Vertebrate Structure and Development
The nervous system is like the control system of an animal. It allows the animal to monitor its environment, stimulates it to orient itself to favorable stimuli, regulates the internal environment of an animal, and is a storage site for information (memory).
Nervous tissue is composed of neurons and associated cells. Neuroglia are cells of the central nervous system (brain and spinal cord). They are involved in maintaining the neurons. Schwann cells are cells that wrap around the axons in the peripheral nervous system. The coating provided by the Schwann cells is called myelin. Nervous tissue surrounded by myelin is white matter. Nonmyelinated tissue is grey matter.
The nervous system can be subdivided as:
CNS Peripheral NS
brain spinal cord Sensory Motor
The sensory (afferent) neurons bring information to the CNS and the motor (efferent) neurons take information from the CNS to effectors (muscles or glands). Information is stored, analyzed, and processed within the CNS.
A typical neuron has a cell body, axon, and numerous dendrites. The axon leaves the cell body and takes the nerve impulse to another part of the nervous system or to an effector. Strands of axons running together make up a nerve. Nerves that carry only sensory information are sensory nerves, those with only motor information are motor nerves (rare), and those with both types of information are mixed nerves.
The epineurium is a sheet of connective tissue which wraps the nerve. The vasa nervorum are the blood vessels that service the nerves. The dendrites and cell body serve as the reception site for incoming nerve impulses.
The cell bodies of neurons are typically found in clumps. If these clumps of cell bodies occur in the CNS we call them nuclei. If the occur outside the CNS they are ganglia.
We often depict an axon as a single long fiber. In reality this isn't quite true. The terminal end of the axon near the synapse sprays out into fine branches called telodendria. The end of each is a synaptic knob which releases neurotransmitter. Some axons have collateral branches which come off the main axon branch.
Mammalian Motor Pathways
If we look at the motor part of the system we see that information travels through 2 or 3 neurons on its way to the gland or muscle. The 1st neuron arises from the CNS. The 2nd forms in the CNS if it is part of the somatic motor system. If it is autonomic it develops in ganglia (outside the CNS).
We refer to the 2 somatic neurons in this pathway as the upper and lower motor neurons. In some cases there is an interneuron too. In the autonomic system they are the preganglionic and postganglionic motor neurons.
Mammalian Sensory Pathways
In many senses the receptor for a sensation is not part of a neuron. This is true for pressure reception or taste. The sensory neuron that synapses with this receptor is called a first-order sensory neuron. It will send the information to a second-order neuron. In other cases the receptor is actually part of the dendritic area. In this case we call the sensory neuron a neurosensory neuron. We see this in smell, pain and temperature reception.
The first sensory neuron runs to the spinal cord. The second sensory neuron runs from the spinal cord to the thalamus. The third sensory neuron runs from the thalamus to the cortex.
It's hard to know when the nervous system actually comes into being. We know that something happens during gastrulation that leads to differentiation. The underlying chordamesoderm under the neural ectoderm induces it to become neural tissue. This has 2 parts. First, the ectoderm is activated and then it grows along a certain pattern.
In an embryo the nervous system starts to develop during neurulation. Initially we get a hollow tube. The neural tube then differentiates into 3 regions. The germinal layer has a great deal of mitosis. The mantle layer develops from cells produced by the germinal layer. The mantle layer has 2 types of cells: the neuroblasts will become the neurons, the spongioblasts will become the neuroglia which act as supportive cells. The outer layer of the neural tube is the marginal layer. It has few cell bodies, but contains axons. Axons become coated in myelin which is a white, fatty substance. It makes the marginal layer look white, so we call it white matter. The mantle layer has the cell bodies and looks gray, so it's called gray matter.
The cell bodies of the neuroblasts start in the mantle layer, but over time the cytoplasm extends out. Initially, the axon has no myelin. As it grows out neuroglia cells migrate along and come to sit next to the new axon. These cells then wrap around the axon and produce myelin. In an adult if the axons inside the CNS are injured they don't regrow. Outside the CNS the part beyond the injury dies and the remaining stub may grow and in some cases reestablish function. The Schwann cells follow this growth and rewrap the new growth.
