Structure of the nervous system

Structure of the
nervous system
• Central nervous system (CNS)
– Brain
– Spinal cord
• Peripheral nervous system (PNS)
– Cranial nerves
–Spinal nerves
– Autonomic nerves

Afferent
Efferent
Efferent

• Extend throughout CNS.
Nerve fibers
– Aggregated into functional units: fiber tracts.
– Cluster of cell bodies: nucleus.
• Bundled into fascicles and nerves in PNS.
– Wrapped in connective tissue:
• Endoneurium, perineurium, epineurium.
• Blood vessels.
– Cluster of cell bodies: ganglion.

Embryonic development
of the neural tube
• Hollow tube, filled with cerebrospinal fluid
(CSF).
• Brain:
– Develops from anterior expansion of hollow tube.
– Differentiated into three major ventricles.
• Spinal cord:
– Develops from remainder of hollow tube.
– Differentiated into three embryonic layers.

Neurulation and
neural crest

Neurula stage

Spinal nerves
develop in
conjunction
with
developing
myotomes

Embryonic development
of the spinal cord
• Differentiated into three embryonic layers:
– Inner germinal layer:
• Active mitotic division.
• CNS neuroblasts proliferate into mantle layer.
– Middle mantle layer:
• Neuron cell bodies congregate.
• Form “gray matter”.
– Outer marginal layer:
• Neurons form dendrites and axons.
• Myelin sheaths form “white matter”.

Structure of the
nervous system
• Central nervous system (CNS)
– Brain
– Spinal cord
• Peripheral nervous system (PNS)
– Cranial nerves
– Spinal nerves
– Autonomic nerves

Embryonic development
of the brain
• Develops embryonically as three vesicles:
– Forebrain (prosencephalon):
• Olfaction and vision.
– Midbrain (mesencephalon):
• Integration of input signals, especially visual.
– Hindbrain (rhombencephalon):
•Hearing
• Coordination of movement, muscle tone, posture.
• Integration in electromagnetic-generating and
electromagnetic-detecting fishes.

Structure of the brain
• 3 embryonic regions form 5 functional regions:
– Prosencephalon:
• Telencephalon (~cerebrum, cerebral hemispheres).
• Diencephalon.
--- Cephalic flexure ---
– Mesencephalon.
--- Pontine flexure ---
–Rhombencephalon:
• Metencephalon (~cerebellum).
• Myelencephalon (=medulla oblongata).
--- Cervical flexure ---
– Spinal cord.

• Neurons:
Human brain development
– Develop in fetus at >250,000/min.
–At birth, ~trillion (10 9 ) neurons.
– Few neurons added after birth.
– Thereafter, neurons die at rate of
thousands/day.
• Glial (neuroglial) cells:
– Outnumber neurons 50:1.
– Regulate internal environment
of brain.
• Blood flow.
– Help to create and destroy
neural circuits.
it

Structure of
adult vertebrate brain
(5) Myelencephalon l (posterior)
(4) Metencephalon
(3) Mesencephalon
(2) Diencephalon
(1) Telencephalon (anterior)

(5) Myelencephalon (medulla oblongata)
• Structure similar to spinal cord:
– External white matter, internal gray matter.
– Central canal expanded: 4th ventricle.
– Roof thin, highly vascularized (posterior
choroid plexus).
• Gas, nutrient exchange.
• Secretions.
• Nerve fibers grouped by function:
– Somatic sensory (dorsal, afferent)
– Visceral sensory
– Visceral motor
– Somatic motor (ventral, efferent)

Afferent
Efferent

(5) Myelencephalon (medulla oblongata)
• Basic function: switchboard for autonomic receptors
and effectors of:
–Eye.
– Pharyngeal arches and derivatives:
• Gills, tongue, larynx
– Visceral organs:
• Heart
• Respiratory organs
•Digestive tract
• Acoustico-lateralis system

(4) Metencephalon
• Roof enlarged into cerebellum.
• Functions:
– Coordination of skeletal muscles.
– Rhythmic locomotory actions.
– Posture.
• Correlation between cerebellum size and amount &
intricacy of body movements:
– Small in lampreys, less active fishes, amphibians,
reptiles.
– Large in active fishes, birds, mammals.

(4) Metencephalon
• Afferent nerves (via medulla oblongata) from:
– Skeletal-muscle proprioceptors.
– Acoustico-lateralis system.
– Eye (via mesencephalon).
– Cutaneous receptors.
• Efferent nerves to skeletal muscles.
• Reversal of gray and white matter:
– Gray cortex.
– Interior white nerve fibers.

(3) Mesencephalon
• Roof enlarged into optic lobes (=optic tectum).
• Function: reception and integration of visual signals.
– Fishes & amphibians:
• Primary visual center.
– Reptiles & birds:
• Visual center, along with cerebrum (forebrain).
– Mammals:
• Secondary visual center.
• Cerebrum is primary visual center.
• Cranial nerves control eye movement.
• Center of learning in lower vertebrates.

(2) Diencephalon
• Subdivided d d into three regions:
a) Roof: epithalamus
b) Walls: thalamus
c) Floor: hypothalamus

Anterior
Posterior

(a) Epithalamus
• Three structures formed by evagination:
–Paraphysis:
• Present in vertebrate embryos.
• Absent in adults.
• Function unknown.
–Parietal(=parapineal) and pineal (=epiphysis) bodies:
• Primitive function: medial eye.
• Pineal operational in lampreys and hagfishes.
• Parietal operational in some lizards.
– Derived function n of pineal body in amniotes:
• Control of internal cycles (e.g., circadian rhythms).

