Lecture 3: Brain Anatomy

Question 1

Divisions of the nervous system

Answer 1

Nervouse system –> central nervous system and peripheral nervous system

CNS - brain and spinal cord

PNS - somatic nervous system and autonomic nervous system

Somatic nervous system - afferent and efferent nerves

Autonomic nervous system - afferent and efferent nerves (within the efferent nerves are the sympathetic and parasympathetic nervous systems)

Question 2

CNS vs PNS

Answer 2

• CNS located within skull and spine

- PNS located outside skull and spine

Question 3

Somatic Nervous System (SNS)

Answer 3

Part of PNS that interacts with external environment

Composed of…
•Afferent nerves - carry sensory signals from skin, skeletal muscles, joints, eyes, ears etc. to CNS
•Efferent nerves - carry motor signals from CAN to skeletal muscles

Question 4

Autonomous Nervous System (ANS)

Answer 4

ANS regulates body’s internal environment

Composed of…
•Afferent nerves that carry sensory signals from internal organs to CNS
(Afferent, Advance, Approach, Arrive)
•Efferent nerves that carry motor signals from CNS to internal organs
(Efferent, Exit, Escape, Embark)

• ANS has 2 types of Efferent nerves (sympathetic and parasympathetic)
• Sympathetic project from lumbar and thoracic regions of spinal cord
• Parasympathetic project from brain and sacral region of spinal cord
• Sympathetic and Parasympathetic nerves are 2 stage neural paths, project from CNS, go part way to target organs before synapsing on other neurons that carry signals rest of the way
• Sympathetic neurons that project from CNS synapse on second-stage neurons at a are at a substantial distance from target organs whereas parasympathetic neurons that project from CNS synapse are near their target organs on very short second-stage neurons

Question 5

3 Functions of Sympathetic and Parasympathetic Neurons:

Answer 5

a) Sympathetic nerves stimulate, organise and mobilise energy resources in threatening situations whereas parasympathetic nerves act to conserve energy
b) Each autonomic target organ receives opposing sympathetic and parasympathetic input and its activity is controlled by relative levels of sympathetic and parasympathetic activity
c) Sympathetic changes are indicative of psychological arousal whereas parasympathetic changes are indicative of psychological relaxation

Question 6

Cranial Nerves:

Answer 6

• Project from brain
• Numbered in sequence from front to back
• Include purely sensory nerves e.g. olfactory nerves (I) and optic nerves (II) but most contain both sensory and motor fibres
• Longest are vagus nerves (X) which contain motor and sensory fibres travelling to gut
• Autonomic motor fibres of cranial nerves are parasympathetic
• Functions are assessed by neurologists for as basis of diagnosis
  > Functions and locations are specific, disruption can give clues about location and extent of tumours and brain pathology

Question 7

The meninges of the brain in detail

Answer 7

Brain and spinal cord most protected organs in body, encased in bone and covered in 3 protective membranes (three meninges)

• **MENINGES:
  1. Outer meninx, tough membrane called dura mater
  2. Arachnoid membrane, immediately inside dura matter, fine weblike membrane
• Subarachnoid space, beneath arachnoid membrane, contains many large blood vessels and cerebrospinal fluid
  3. Pia mater, innermost meninx, delicate, adheres to surface of CNS

Question 8

What are choroid plexuses?

What do the choroid plexuses do?

Answer 8

Networks of capillaries that protrude into ventricles from pia mater in the brain, fills subarachnoid space – central canal of spinal cord and cerebral ventricles of brain

They continually produce cerebrospinal fluid

Question 9

What are the meninges?

Answer 9

The meninges refer to the membranous coverings of the brain and spinal cord. There are three layers of meninges, known as the dura mater, arachnoid mater and pia mater.

Question 10

What are the ventricles of the brain?

Answer 10

The ventricles of the brain are a communicating network of cavities filled with cerebrospinal fluid (CSF)

Question 11

Cerebrospinal fluid in detail

Answer 11

***CEREBROSPINAL FLUID:
Cerebrospinal fluid protects CNS, continuously produced by choroid plexuses – networks of capillaries that protrude into ventricles from pia mater, fills subarachnoid space – central canal of spinal cord and cerebral ventricles of brain

• Patients who have had some fluid drained suffer raging headaches, stabbing pain each time they jerk heads
• Excess fluid continuously absorbed from subarachnoid space into large blood filled spaces (dural sinuses) which run through dura mater and drain into large jugular veins of neck

Question 12

The brain ventricles in detail

Answer 12

***VENTRICLES

• Central canal is small central channel that runs length of spinal cord
• Cerebral ventricles are 4 of large internal chambers (2 lateral ventricles, third ventricles and fourth ventricles)
• Occasionally flow of fluid blocked by tumour near narrow channels that link ventricles e.g. near cerebral aqueduct connecting 3rd and 4th ventricles causing brain to expand –> hydrocephalus (water head), treated by draining excess from ventricles
• Subarachnoid space, central canal and cerebral ventricles are interconnected by a series of openings and form a single reservoir

Question 13

Blood-Brain Barrier:

