Central Nervous System    

●     CNS
The CNS is where messages are received (from the PNS) and processed and outgoing messages are initiated (to the PNS)
Consists of the brain and spinal cord

○     Functions of the CNS

■     High order functioning

■     Learning, reasoning and memory

■     Controlling voluntary actions/activities

■     Controls some involuntary actions

■     Perceives pain and pleasure

■     Processes and integrates sensory information

■     Maintains homeostasis

■     Controls autonomic nervous system

■     Releases releasing and inhibiting factors (hormones)

■     Monitors blood composition

■     Controls feeding, sleeping and aggression

■     Connects left and right hemispheres of the brain

■     Controls auditory and visual reflexes

■     Controls basal functions like heart rate, blood pressure and ventilation

■     Controls muscular movement and posture

■     Coordinated fine muscle contraction

○     Protection of CNS

■     As the structures of the CNS are very delicate and important for body function, they must be protected from damage.

■     There are three structures that provide this protection:

■     Bone - Cranium + Vertebrae

■     The bone is the outermost protection for the brain and spinal cord.

■     Bone is ideal for protecting these structures as it is hard and solid, making it difficult to penetrate.

■     The brain is protected by the cranium (not the skull as this contains the jaw bone)

■     The spinal cord is protected by the vertebrae. The spinal cord runs through the middle of the vertebrae (in the vertebral canal) that make up the spine.

■     Meninges (membranes)

■     Located inside the bone and covering the surface of the brain and spinal cord. Meninges are made up of a tough fibrous connective tissue.

■     There are three layers of the meninges:

■     Outer layer (Dura Mater): This layer is tough and fibrous. Is close fitting to the bones in the skull, but not in the vertebrae.

■     Middle layer (Arachnoid Mater) : This layer is made up a loose mesh fibers, much like a spider web in appearance. Acts as a cushion for the CNS.

■     Inner layer (Pia Mater) This layer is a very thin and delicate layer of fibrous tissue which adheres to the surface of the brain and spinal cord.

■     From inside – outside = PAD

■     Cerebrospinal fluid (CSF)

■     CSF is a clear watery fluid which contains a few cells, glucose, protein, urea and salts.

■     The CSF occupies space between the middle and inner layers of meninges as well as circulating through the cavities (ventricles) in the brain and the canal in the center of the spinal cord.

■     The CSF is a fluid which acts as a shock absorber for the CNS. This is the primary protective role of the fluid.

■     The fluid also has two other functions (support and transport):

■     Provides nutrients and removes wastes from the brain and spinal cord.

■     CSF is formed from blood, meaning after it has circulated through the CNS it returns to the blood capillaries.

●     Parts of the brain

○     Whilst the brain can be divided into parts with specific functions, the brain itself works as an integrated whole.

■     This means that damage to any part of the brain could effect the way the whole structure functions.

○     We will be taking a closer look at the following parts of the brain:

○     Cerebrum

■     This is the biggest part of the brain

■     Contains an outer surface of grey matter (approx. 2-4mm think) called the cerebral cortex and and inner section of grey matter called basal ganglia. These two layers are separated by a section of white matter.

■     The cerebrum contains 70% of all neurons found in the brain.

■     To increase surface area the cerebrum is folded. These folds produce ridges called convolutions (gyrus).

■     The grooves between the folds can either be shallow or deep.

■     Shallow grooves are called sulci

■     Deep grooves are called fissures

■     Cerebral Hemispheres

■     The deepest ****fissure is called the longitudinal fissure. This divides the cerebrum into two hemispheres (left and right).

■     The two hemispheres are joined at the base of the longitudinal fissure by the corpus callosumwhich is an area of white matter that contains a large bundle of transverse fibres.

■     The corpus callosum allows for communication between the two hemispheres.

■     Cerebral Lobes

■     The two hemispheres of the cerebrum can be further divided into four lobes.

■     There is also a fifth lobe called the insula found deep inside the brain, responsible for recognition of different senses & emotions, addiction and psychiatric disorders.

■     Each lobe is thought to be responsible for certain functions of the brain.

■     Cerebral Cortex

■     Scientists have used electroencephalograms (EEG’s) to study the function of the cerebral cortex.

■     The cerebral cortex has been divided into three main functional areas which allow for thinking, reasoning, learning, memory, intelligence and coordinating the body’s voluntary activities as a response to senses.

■     These three function areas are:

■     Sensory areas: Receive and process nerve impulses from the afferent pathway (from the receptors to brain)

■     Motor areas: Send impulses to the effectors (muscles) via the efferent pathways (brain to muscles)

■     Association areas: Interpret information from the senses to make it useful.

■     White Matter in the Cerebrum

■     The white matter of the cerebrum found between the cerebral cortex and the basal ganglia is composed of bundles of nerve fibres. In the CNS we call these bundles of nerve fibres, tracts.

■     These tracts in white matter are myelinated (similar to the nerve fibres you would find in the PNS)

■     The tracts in the grey matter are unmyelinated.

■     There are three types of tracts found in white matter:

■     Tracts connecting various areas of the cortex within the same hemisphere

■     Tracts carrying impulses between the hemispheres

■     Tracts that connect the cortex to other parts of the brain or spinal cord

• Other Parts of the Brain

○     Spinal Cord

■     Spinal Cord Structure

■     A dorsal cylinder of nervous tissue running from the bottom of the brain, passing through the foramen magnum (opening at the base of the skull) and travelling down the length of the body to the second lumbar vertebrae.

■     The spinal cord is protected, much like the brain by bone, meninges and CFS.

■     The outer meningeal layer is not connected the vertebrae (like the brain is connected to the skull), instead there is a space containing fat, connective tissue and blood vessels. This provides flexibility and movement with the spine.

■     A cross section of the spinal cord shows two sections:

■     Grey matter (nerve cell bodies and unmyelinated nerve fibres)

■     White matter (myelinated nerve fibres)

■     Unlike the cerebrum and cerebellum, the grey matter in the spinal cord is surrounded by the white matter.

■     Grey Matter of Spinal Cord

■     The grey matter in the middle of the spinal cord appear in the shape of the letter H.

■     In the cross bar of the H is a small space called the central canal which contains the CSF.

■     The myelinated fibres of the white matter are arranged in bundles. These are known as ascending and descending tracts.

■     Ascending tracts are sensory (afferent) neurons which carry impulses up the spinal cord, towards the brain.

■     Descending tracts are motor (efferent) neurons which carry impulses down the spinal cord away from the brain.

■     The dorsal (upper) root of the spinal cord carries sensory neurons. It also has a dorsal root ganglia which is a swelling of the root containing cell bodies of sensory neurons.

■     The ventral (lower) root of the spinal cord carries motor neurons.