Comparative Anatomy and Physiology

VT 105

Comparative Anatomy and Physiology

Nervous System

Functions of the Nervous System

sensory function – sensesstimuli (changes in internal or external environment)

integrative function – processes sensory inputs and decides onappropriate responses

motor function – sends signals to effector cells, which respond to the stimuli

Divisions of the Nervous System

Central Nervous System (CNS) – brain and spinal cord

main integrative center contains the cell bodies of most neurons

Peripheral Nervous System (PNS) – all nervous tissue outside the CNS

nerves – bundles of axons following specific paths outside the CNS

cranial nerves (12 pairs) – arise from the brain

spinal nerves (many pairs) – arise from the spinal cord

ganglia – small clusters of neuroncell bodies outside the CNS

sensory receptors – dendrites of neurons or specialized cells

Functional Divisions of the PNS

somatic nervous system (SNS) – voluntary

sends sensory information about the external environment or body

position to the upper brain, where the inputs are consciously perceived

sends motor impulses to skeletal muscles to cause body movements

autonomic nervous system (ANS) –involuntary (self-regulated)

sends sensory information about the internal environment to the

lower brain (not consciously perceived)

sends motor impulses to effectors such assmooth muscle, glands, and cardiac muscle

HISTOLOGY OF NERVOUS TISSUE

Neurons – 3 basic parts

1) cell body – contains thenucleus and cellular organelles

carries out vital functions of the cell 2) dendrites – branched receiving portion of neuron

receive stimuli from the environment or other neurons

vary in number (more dendrites = more stimuli can be received) 3) axon – single, long sending portion of neuron

synaptic end bulb– bulb at end of axon which synapses with effectorcell

synaptic vesicles – store neurotransmitters

sensory (afferent) neuron – axon sends impulses to the CNS

motor (efferent) neuron – axon sends impulses away from the CNS

synapse – siteof communication between a neuron and an effector cell

(neuron, muscle fiber, gland)

Neuroglia

Types in CNS:

astrocytes – surround and support neurons, structurally and functionally

help form the blood-brain barrier

oligodendrocytes – produce myelin sheath in the CNS

Types in PNS:

Schwann cells – produce myelin sheath in PNS

satellite cells – surround and protect cell bodies in ganglia

Myelination

myelin sheath – layers of cell membrane(lipid)wrapped around the axon

electrically insulates the axon increases rate of impulseconduction

PNS – Schwann cells wrap segments of axons

cytoplasm and nucleus of Schwann cell form the outermost layer

nodes of Ranvier – gaps in myelin sheath between Schwann cells

CNS – oligodendrocytes have multiple, flat processes which wrap several

adjacent axon segments

White and Grey Matter

white matter – areas of CNS that appear white and shiny

containsmanymyelinated axons – myelin is white (lipid)

grey matter – grey areas of CNS

composed of neuron cell bodies, neuroglia, unmyelinated axons

NEURON PHYSIOLOGY – Production of Electrical Impulses

electrical current – flow of charged particles (ions in cells)

Ion Channels in the Cell Membrane

chemically-gated channels – open or close when a particular molecule binds

(eg. taste & smell molecules, neurotransmitters)

mechanically-gated channels – open or close in response to mechanical forces

(eg. touch & pressure, sound waves)

voltage-gated channels – open or close in response to change in membrane

potential (charge inside cell)

Resting Membrane Potential – at rest, neuron cell membrane is polarized

(different charges on insideand outside of membrane)

Na+/K+ pumps

pumpNa+ out of neuron (high Na+ concentration outside neuron) K+

positive Na+ balanced by negative Cl-

pump K+ into neuron (high K+ concentration inside neuron)

positive K+ balanced by negative protein molecules

Neuron has different permeability to ions

K+ permeability is 50-100 times greater than Na+

(many K+ leakage channels, almost no Na+ leakage channels)

K+ leaks out of neuron, down its concentration gradient

Interior of neuron becomes increasingly negative

(negative proteins in neuron too large to diffuse out – impermeable)

Negative charge in neuron draws some K+ back into cell

At equilibrium,resting membrane potential is about -70mV

(70mV more negative inside cell than outside cell)

Stimulation of Neuron – small changes in resting membrane potential caused by

opening chemically- or mechanically- gated channels on dendrites

depolarization– membrane becomes less polarized(less negative inside)

