14. Nervous System II Flashcards
Central Nervous System
The central nervous system (CNS)
consists of the brain and spinal cord.
Spinal Cord
Protected by the vertebral column & meninges.
Brain
Protected by the cranium, cranial meninges, cerebrospinal fluid (CSF) & the
blood brain barrier (BBB).
Meninges
Three layers of connective tissue that cover the brain & spinal cord:
Dura Mater
Arachnoid Mater
Pia Mater
Dura Mater
The tough outer layer divides into 2 layers in the brain to create venous sinuses that allow blood to drain out of the brain into the jugular veins.
Arachnid Mater
- Collagen and elastic fibres.
* Subdural space between arachnoid mater and dura mater filled with interstitial fluid.
Pia Mater
- Thin transparent layer that contains blood vessels that supply the spinal cord.
- Between the pia mater and arachnoid mater is the sub arachnoid space filled with cerebrospinal fluid.
Haemorrhages
Subdural haematoma
Subarachnoid haemorrhage
Subdural haematoma
Venous bleed (slowly develops).
Subarachnoid haemorrhage
Arterial bleed (rapid).
Cerebrospinal Fluid (CSF)
• CSF is a clear liquid that protects the brain & spinal cord from mechanical and
immunological trauma.
•It is made from filtered blood & is mostly water with ions & glucose. Blood cells are too large to filter through, making CSF colourless.
• CSF occupies the subarachnoid space &
the ventricles (cavities) in the brain, as well as the central canal of the spinal cord.
• Produced by ependymal cells continuously at a rate of 500 ml/day . However, the brain can only contain about 150 ml, so large amounts are gradually absorbed back into the blood.
Cerebrospinal Fluid (CSF): Functions
- Supports & protects the brain and spinal cord, acting as a shock absorber.
- Maintains uniform pressure around the brain in case of brain enlargement compensation occurs by reduction of CSF.
- Keeps brain & spinal cord moist.
- Circulates nutrients and waste.
- Optimal chemical environment for nerve signalling i.e. ions.
Blood Brain Barrier (BBB)
• Blood flows to the brain from the vertebral and carotid arteries.
• Brain uses 20% of the body’s oxygen and glucose at rest neurons die within minutes of being deprived of oxygen.
• The BBB protects the brain from toxins,
harmful substances and bacteria that enter the bloodstream by maintaining tightness of the capillaries and the junctions between cells.
• The BBB is maintained by ‘astrocytes’.
Transport across the BBB
- Lipid soluble substances cross easily e.g. alcohol, anaesthetics.
- Glucose crosses by active transport.
- Gases diffuse easily e.g. oxygen, carbon dioxide.
- Ions diffuse slowly across.
- Proteins and some drugs do not pass.
The Brain
- Brainstem:
• Divided into the medulla oblongata, pons & midbrain. - Diencephalon (‘ interbrain’):
• Divided into thalamus, hypothalamus & epithalamus - Cerebrum.
- Cerebellum (‘little brain’).
Brainstem
• The brainstem is a continuation of the spinal cord in the cranium (beyond the ‘foramen magnum’).
Medulla oblongata
Pons
Midbrain
Diencephalon
Thalamus
Hypothalamus
Epithalamus
Medulla oblongata
- Contains the respiratory and cardiac centres (regulates heartbeat & breathing). • Controls reflexes such as vomiting, sneezing & hiccupping.
- Contains four cranial nerves ( 9-12)
- Contains motor tracts (corticospinal tract).
Pons
- Helps to control breathing.
- Four pairs of cranial nerves (5-8)
- Relays motor tracts.
Midbrain
• Contains the ‘substantia nigra’ (large, darkly pigmented area that contains dopamine releasing neurons that help control subconscious muscle activities • Eye reflexes and tracking. • Two cranial nerves (3 & 4). • Auditory tracts. • Movement of head and neck. • Transfers motor tracts.
Thalamus
• The core of the brain, making up 80% of the diencephalon. • ‘Major relay centre’ relays sensory information to the cerebrum. • Integrates sensory and motor information.
Epithalamus
• Connects limbic system to other
parts of the brain.
• Contains the pineal gland which
secretes melatonin.
Hypothalamus
• Hormone regulation and homeostasis. • Receives sensory information (i.e. from viscera, smell). • Controls the ANS (a major regulator of visceral activities). • Controls body temperature. • Regulates emotional and behavioural patterns (limbic), eating & drinking (appetite/satiety) & regulation of circadian
Cerebellum
• Located in the inferior and posterior
aspect of the cranium.
• Of the 100 billion neurons in the brain, half of them are packed into the cerebellum, despite it only accounting for 1/10 th of brain mass.
Cerebellum: Fucntions
• Maintenance of posture and balance.
• Functions to co ordinate and smooth complex (‘skilled’) sequences of movements.
• Correction of errors during on going movement.
Discrepancies are detected by cerebellum and sends feedback to cerebrum via the thalamus.
• E .g. learning to walk, learning an instrument.
• Also thought to play a role in cognition, thought and language processing.
• It alters motor behaviour by altering circuits that influence motor pathways (corticospinal tracts).
• The cerebellum is susceptible to injury from chronic alcohol abuse . Cerebellar damage may also result from coeliac disease or a thiamine (vitamin B1)
• Cerebellar disease produces symptoms of ‘ataxia’: lack of muscle co ordination. Commonly results in stumbling, unsteadiness, slurred speech
and an intention tremor (e.g. hand tremor when putting key in door).