In the neural tube the dorsal and ventral parts of the tube show differences in where the information is travelling. The alar plate develops in the dorsal part of the tube. Neurons connecting with this area bring sensory information to the CNS. The basal plate develops in the ventral part of the tube and connects with motor neurons carrying information from the CNS.
New nervous tissue is generated only in an embryo. As development continues cell division in the germinal layer stops. The cells left next to the hollow center of the nerve tube act to line the tube. These are ependymal cells.
Development: In the embryo the 1st part of the brain to develop is the prosencephalon and deuteroencephalon. The prosencephalon is induced by the prechordal plate and the deuteroencephalon is induced by the notochord. Next the prosencephalon, mesencephalon and rhombencephalon develop. As development progresses the prosencephalon -> telencephalon and diencephalon. The rhombencephalon -> metencephalon and myelencephalon.
The telencephalon becomes the cerebrum (cerebral hemispheres) of the adult. In amniotes it is divided into 2 cerebral hemispheres. A prominent part of the underside of the cerebrum are the olfactory bulbs.
Another part is the corpus striatum = basal ganglia = basal nuclei. It integrates sensory information, deals with stereotyped movement (walking) and can initiate automatic movements. It is divided into the archistriatum, paleostriatum and neostriatum.
The last part is the cortex (pallium). In lower vertebrates the grey matter is internal to the white matter. In reptiles it starts to move out. In mammals the grey matter surrounds the white matter. In fishes and amphibians the cerebral hemispheres contain the corpus striatum and a paleopallium and archipallium. These are retained in reptiles, birds, and mammals. The archipallium becomes the hippocampus in mammals. It is involved in instinctive behaviors and maybe storage of recent memory. In reptiles, birds, and mammals there is a new brain region, the neopallium. There is very little in reptiles and birds. In mammals it becomes the predominant part of the cerebral hemispheres. It is so large that it has to be folded up into ridges (gyri) and folds (sulci). In birds we see the outer gray matter in another structure, the hyperstriatum.
Several structures in the cortex and the diencephalon form the limbic system. It is important in sexual and emotional behavior, feeding, memory, learning and motivation. It also is inhibitory to stereotyped behavior.
The diencephalon becomes a number of structures including the epithalamus, thalamus, hypothalamus and optic chiasma. The epithalamus contains the pineal gland. This is involved in thermoregulation, reproduction and timing of events.
The thalamus is a relay center. There are many synapses for neural pathways here. It can turn on or off the cortex.
The hypothalamus is a major autonomic regulator. Thermoregulation, thirst, appetite, sex behavior, emotions and so forth are regulated here. It releases a number of hormones which act on the pituitary. The optic chiasma is where the 2 optic nerves cross.
The mesencephalon or midbrain contains the tectum which is involved in visual processing. The red nucleus and substantia nigra are relay stations - involved in muscle function.
The metencephalon becomes the cerebellum. The external part is grey and the internal part is white. Its function is to control motor coordination and balance. It is not conscious coordination, but is automatic fine tuning. It coordinates sensory and motor information and sends impulses to other parts of the brain. It along with the basal ganglia smooth complex behaviors. It compares ingoing sensory information with outgoing motor commands to see if the movement is going as planned. Distance judging occurs here.
The myelencephalon is the medulla oblongata and pons (birds and mammals). These are not under conscious control. There are centers for respiration and heart function. The reticular formation is needed for maintaining consciousness. It is also involved in motor functions.
The brain has fluid filled areas called ventricles. The 1st 2nd sit opposite each other and are called lateral ventricles. These are in the telencephalon. The 3rd ventricle is in the diencephalon between the right and left side of the thalamus. The 4th ventricle is at the level of the cerebellum and medulla oblongata. Choroid plexus is a network of tissue that produces cerebrospinal fluid.
The brain is surrounded by meninges. In fishes there is 1 layer, the primitive meninx (meninx primitiva). In amphibians and reptiles there are 2, the dura mater on the outside and the pia-arachnoid (leptomeninx) on the inside.
Birds begin to have a 3rd layer and mammals have 3 distinct layers. The dura mater is on the outside, it sticks to the cranium. Underneath is the arachnoid. This is a thin delicate layer. The pia mater is the inner layer. It sticks to the brain.