Action Potentials (nerve impulses) – large change in resting membrane potential

caused by opening voltage-gated channels on axons

begins near the cell body and travels down axon to synaptic end bulbs

1) Neuron Stimulated to Threshold – specific level of depolarization that triggers

opening of voltage-gated Na+channels(about -55mV)

all-or-none response – if neuron reaches threshold, an action potential

occurs (a signal is sent)

if threshold isn't reached, no action potential

2) Depolarization Phase – neuron becomes less polarized(less negative inside)

voltage-gated Na+ channels open – Na+ rushes into neuron (charge inside neuron becomes positive)

3) Repolarization Phase – neuron becomes polarized again (negative inside)

voltage-gated K+ channels open – K+ rushes outof cell (charge inside neuron becomes negative again)

(voltage-gated Na+and K+ channels close again)

4) Na+/K+ pumps restore resting membrane potential

Refractory Period – time after action potential begins when cell can’t generate another

action potential because voltage-gated channels are not reset

Conduction of Action Potentials – traveling of nerve impulse down the axon refractory period results in one-way conduction

continuous conduction– step-by-step depolarization of the entire length of an

unmyelinated axonrelatively slow

saltatory conduction – occurs along myelinated axons

depolarization leaps from one node of Ranvier to the next

the entire axon does not completely depolarize

impulse is conducted very rapidly

opening ion channels only at nodes means less Na+ and K+ pass through

membrane and less ATP (energy) is used to pump them back

Synapses Between Neurons

presynaptic neuron – sending neuron (axon synaptic end bulb)

postsynaptic neuron – receiving neuron (dendrite)

synaptic cleft – small space between 2 communicating neurons

an action potential in the presynaptic neuron triggers release of neurotransmitter

from synaptic vesicles neurotransmitter diffuses across synaptic cleft and binds to receptors

(membrane proteins on the postsynaptic neuron that cause change in charge)

excitatory neurotransmitter – depolarizes the postsynaptic neuron

brings it closer to threshold (may cause an action potential)

inhibitory neurotransmitter – hyperpolarizes the postsynaptic neuron

postsynaptic neuron becomes more negative (farther from threshold)

the postsynaptic neuron can have many synapses

summation of all of the excitatory and inhibitory synapses determines whether the

postsynaptic neuron reaches threshold and produces an action potential

Neurotransmitters – there are many different kinds of neurotransmitters

Acetylcholine (ACh) – acts in PNS and CNS

excitatory at skeletal muscles – causes contraction

inhibitory in the heart – decreases heart rate

acetylcholinesterase – enzyme inactivates acetylcholine in synaptic cleft

gamma aminobutyric acid (GABA) – common inhibitor in CNS

some tranquilizers (valium) enhance action of GABA

Catecholamines – excitatory or inhibitory depending on the receptors

norepinephrine (NE) – “fight-or-flight” responses

epinephrine(E)– hormone from adrenal gland (similar to NE)

Removal of Neurotransmitter – effect of neurotransmitter continues until it is

removed from the synaptic cleft

3 mechanisms of removal:

enzymatic degradation (eg. acetylcholinesterase)

uptake by cells – neuron that released it or neuroglial cells

diffusion away from synaptic cleft, degraded by other cells

How Drugs and Toxins Modify Nervous System Function

stimulate or inhibit neurotransmitter synthesis

stimulate or inhibit neurotransmitter release

block or activate neurotransmitter receptors

agonists activate receptors (mimic neurotransmitter)

antagonists block receptors (prevent neurotransmitter function)

stimulate or inhibit neurotransmitter removal

THE BRAIN

Protection and Nourishment of the Brain

Cranium – bones surrounding and protecting the brain

Cranial Meninges – 3 connective tissue membranes around brain

pia mater – inner membrane which adheres to surface of brain contains blood vessels which supply the brain

arachnoid mater – delicate middle membrane has web-like collagen and elastic fibers that extend to the pia mater

subarachnoid space – space between arachnoid and pia that contains cerebrospinal fluid (CSF)

dura mater – tough, protective outer membrane

fuses with periosteum of cranium

folds between cerebral hemispheres and between cerebrum and

cerebellum help secure brain's position

contains large, open veins that collect excess CSF

Blood-Brain Barrier – protects brain by preventing passage of many substances

from the blood to brain tissue

brain capillaries have tight junctions between cells

astrocyte processes surround capillaries – selectively pass some substances to neurons but block others

glucose crosses by active transport – main energy source for neurons

Cerebrospinal Fluid (CSF) – clear fluid which circulates through cavities in brain, spinal cord, and in subarachnoid space

Functions of CSF:

chemical content helps regulate autonomic functions cushions delicate neurons of brain and spinal cord

