Bio 321 Neuroanatomy

Dr. M. Yu

Nervous System Introduction

Bio 321 Neuroanatomy

Medical Significance

• The brain and nervous system control all other functions of the body.

• The extreme importance of the nervous system in medicine is based on the serious nature of the many disorders affecting its structures (more than 1000 disorders).

• Causes more hospitalization than any other diseases, including heart diseases and cancers.

• Neurological diseases affect 50 million Americans and costs us about $400 billions annually

Introduction• In this country alone, the numbers are

overwhelming: – 1. Cerebrovascular Disease - is the 3rd ranking cause of

death - vascular conditions of brain & spinal cord annually kill ~500,000

– 2. Epileptics seizures ~ 1,500,000 – 3. Movement disorders affect another one million

people – 4. There are ~ 2 million totally blind individuals; &

over 13 million with visual impairments – 5. There are ~ 17 million totally or partially deaf

persons

Introduction

– 6. Over 3 million people are afflicted with Alzheimer’s disease

– 7. At least 700,000 have cerebral palsy

– 8. More than 250,000 have multiple sclerosis

– 9. In addition, there are over 500,000 accidental head and spine injuries annually; fortunately only a minority of which actually injure the brain or spinal cord

– 10. Acute head injury is the leading cause of death or disability between ages 2 & 40 (as of 1995)

Cellular Components of the Nervous System

• Neurons - the primary functional cells in the nervous system (- approx. 100 Billion in CNS) – 1. responsible for initiating & conducting

electrical signals by which nervous system communicates

– 2. size & shape varies greatly between regions of the nervous system & with respective functions

– 3. mature neurons do NOT divide or replicate, do NOT regenerate following injury

Anatomic features (common to all neurons)

• 1. Soma - cell body – metabolic center of cell – typical cellular organelles: large clear nucleus,

nucleolus, mitochondria, ER, Golgi Apparatus, lysosomes

• 2. endoplasmic reticulum - a characteristic feature of neurons: - high concentration of RER (granules, form Nissl substance, stain basic)

Neurons

• 3. Cytoplasmic fibrils - another distinguishing feature of neurons, make up cytoskeleton – a. Neurofilaments: intermediate cytoskeletal filaments -

abundant, found throughout soma & along processes (10 nm in diameter) – a principle support system component

– b. Neurotubules (Microtubules): especially abundant in dendrites - found in cytoplasmic, axonal transport, movement of organelles throughout cell - (20-30 nm in diameter) = microtubules, oriented lengthwise

– c. Microfilaments: thinnest, associated with external membrane & dendritic spines - anchor membrane constituents, hold Receptors in place - (5 nm in diameter) = microfilaments in other cells Processes of neurons

Dendrites

• Dendrites - number, length, degree of ramification varies between neuronal types – a. usually many per neuron, typically branch

extensively – b. carry incoming signals toward soma – c. neuron signal receptors (the dendritic field of

a cell = its receptive field)

Axon

• Axon - one per neuron – a. conducting process - carries info away from

soma, toward other neurons or effectors – b. length varies greatly, can be very long -

sciatic axons > 1 meter (spine -> foot) – c. no ribosomes, no protein synthesis, depend

on axonal cytoplasmic transport from soma – d. collaterals = branches of axon, degree of

branching varies greatly

Axon

• Axon Hillock = Initial segment (of axon) – a. base of axon as it leaves soma - looks pale (no Nissl

staining)

– b. specialized segment of membrane for action potential initiation due to lower threshold than rest of cell

– c. also has an extremely high concentration of voltage-gated ion channels

– d. allows action potential conduction to travel in ONE direction

Terms• 1. Nucleus - a group of neuronal cell bodies within

the central nervous system, eg lateral geniculate nucleus

• 2. Ganglion (ganglia, pl) - group of cell bodies in peripheral nervous system

• 3. Nerve - collection of axons in the PNS (tract, fasciculus = a bundle of nerve fibers)

• 4. White matter - axons, myelinated• 5. Gray matter - concentrations of cell bodies and

unmyelinated dendrites

Classification of neurons • A. based on processes:

– 1. Unipolar neurons a.• a single primary process extends from soma, which can branch

into dendrites & axon • b. the principle neuronal cell type of invertebrates• c. Pseudounipolar neurons, e.g. dorsal root ganglion cells, appear

unipolar, in embryo are actually bipolar; axon & dendrite extensions fuse

– 2. Bipolar neurons - e.g. bipolar cells in retina • - axon & dendrite both extend from soma, from opposite ends

– 3. Multipolar neurons • a. multiple dendritic branches from all parts of soma, & one axon • b. many variations due to length, number of dendrites, length of

axon • c. e.g. pyramidal neuron, spinal motor neuron, purkinje neuron

Classification of neurons

• B. Based on functions– Sensory neurons: carry sensory information

from– Motor neurons: carry motor information to

muscle sand glands– Interneurons: in between sensory and motor– Projection neuron / relay neuron: has long

fibers and project information from on region to another

Classification of neurons

• C. Based on orientation– 1. Afferent - refers to the neuron or process

extending toward the cell in question – 2. Efferent - refers to the neuron or process

extending away from the cell in question

Neuroglial (Glia)

• supporting cells - Do not conduct action potential, body has 10-15 times more glia cells than neurons (about one trillion)

• Derived from Neuroectodermal and mesodermal origin

• Neuroectodermal origin: #1-4 – 1. Schwann cells

• - form myelin sheath which insulates an axon in peripheral nerves

• - cell winds around axon, inside its own layers, piling up layers of lipid/protein cell membranes

• - one Schwann cell associates with and myelinates a segment of only one axon

• - Schwann cell, myelin, axon are all surrounded by a basement membrane (covers whole unit)

