Central Nervous System - Brain & Cranial Nerves

Central Nervous System

- Brain & Cranial Nerves

Chapter 14Part A

Central Nervous System

Central nervous system (CNS) is responsible for:Receiving impulses from receptorsIntegrating informationSending impulses to the effectors

Central Nervous System

Central nervous system (CNS) is responsible for:Receiving impulses from receptorsIntegrating informationSending impulses to the effectors

It is composed of:BrainSpinal cord

Brain

Brain is one of the largest organs of the body.Located in the cranial cavity….formed by 8 cranial bones.Connected to spinal cord at foramen magnum.

Foramen magnum

Cranium

Brain

Spinal cord

Brain

Brain rapidly grows in fetus and then during early childhood years.Growth of the brain:

Neurons stop dividing in an infant.Brain continues to grow due to increase in:

Size of the neurons.Number of glial cells.Myelination.Branching of dendrites and axons.Number and size of synapses.

Brain - Functions

Brain has the following functions:1. Receives sensations: receives impulses from all over

the body to make it aware of the environment.2. Integrates information: makes decisions on major

functions of the body.3. Control Center: for temperature, movement, balance,

cardiac/respiratory systems, hormones, hunger, thirst, emotions, intelligence, memory, dreaming, learning.

Brain - Protection

Brain is protected by bones and 3 connective tissue membranes called meninges.From outside inside:1. Boney protection: cranial bones form cranial cavity houses brain.2. Dura mater: outermost tougher meninx splits into 2 layers:

Outer dura mater – periosteal layer: stick to the periosteum of the cranial bones.Inner dura mater – meningeal layer: wraps around the brain and its crevices.

3. Arachnoid mater: middle avascular meninx.4. Pia mater: innermost meninx that sticks to the brain.

Brain - Protection

There are spaces between the protective layers:-1) Outer periosteal layer is fused to the periosteum of cranial bone- no epidural space.

2) Dural venous sinuses- located in the space between periosteal and meningeal layer-contains veins that carry deoxygenated blood from brain to heart (via internal jugular vein).

3) Subdural space- potential space between dura mater & arachnoid (space appears when there is trauma or underlying pathological conditions).

4) Subarachnoid space- space between arachnoid and pia mater-contains CSF and network of blood vessels.

Brain - Ventricles

Brain is not a solid structure…..it has fluid-filled cavities inside.Ventricles: cavities in the brain…filled with cerebrospinal fluid (CSF).

Continuous with the central canal of the spinal cord.Lined with ciliated ependymal cells help form and circulate CSF in the brain and spinal cord.

Two lateral ventricles through interventricular foramina into third ventriclethrough cerebral aqueduct into the fourth ventricle through median apertureinto central canal of the spinal cord.Fourth ventricle through 2 lateral apertures into subarachnoid space around brain and spinal cord.

Lateral ventriclesInterventricular

foramen

Third ventricle

Cerebral aqueduct

Fourth ventricle

Central canal

Cerebrospinal Fluid - CSFCranium

Brain

Pia mater Subarachnoidspace

Dura mater (Outer)

Dural sinus

Dura mater (Inner)

Subdural spaceArachnoid mater

CSF reabsorption

Cerebrospinal fluid (CSF):It is filtered blood.It circulates in:

Central canal of the spinal cord, Ventricles of the brain, Subarachnoid spaces around the brain and spinal cord.From subarachnoid space-via arachnoid villi-CSF drains into dural sinus and gets

reabsorbed into blood. Any interruption in the normal circulation/reabsorption of CSF-over accumulation of CSF- e.g.

hydrocephalus in infants (abnormal widening of ventricles-enormously expanded skulls).

Outer, InnerDura Mater

Arachnoidvilli

Cerebrospinal Fluid - Functions

Functions of CSF:Mechanical – cushions, floats and protects the brain inside the cranial cavity.Chemical – maintains ion concentration and pH.Circulates to provide nutrient and pick up waste products.

Formation of CSF:Formed by choroid plexus.Choroid plexus:

Tissue present in the wall of the ventricles.Composed of blood capillaries covered by ependymal glial cells.Forms the Blood-CSF Barrier…..filters blood forms CSF protects brain from chemicals/infections blood may carry.

Cerebrospinal Fluid - Functions

Blood-CSF Barrier:Choroid plexus – a combination of blood capillaries and ependymal cells.Screens blood:

Allows rapid passage of glucose, O2, CO2, water, lipid soluble substances (alcohol, caffeine, nicotine, heroin, anesthetics).Slow passage of urea and ions.Do not allow proteins and most antibiotics to pass.

Cerebrospinal Fluid - FunctionsChoroid plexus of lateral ventricle