Answer 13

Function of brain can be severely disturbed by introduction of certain kinds of chemicals

• Blood-brain barrier is mechanism that impedes passage of toxic substances from blood to brain
• The barrier is a consequence of special structure of cerebral blood vessels, walls are tightly packed, form barrier – particularly from proteins and other large molecules, unlike blood vessels in rest of body which are loosely packed allowing passage of molecules
• Degree to which therapeutic or recreational drugs can influence brain activity depends on ease with which they penetrate blood-brain barrier
• Does not impede passage of all large molecules, some areas allow e.g. glucose to pass – required for brain function

Question 14

Cells of Nervous System:

Neurons

Answer 14

Cells of nervous system fundamentally neurons and glial cells (glial cells surround neurons and provide support for and insulation between them)

Neurons: Cells specialised for reception, conduction and transmission of electrochemical signals

Question 15

Neuron cell membrane

Answer 15

• Lipid bilayer/2 layers of fat molecules
• Protein molecules are embedded in lipid bilayer, used for cells functional properties
• Channel proteins mean certain molecules can pass
• Signal proteins allow signals to be transferred to inside neuron when particular molecules bind to them on outside of neuron

Question 16

Neurons and Neuroanatomical Structure:

Answer 16

• CNS: Clusters of cell bodies are nuclei (singular nucleus), bundles of axons called tracts
• PNS: Clusters of cell bodies are ganglion (singular ganglion), bindles of axons called nerves

–> Nucleus has 2 different anatomical meanings, structure in neuron cell body and cluster of cell bodies in CNS

Question 17

Classes of Neurons:

Answer 17

Classification of neurons based on number of processes (projections) emerging from cell bodies

• Multipolar Neuron: Neuron with more than 2 processes extending from cell body
• Unipolar Neuron: Neuron with 1 process extending from cell body
• Bipolar Neuron: Neuron with 2 processes extending from its cell body
• Interneurons: Neurons with short axon or no axon at all, function is to integrate neural activity within a single brain structure, not to conduct signals from one structure to another

Question 18

Glial cells

4 types

Answer 18

The glial cells surround neurons and provide support for and insulation between them… there are lots of types of glial cells

1. Oligodendrocytes
2. Schwann cells
3. Microglia
4. Astrocytes

Question 19

Glial cells

1. Oligodendrocytes

Dendro means tree

Answer 19

Oligodendrocytes are glial extensions that wrap around the axons of some neurons of CNS, these extensions are rich in myelin, myelin sheaths they form increase speed of efficiency of axonal conductance

Question 20

Glial cells

2. Schwann cells

Answer 20

Schwann cells are a second class of glial cells; each constitutes one myelin segment whereas each oligodendrocyte provides several myelin segments often on more than one axon

Only Schwann cells can guide axonal regeneration (regrowth) after damage, that is why effective axonal regeneration in mammalian nervous system is restricted to PNS (Schwann PNS, Oligodendrocytes CNS)

Question 21

Glial cells

3. Microglia

Answer 21

Microglia are third class of glial cells, smaller than other glia, respond to injury or disease by multiplying, engulfing cellular debris and triggering inflammatory responses

Question 22

Glial cells

4. Astrocytes

Answer 22

Astrocytes - fourth glass of glial cells, largest glial cells, star shaped, extensions of some cover outer surfaces of blood vessels through brain, make contact with neuron cell bodies, role in allowing passage of some chemicals from blood into neurons and in blocking other chemicals

shown to send and receive signals from neurons and other glial cells –> control the establishment and maintenance of synapses between neurons to modulate neural activity and participate in glial circuits

Question 23

Neuroanatomical techniques and directions

Neuroanatomical techniques, why do we use them?

Answer 23

Problem with visualising neurons is that they are so tightly packed, axons and dendrites are so intricately intertwined that looking through microscope at unprepared neural tissue reveals almost nothing

• Key is to prepare neural tissue in variety of ways in order to have clear view of different neuronal structures

Question 24

Neuroanatomical techniques and directions

Golgi stain

Answer 24

Exposed meninges to potassium dichromate and silver nitrate (silver chromate), chemical reaction invaded few neurons, each went entirely black, made it possible to see individual neurons by silhouette

• Commonly used when overall shape of neurons is of interest
• Provides no indication of number of neurons in area or nature of inner structure

Question 25

Neuroanatomical techniques and directions

Nissle stain

Answer 25

• Indicates number of neurons in area and nature of inner structure
• Most common dye is cresyl violet, penetrates all cells on slide, bind effectively only to structures in neuron cell bodies, used to estimate number of cell bodies in an area by counting number of Nissl stained dots, only layers composed mainly of neuron cell bodies are densely stained

Question 26

Neuroanatomical techniques and directions

Electron microscopy

Answer 26

• Provides info about the details of neuronal structure
• Magnification limit is 1500, insufficient to reveal fine anatomical details of neurons
• Greater detail obtained by first coating thin slices of neural tissue with electron absorbing substance taken up by different parts of neurons to different degrees them passing beam of electrons through tissue onto photographic film –> electron micrograph
• Images so detailed difficult to visualise general aspects of neuroanatomical structure
• Scanning electron microscope produces 3D electron micrographs but is not capable of as much magnification