Formation and Circulation of CSF

ventricles – 4 cavities in brain filled with CSF

capillary networks in each ventriclefilter blood to form CSF neuroglial cells lining ventricles regulate content of CSF

CSF circulates from ventricles to central canal of spinal cord and the

subarachnoid space

CSF returns to blood in veins within dura mater

hydrocephalus – excess accumulation of CSF resulting in increased

pressure on the brain

4 Divisions of the Brain - brainstem, diencephalon, cerebellum, cerebrum

1) Brainstem – connects to the spinal cord

controls vital autonomic functions and autonomic reflexes such as; swallowing,

coughing, sneezing, vomiting

Medulla oblongata – caudal brainstem

vital centers – control vital autonomic functions

cardiovascular center – regulates heart and blood vessels

respiratory center – controls respiratory muscles

Pons – middle region of brainstem

regulates respiratory rhythm

Midbrain (Mesencephalon) – cranial brainstem

contains reflex centers for vision and hearing

2) Diencephalon – (between brain) between brainstem and cerebrum

thalamus – 80% of diencephalon

relay station for sensory impulses traveling to the cerebrum

hypothalamus – ventral to (below) thalamus

has no blood-brain barrier – senses changes in blood and CSF

regulates the ANS – involuntary organ functions

regulates eating and drinking – thirst center, feeding center

regulates body temperature via ANS

link between the nervous and endocrine systems

produces hormonesthat regulate anterior pituitary gland

produces oxytocin and antidiuretic hormone

participates in emotional behavior (eg. fight-or-flight responses)

3) Cerebellum – attached to dorsal brainstem

coordinates skeletal muscle movements

receives voluntary motor impulses from cerebrum

receives sensory impulses related to body position and balance

the cerebellum compares intended movements with actual movements

sends feedback to cerebrum for corrections

disorders result in hypermetria – voluntary movements are jerky and exaggerated

4) Cerebrum – largest, most dorsal portion of brain

origin ofvoluntary actions, site ofconscious perceptions, center of intellect

longitudinal fissure – deep groove that divides cerebrum into 2 hemispheres

sulci – shallower grooves that divide hemispheres into lobes

Cerebral Cortex – outer gray matter

contains neuron cell bodies controlling conscious functions

Functional Areas of the Cerebral Cortex

Sensory areas – caudal cerebrum

primary sensory cortex– receives sensations of pain, touch,

temperature from the opposite side of the body

parietal lobe

visual cortex – receives visual sensations occipital lobe

auditory cortex – receives sensationsof hearing temporal lobe

Motor areas – cranial cerebrum

primary motor cortex – controls voluntary contractions of

skeletal muscles

frontal lobe

the cortex sends motor impulses to the opposite side of the body

Association areas – located within or near motor and sensory areas

allow recognition of sensations

control complex, learned motor skills

performs abstract functions – prediction, reasoning

Cerebral White Matter – deep to cortex

contains axon tracts running to and from spinal cord, between the cerebral

hemispheres, and within the same hemispheres

corpus callosum – main tracts connecting the 2 cerebral hemispheres

Cranial Nerves– 12 pairs arising mainly from brainstem

sensory nerves – only sensory axons

motor nerves – only motor axons

mixed nerves – sensory and motor axons

Cranial nerve I – olfactory nerve

sensory – olfaction (smell)

Cranial nerve II – optic nerve

sensory – vision

Cranial nerve III – oculomotor nerve

motor – somatic – most eyeball movements autonomic– inner eye movements (pupil size, focusing lens)

Cranial nerve IV – trochlear nerve

motor – eyeball movements

Cranial nerve V – trigeminal nerve

mixed – sensory from face, jaw, and teeth

motor to muscles of mastication (chewing)

has 3 branches; ophthalmic nerve – sensory

maxillary nerve – sensory

mandibular nerve – mixed

Cranial nerve VI – abducens nerve

motor – eyeball movements

Cranial nerve VII – facial nerve

mixed– sensory from taste buds

motor somatic – facial expressions, lip movement

autonomic – secretion of tears, saliva, nasal secretions

Cranial nerve VIII – vestibulocochlear nerve (auditory, acoustic)

