuations from pulsatile flow.

Describe the blood supply to the spinal cord

Segmental arteries:

• Cervical- vertebral artery

• Thoracic- intercostal arteries

• Lumbar- aorta

Describe the venous drainage of the brain and spinal cord

The brain and spinal cord are drained by a communicating system of sinuses.

Segmental ventral spinal vein draining:

• Cervical- vertebral vein

• Thoracic- azygous vein

• Lumbar- azygous/caudal vena cava

What is the blood brain barrier?

This is a selective barrier between the circulating blood, and the parenchyma of the nervous system

Capillaries in the brain are surrounded by:

• Closed, overlapping endothelium

• Dense basement membrane

• Processes of astrocytes

• Selective barrier for exchange of substances

The blood brain barrier acts as a selective block to large molecules. Molecules can move across mainly by active transport, unless they are lipid soluble.

What is syringohydromyelia?

Syringohydromyelia is a condition characterised by a fluid-filled cyst (syrinx) within the spinal cord. It combines syringomyelia (a cavity in the spinal cord tissue) and hydromyelia (an expansion of the central canal). It disrupts normal cerebrospinal fluid (CSF) flow and requires medical monitoring or surgery.

What are the clinical uses of the meninges?

The meninges allow for epidural anaesthesia, collection of CSF and provides for the blood brain barrier

What are the denticulate ligaments and boundaries in relation to the meninges?

Denticulate ligaments are the focal firm attachment between the pia-arachnoid and the dura mater

Boundaries are continuous around the brain and spinal cord

Describe the layers of the meninges

Dura mater

• Intracranially adherent to the periosteum of the skull

• Spinal- separated from the periosteum by the epidural space, which is filled with fat

• This is a tough layer, composed mainly of dense connective tissue

Arachnoid mater

• This is a fine layer, pressed up against the dura mater with fine, wispy filaments which extend to and blend with the pia mater

Pia mater

• This is a thin layer, which is adherent to the underlying brain/spinal cord

• It blends with the fibres of the arachnoid

What is the function of the meninges?

The meninges function as protection, CSF containment, support and maintenance of the blood brain barrier

What is the function of the arachnoid villi in the meninges?

Arachnoid villi act as one-way valves to regulate pressure within the CSF. This drains into sagittal and other venous sinuses

Describe the formation and function of CSF

CSF is produced by the choroid plexus (a vascular proliferation of the ependymal layer) in lateral and third and fourth ventricles.

CSF is formed by dialysis of blood in the choroid plexus (ultrafiltration of plasma, and active transport). There is further production from ependymal linings and vessels within the pia mater.

The CSF functions as protection, support and nutrition.

Draw out the ventricular system of the brain

• From above

• From the side