Question 27

Neuroanatomical techniques and directions

Neuroanatomical tracing techniques

(2 types)

Answer 27

1. Anterograde (forward) tracing methods
   - used when investigator wants to trace paths of axons projecting away from cell bodies located in particular area, investigator injects into the area one of several chemicals commonly used for anterograde tracing, chemicals taken up by cell bodies, transported
2. Retrograde (backward) tracing methods
   - used when investigator wants to trace paths of axons projecting into a particular area, investigator injects into the area one of several chemicals commonly used for retrograde tracing – chemicals that are taken up by terminal buttons and then transported backward along their axons to their cell bodies, after few days’ brain removed, sliced, slices treated to reveal locations of injected chemicals

Question 28

Neuroanatomical techniques and directions

Directions in Vertebrate Nervous System: Cat

Answer 28

• 3D system of directional co-ordinates used by neuroanatomists
• Directions in vertebrate nervous system are described in relation to the orientation of spinal cord

> Has 3-axes:
- Anterior (rostral) – Posterior (caudal)
(anterior means towards nose end, posterior is towards tail end)

• Dorsal – Ventral
  (dorsal means toward surface of back or top of head – dorsal surface, ventral means towards surface of chest or bottom of head – ventral surface)
• Medial – Lateral
  (medial means toward the midline of the body, lateral means away from the midline toward the body’s lateral surface)

Question 29

Neuroanatomical techniques and directions

Directions in Vertebrate Nervous System: Humans

Answer 29

Humans complicate simple three-axis system, we walk on hind legs - changes orientation of cerebral hemispheres in relation to spines and brain stems, system of vertebrate neuroanatomical directions however adapted for use in humans, body surfaces are the same in humans as they are in more typical non-upright vertebrates, top of human head and back of human body are both referred to as dorsal even though they are in different directions

Superior and inferior are used to refer to top and bottom of primate head

Proximal and distal used, proximal means close and distal far, used with regard to peripheral nervous system, proximal means closer to CNS, distal means farther from CNS

Question 30

Neuroanatomical techniques and directions

3 Different Brain Planes

Answer 30

Horizontal Sections, Frontal Sections (coronal sections) and Sagittal Sections

• Horizontal like a flat piece of paper, frontal like a window
• Section cut down centre of brain between two hemispheres is called a midsagittal section
• Section cut at a right angle to any long narrow structure e.g. spinal cord or a nerve is a cross section

Question 31

Spinal Cord

Answer 31

In cross section, spinal cord comprises 2 different areas –> Inner H shaped core of gray matter and a surrounding area of white matter

• Gray matter is composed largely of cell bodies and unmyelinated interneurons
• White matter is composed of largely myelinated axons, it is the myelin which gives white matter its glossy white sheen
• 2 Dorsal arms of spinal gray matter are dorsal horns
• 2 Ventral arms are called ventral horns
• Pairs of spinal nerves are attached to spinal cord, one on left and one on right at 31 different levels of spine
• Each of these 62 spinal nerves divides as it nears the cord and its axons are joined to the cord via one of 2 roots, dorsal root or ventral root
• All dorsal root axons whether somatic or autonomic are sensory (afferent) unipolar neurons with their cell bodies grouped together just outside the cord to form the dorsal root ganglia
• Many of their synaptic terminals are in the dorsal horns of the spinal gray matter
• Neurons of ventral root are motor (efferent) multipolar neurons with their cell bodies in the ventral horns
• Those that are part of somatic nervous system project to skeletal muscles
• Those that are part of the autonomic nervous system project to ganglia where they synapse on neurons that in turn project to internal organs (heart, stomach, liver etc.)

Question 32

The five major divisions of the brain

overview

Answer 32

Early Development:

• In Vertebrate embryo, tissue that eventually develops into CNS is recognizable as a fluid filled tube (neural tube)
• First indications of developing brain are 3 swellings that occur at anterior end of this tube
• 3 swellings eventually develop into adult forebrain, midbrain and hindbrain
• Before birth, initial three swellings in neural tubes become 5, occurs because forebrain and hindbrain swellings each grow into 2 different swellings –> Order of 5 swellings that compose developing brain at birth (below)
• -> Encephalon means “in the head”
• -> Telencephalon is on top and other 4 divisions are below it in alphabetical order
• -> Directional coordinates are the same for the brain stem as for the spinal cord but they are rotated 90 degrees for the forebrain

1. telencephalon
2. diencephalon
3. mesencephalon
4. metencephalon
5. myelencephalon (medulla)

Question 33

The five major divisions of the brain

Telencephalon

Answer 33

• **Overview:
• Forebrain
• Left and right cerebral hemispheres
• Undergoes the greatest growth during development
• Largest division of human brain
• Mediates brain’s most complex functions
• Initiates voluntary movement, interprets sensory input, mediates complex cognitive processes such as learning, speaking and problem solving

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