sensory – 2 branches

vestibular nerve – balance

cochlear nerve – hearing

Cranial nerve IX – glossopharyngeal nerve

mixed – sensory from taste buds, throat

motor somatic – swallowing and tongue movement

autonomic – secretion of saliva

Cranial nerve X – vagus nerve

mixed – sensory from larynx, visceral organs, carotid artery

motor somatic – swallowing

autonomic parasympathetic to most viscera

Cranial nerve XI – accessory nerve

motor – head and shoulder movements

Cranial nerve XII – hypoglossal nerve

motor – tongue movements

SPINAL CORD AND SPINAL NERVES

External Anatomy of Spinal Cord

extends from brainstem to lumbar vertebrae in adult

cauda equina – bundle of nerve roots in caudal vertebral canal after spinal

cord ends

spinal nerves – emerge in pairs through the intervertebral foramina

most emerge caudal to corresponding vertebra (except cervical nerves)

cervical nerves – 1 pair/cervical vertebra

+ 1 pair between skull and atlas vertebra

thoracic nerves – 1 pair/ thoracic vertebra

lumbar nerves – 1 pair/lumbar vertebra

sacral nerves – 1 pair/sacral vertebra

coccygeal nerves – variable numbers

spinal meninges – 3 connective tissue membranes surrounding the spinal cord

similar to, and continuous with cranial meminges

pia mater – thin, inner membrane on surface of spinal cord with many blood

vessels supplying the spinal cord

arachnoid mater – thin, middle membrane with a spider’s web of collagen and

elastic fibers extending to the pia mater

subarachnoid space – space beneath the arachnoid mater containing cerebrospinal fluid (CSF)

site for spinal tap to collect CSF

dura mater – outer, dense connective tissue sheath surrounding spinal cord and

cauda equina

suspends spinal cord within vertebral canal

epidural space – space above dura mater contains adipose tissue and blood vessels

cushions spinal cord site for epidural anesthetic injections

Spinal Nerves – mixed nerves (contain sensory and motor axons)

spinal nerves arise at specific spinal cord segments

dorsal root – contains sensory axons

dorsal root ganglion – swelling on dorsal root containing cell bodies of sensory neurons

ventral root – contains motor axons

Distribution of Spinal Nerves – spinal nerves branch immediately after passing through intervertebral foramina (gaps between vertebrae)

phrenic nerve (C5-C6) – innervates diaphragm

thoracic nerves(intercostal nerves)

innervate muscles and skin of thorax and abdominal skin

nerveplexuses – complex networks of nerves on either side of body

brachial plexus (C6-C8 and T1 in cat) innervates shoulder and forelimb

musculocutaneous, radial, ulnar, median nerves

lumbosacral plexus (L4-L7 and S1-S3 in cat) innervates abdominal muscles, perineum (genitals, anal and urethral sphincters) and lower limb

sciatic, femoral, and obturator nerves

spinal cord damage causes loss of sensation and voluntary muscle control

caudal to site of injury

Internal Anatomy of Spinal Cord

Gray Matter – butterfly or H shape located centrally

contains nuclei – clusters of cell bodies in CNS

ventral gray horns – somatic motor nuclei

dorsal gray horns – sensory nuclei

central canal – contains CSF, lined by ependymal cells

White Matter – mainly myelinated axons located peripherally

sensory (afferent) tracts – axons carrying sensory impulses to the brain

motor (efferent) tracts – axons carrying impulses fromthe brain to skeletal muscles or autonomic effectors

Sensory and Motor Pathways

sensations – nerve impulses stimulated by internal or external stimuli

perception – conscious awareness and interpretation of sensations (occurs in

cerebral cortex)

SOMATIC Sensory Pathway

Sensory Receptors – specialized cell or dendrites that detect stimuli in the internal or

external environment

touch receptors – have mechanically-gated channels stimulated by touch

pain receptors – have chemically-gated channels stimulated by chemicals

released by tissue damage or inflammation

First-order neurons – carry nerve impulses from receptors to CNS

cranial nerves – from face and mouth to brainstem

spinal nerves – from head, neck, thorax, abdomen, and limbs to spinal cord

Second-order neurons – carry nerve impulse from brain stem or spinal cord to thalamus

cross over in medulla or spinal cord before going to thalamus

almost all sensory information from one side of the body goes to the opposite

cerebral cortex

Third-order neurons – carry nerve impulses from thalamus to cerebral cortex

impulses transmitted to the appropriate sensory area of the cortex

Somatic Motor Pathways – from cerebral cortex to skeletal muscles

Upper motor neurons (UMNs) – from motor area of cerebral cortex to brainstem

or spinal cord

initiate voluntary movements

Lower motor neurons (LMNs) – from CNS to skeletal muscles

cranial nerves – brainstem to face, mouth, and neck