The Cerebrum
- The cerebrum is divided into two hemispheres (left & right
- The cerebrum contains an outer cerebral cortex that contains grey matter . It has multiple folding’s (fissures).
- The cerebrum has a deeper region of cerebral white matter
- The two cerebral hemispheres are connected by the ‘corpus callosum’, allowing coordination of movements between sides (e.g. walking)
- The brain does not decide what a person does. The individual uses the brain like a computer!
The Cerebrum: Functions
- Sensory perception.
* Motor control of skeletal muscles (movement).
Cerebrak Hemispheres
• Motor neuron axons join to form the corticospinal tract. These motor tracts cross over in the medulla oblongata (decussating).
• This means the left side of the brain co-ordinates the right side of the body
• The right side of the brain co-ordinates
the left side of the body.
The Cerebrum Structure
• Each cerebral hemisphere can be further
subdivided into four lobes, which are named after the bones that cover them:
• Frontal, parietal, temporal, occipital.
• Each of these lobes performs different
functions:
Frontal Lobe
The frontal lobe is involved in motor
skills.
Temporal Lobe
The temporal lobe is involved in hearing.
Parietal Lobe
The parietal lobe contains the sensory cortex.
Occipital Lobe
The occipital lobe plays a role in vision.
The Limbic System
• The limbic system is area of the brain that is active with different emotions but again, remember this does not control your emotions (you do).
• The limbic system is formed of several functionally and anatomically interconnected structures including parts
of the cerebrum, diencephalon and midbrain.
• Key structures within the limbic system include the hypothalamus, hippocampus & amygdala.
• The limbic system becomes active when dealing with emotional responses to our environments.
• The amygdala is a key area in ‘emotions’ whilst the hippocampus plays an essential role in encoding and retrieving memories.
Brain Blood Supply
• The brain has a very limited ability to store oxygen or glucose, therefore a constant supply is necessary.
• Brain receives about 20% of cardiac
output (about 750ml per minute), despite accounting for 2% of body mass.
• Cardiovascular health is important for
brain health.
• The blood supply to the brain is from two
major arterial systems: carotid & vertebral.
• The two arterial systems meet to form the
‘Circle of Willis’. This is a common location
for aneurysms to develop: “berry aneurysms”.
Cranial Nerve I
Ofactory - sense of smell
Cranial Nerve II
Optic - vision
Cranial Nerve III
Oculomotor - eye movements, eyelid opening, pupil size
Cranial Nerve IV
Trochlear - eye movements
Cranial Nerve V
Trigeminal - facial sensations, chewing
• CN V has 3 branches: Ophthalmic, maxillary and mandibular.
Cranial Nerve VI
Abducens - Eye Movement (abducts eye)
• Damage to CN III, IV, VI can lead to strabismus (one eye crossed), ptosis (drooping of upper eyelid) & diplopia (double
Cranial Nerve VII
Facial - Taste, facial expressions, salivary glands
Cranial Nerve VIII
Vestibulocochlear - Hearing, balance
Cranial Nerve IX
Glossopharyngeal - taste, swallowing, tongue sensations
Cranial Nerve X
Vagus - Sensory and motor functions in chest and abdomen
• ALL sensory information from the chest and abdomen (Heart, GI tract, lungs) e.g.
Sensation from epiglottis, detects gas levels in arteries.
• The largest nerve of the autonomic NS
mostly parasympathetic but some sympathetic (Increased BP).
• Both sensory & motor function but 90% of
fibres are afferent (sensory).
• Injuries may result in tachycardia, dysphagia. Vaso vagal syncope (fainting).
Cranial Nerve XI
Accessory - Neck and shoulder movement
Cranial Nerve XII
Hypoglossal - Tongue mvoement and speech
Spinal Cord
• The spinal cord begins at the foramen magnum and terminates at the vertebral
level L2 as the ‘conus’.
• The meninges continue beyond L2 to the level of S2 (sacrum).
• The ‘cauda equina’ (“horses tail”) consists of spinal nerves that continue beyond the conus.
Spinal Cord Physiology
• The spinal cord consists of white
matter around a core of grey matter.
White Matter
• Consists of bundles of myelinated axons. Made up of ‘ tracts’ (“highways): 1. Motor (descending) tracts : Voluntary & involuntary movement (coordination, posture, balance). 2. Sensory (ascending) tracts Transmit impulses from skin, tendons, muscles & joints (e.g. touch, vibration, proprioception).
Grey Matter
• Consists of cell bodies & dendrites. Receives and integrates incoming and outgoing information. Divided into: 1. Posterior (Dorsal) Horn: Receives sensory impulses. 2. Anterior (Ventral) Horn: Sends out motor impulses.
Spinal Cord Tracts
- Dorsal columns
- Spinothalmic tract
- Corticospinal Tract
• Damage to any of the spinal tracts would
lead to loss of the tracts function below
that level in the body.
Dorsal Columns
Ascending Tract
Light Touch, vibration, proprioception
Spinothalmic tract
Ascending Tract
Pain and temperature
Corticospinal tract
Descending tract
Voluntary movements
Reflex Arcs
• A reflex is a fast, automatic, unplanned sequence of actions that occurs in response to a stimulus. • If the integration takes place in the spine it is known as a ‘ spinal reflex’. 1. Sensory receptor 2. Sensory neuron 3. Integrating centre (interneuron) 4. Motor neuron 5. Effector organ
Spinal Nerves
• The spinal cord contains extensions called ‘spinal nerves’, which exit the vertebral column through holes in the vertebra called vertebral foramina (holes). • Each spinal nerve corresponds with a vertebral foramina.