Pineal body
• Transduces neural information into endocrine
information.
• Secretes the hormone melatonin on circadian cycle:
– High concentrations at night.
– Low during day.
• Tracks secondary lunar cycles:
– Mating and reproductive cycles.
– Immunocompetence.
–Aging.
g

(b) Thalamus
• Important center for relay and integration of
signals between the cerebrum and midbrain /
hindbrain.
n.
• Most important in mammals due to enlarged
cerebrum.

(c) Hypothalamus
• Three important functional regions:
– Optic chiasma: optic stalks.
– Control of the autonomic (“involuntary”) nervous
system:
• Temperature regulation, O 2 / CO 2 balance.
• Heart rate, breathing rate.
• Biological clock: sleep rhythms, hormone
cycles.
– Interacts with epithalamus.
• Emotional responses.
– Neurohypophysis:
• At tip of infundibulum.
• “Posterior” (dorsal) portion of pituitary gland.

Formation of optic stalks

Neurohypophysis
• Secretes many important hormones, such as:
–Vasopressin: salt-water balance.
– Oxytocin: stimulates t contraction ti of smooth muscle.
• Mammary glands, uterus.
• Humans: related to emotional, pair-bonding and
stress-related responses.
• Corresponding “anterior” (ventral) pituitary gland:
– Adenohypophysis.
– Derived from endoderm (roof of mouth).
– Many hormones.

(1) Telencephalon
• Ancestral function: primary olfactory center.
– Secondary center: memory, learning, orientation.
• Derived function in mammals and birds:
– “Higher” brain functions.
– Expanded d cerebral hemispheres.
h
• Wrap around posterior brain regions.
• Reversal of gray and white matter.
– Taken over control of many other functions:
• Vision: color vision.
• Coordination: highly integrated movement.
• Memory: learning, emotional responses.

(1) Telencephalon
• Subdivided into three regions:
–Paleopallium: olfaction.
• Dominant in fishes.
• Reduced in amniotes.
– Archipallium: memory and navigation.
• Hippocampus in all vertebrates.
• Largest in cetaceans, primates.
– Neopallium: cerebral hemispheres.
• Thin layer in fishes, amphibians.
• Enlarged in reptiles.
• Dominant in birds, mammals (neocortex).

Functional regions
of the neocortex in mammals
• Regions:
(1) Frontal: includes speech center
(Broca’s area), coordinates
breathing and vocalization.
(2) Parietal: includes somatosensory
integration area.
(3) Temporal: includes auditory region.
(4) Occipital: includes visual region.
• Allocations of particular functions to
regions are overly simplistic.

Somatosensory cortex:

Ventral view

Limbic system
• = “Reptilian brain”.
• Functions: fear, hunger, fighting/fleeing, memory,
sensory input, and sexual behavior.
• Structure is progressive in vertebrates:
– Primitively, main function is processing olfactory
stimuli.
– Somewhat developed in fishes, amphibians.
– Full development in the early reptiles.
• Mammalian and avian limbic systems st not much
different from crocodilian.

Limbic system
– Derived function in humans, primates, and many
other mammals:
• Regulation of emotional processes.
– Humans: join “higher” mental functions (reasoning)
with “lower” functions (fear, pleasure).
• Accounts for why:
– Sexual behavior and eating seem pleasurable.
– Mental stress causes high blood pressure.
–Etc.
• Disorders in regulating mechanisms lead to craving and
addiction behavioral disorders.

Evolutionary development
of the brain
• Cephalochordates (e.g., amphioxus) have slight dilation
of neural tube in head: cerebral vesicle.
• Gene expression patterns and microanatomy indicate:
–Diencephalon:
• Small region homologous with vertebrate lateral
eyes.
• Lamellar ar organ homologous ogous with vertebrate rat pineal
photoreceptors.
– Small midbrain.
– Hindbrain.

Trends in vertebrate brains
• Ancestral (“lower” vertebrates):
– Posterior brain well developed.
– Straight tube, large ventricles.
– Cerebellum enlarged.
• Derived (“higher” vertebrates):
– Anterior brain well developed.
– Ventricles reduced, walls thickened. Cerebral
hemispheres enlarged .
–Brain highly folded and elaborated.
– Increased regionalization; functions shifted
forward.
– Enhanced limbic system.

Trends in vertebrate brains
• New elements added to existing elements, rather
than replacing them.
– Newer elements control older elements.
– “Higher” nervous system:
• Superimposed control circuits.
• More complicated behaviors.
• Repeated and parallel increases in relative brain
size within major groups.
– Teleost brains almost 2x size of shark brains.
– “Higher” teleost brains almost 2x size of “lower”
teleost brains.
– Etc.

Brain size
• Brain size is relatively larger in more derived
vertebrate groups.
• But brain size is highly hl correlated with body size
(allometry).

• Increases in brain size
over evolutionary time:
– Average increases.
– Widening size
distributions.
– Few replacements of
smaller-brained by
larger-brained
animals.
Brain size

Brain size in hominoids
Homo sapiens
Homo erectus
Australopithecus afarensis
olume
Brain vo
Body weight

Brain size in hominoids