• - help to buffer excess extracellular K+ (prevent rampant depolarization)

• - myelin sheath insulation greatly speeds conduction

• 2. Oligodendroglia – same function as Schwann cells, but for axons

of neurons in central nervous system– have small, round, dense nuclei – unlike Schwann cells, can myelinate a segment

of several axons – no basement membrane surrounds the axon like

Schwann cells do in PNS – these features affect ability of CNS cells to

regenerate following injury (vs. PNS)

• 3. Astrocytes – named from ‘star’ appearance of processes

radiating out from soma – oval nuclei (larger & much less dense than

oligodendroglia)

Function of Astrocytes– i. nutritive: form significant portion of brain blood

barrier (BBB), surrounding CNS vasculature • thought to be important communicators between neurons and

capillaries • transport nutrients, ions, molecules from capillary to neurons

– ii. healing: act as scavengers, remove neuronal debris, seal off area for healing

– iii. buffers excess extracellular K+ , preventing excess depolarization

– iv. remove chemical neurotransmitters from synaptic clefts - high affinity for specific neurotransmitters

– v. support: provide structure, stability, act similarly to connective tissues

– vi. in development: act as guides for outgrowth & migration in the developing NS

Types of astrocytes

• Distinguished by localization, not on any functional difference– i. fibrous astrocytes - name for those in, around

white matter (areas rich in axons) – ii. protoplasmic astrocytes - those in gray

matter, near synapses, dendrites

• 4. Ependymal cells – layer of ciliated columnar epithelial cells with

tight junctions which line cavities of the neural tube (cerebral ventricles, spinal cord central canal) • a. this layer forms a selective barrier between

nervous tissue & ventricular fluid• b. also forms choroid plexus - produces

cerebral spinal fluid by filtering its components from blood

Glial cells of mesodermal origin

• Microglia • - cells from mesoderm which migrate into

CNS • - normally in small numbers, proliferate as

needed • - become macrophages in response to injury

or damage, • - act as scavengers, have a phagocytic role

in removing debris, damaged cells

Synapse

• A. Chemical synapse– Presynaptic membrane, postsynaptic membrane, and

synaptic cleft– can also be classified as: axodendritic, axosomatic,

axoaxonal, and dendrodendritic– neurotransmitter (adrenaline, acetylcholine, dopamine,

serotonin, glutamic acid…)– neuro-modulators (excitable or inhibitive

neurotransmitters)

• B. Electrical synapse – neuron close together, rare in mammalian nervous

system

Meninges

• There is very little connective tissue in brain tissue, except in linings of vasculature – A. CNS = meninges: Connective tissue

membranes surrounding the NS • 1. pia mater - immediately next to the nervous

tissue, thin & delicate

• 2. arachnoid - middle layer

• 3. dura mater - outermost meninges, thicker & very tough

PNS:

• 1. endoneurium: thin collagenous layer, immediately surrounds a myelinated nerve fiber

• 2. perineurium: Connective tissue covering surrounding a fascicle of nerve fibers

• 3. epineurium: thick connective tissue layer surrounding many fascicles which make up a nerve trunk

• * the 3 CNS meningial layers are continuous with the connective tissue layers around PNS nerves

f asciclecovering around f ascicle is the perineurium

covering around all the f ascicles is the

epineurium

Axonal Transport

• Fast Axonal Transport - involves microtubules as a track -some organelles move along in stepwise fashion - especially synaptic vesicles, precursors, etc - requires oxidative metabolism, energy utilization

• 1. Anterograde: = orthograde, from cell body to terminal – a. mechanism by which synaptic vesicles move from

soma to terminal along microtubules – b. speed = 410 mm/day

• 2. Retrograde: from terminal to soma – a. mechanism for returning worn out materials to cell

body for degradation or recycling– b. returns to cell body information from, about the axon -

communicates with neighbors– c. nerve growth factor (NGF) travels from source to

soma, stimulates neuronal growth– d. toxin transport, e.g. neurotropic viruses get to

CNS: herpes simplex, rabies, polio– e. speed = approx 1/2 - 2/3 that of anterograde transport

Axonal Transport

• Slow Axoplasmic Flow – 1. predominant means of organelle movement

within the neuron – 2. movement is through cytoplasm of axon, not

along microtubules – 3. speed = approx. 10 mm/day

Main regions of the central nervous system

• Cerebrum– Frontal, parietal, occipital, temporal and insula– Interpretation– initiating voluntary movements– storing memory– retrieving memory– Reasoning– center for intelligence and personality

• Cerebellum– Large in human, receiving information from sensory

systems and the cerebral cortex, main function to maintain balance, posture, mostly not initiated from the cerebral cortex.

Brain stem• Medullla, pons and midbrain are generally

referred as brain stem• Medulla Oblongata

– Continuation from spinal cord into the brain, part of the brain stem.

• Pons– Dorsal part consists of sensory and motor tracts – Ventral part contains connection between two

hemispheres, contribute to motor efficiency

• Midbrain– Involved with visual and auditory system, red nucleus

and substantia nigra are also located here (more later)

• Diencephalon:– Forms the central core of the cerebrum, including

thalamus, hypothalamus, epithalamus, and subthalamus, controls automatic nervous system, endocrine function via hormones and nervous impulses.

• Telencephalon– Includes cerebral cortex, corpus striatum, and

medullary center. Areas of cerebral cortex (paleocortex) receive primitive function from olfactory system, which is common to lower vertebrates. Other areas are called archicortex, includes limbic system (emotions, and some memories, early vertebrates). Most areas of the cerebral cortex (90%) in human are referred as neocortex, which controls all sensations (except smell), involves emotions, memories and intellectual activities.