Between the dura mater and the arachnoid is the subdural space. It's filled with cerebrospinal fluid. The subarachnoid space sits between the pia mater and arachnoid. It is filled with cerebrospinal fluid.
The typical vertebrate spinal cord has both grey and white matter. Agnathans have this mixed. Other vertebrates have distinct grey and white matter. The cord is enlarged where the nerves to and from the limb enter.
In most vertebrates the nerves leaving the dorsal and ventral areas join to form a single spinal nerve. There is 1 per side in each segment. The portion leaving the dorsal area is the dorsal root and the portion leaving the ventral area is the ventral root. In lower vertebrates sensory and motor fibers are mixed to some degree in the roots. In amniotes the dorsal root is strictly sensory and the ventral root is motor.
The meninges around the spinal cord are the same as those around the brain. In mammals there is cerebrospinal fluid between the meninges. That and a layer of peridural fat help protect the spinal cord.
The spinal cord begins at the foramen magnum. There isn't an abrupt change between brain and spinal cord, rather it is a gradual rearrangement and the anterior end of the cord is a transitional area between brain and cord.
In spinal cord shows cervical and lumbar enlargements where the front and hind limb nerves come off. This is caused by the large number of cell bodies that come off to serve the nerves of the limbs. The more robust the limb, the larger the enlargement.
A pair of spinal nerves leaves the spinal cord at each segment. Dorsal rootlets carry sensory information from the spinal nerve to the dorsal gray matter. Ventral rootlets carry motor information from the CNS to the spinal nerve. The rootlets combine to form the spinal nerve - except in lampreys. Along the dorsal root is the dorsal root ganglion. The cell bodies of unipolar neurons lie in the ganglion.
Just a little ways from the spinal cord, the spinal nerve splits. It forms at least 2 branches. 1) dorsal ramus goes to the muscles and skin of the back. 2) ventral ramus, the main branch goes to the muscles of the lateral and ventral area of the trunk and to the skin. In the thoracic and lumbar regions the autonomic nerves join with the spinal nerves near the spinal cord. They are connected by rami communicantes.
0 - Terminal - sensory - involved in responses to sex pheromones. Not present in Agnathans, birds and some mammals.
I - Olfactory - sensory - olfaction. In many vertebrates a large portion of the brain is devoted to olfaction. In some vertebrates with a vomeronasal organ there is a separate branch - the vomeronasal nerve.
II - Optic - sensory - vision. Axons may or may not cross over in the brain. The optic chiasma is where the fibers cross over (decussate).
III - Oculomotor - motor - moves eyeball. For this function it goes to the inferior oblique muscle and the superior, medial and inferior rectus muscles. Also, fibers go to the muscles that control the pupil size and to muscles that allow us to focus.
IV - Trochlear - motor - moves eyeball. This is the only cranial nerve that comes from the posterior part of the brain. It is one of the smallest. It goes to the superior oblique muscle.
V - May be 1 or 2 nerves. If it's 2 nerves: V1 - Deep ophthalmic - sensory to skin. Not separated in vertebrates higher than primitive Osteichthyes.
If it's 1 nerve: Trigeminal V1 - deep ophthalmic - sensory.
V2 - maxillary (infraorbital in fishes) - sensory to teeth gums and skin of maxillary area.
V3 - mandibular - sensory and motor to mandibular area (visceral arch 1).
VI - Abducens - motor - abducts eyeball. It is one of the smallest cranial nerves. It goes to the lateral (external) rectus muscle.
VII - Facial - sensory to mouth and face. Motor to muscles from visceral arch 2. Also innervates tear glands and salivary glands.
VIII - Vestibulocochlear - sensory - to inner ear for balance and hearing. In tetrapods the branch that is involved in hearing goes to the cochlea - cochlear nerve, and the branch that is involved in balance goes to the vestibular apparatus - vestibular nerve. The lateral line of fishes and amphibians is innervated by C.N. VIII
IX - Glossopharyngeal - mixed - sensory to throat and posterior taste buds. Motor to small throat and mouth muscles. In fishes it splits into 2 main branches: pretrematic nerve and posttrematic nerve The pretramatic nerve goes to the anterior wall of the gill chamber and monitors the condition of the gill filaments. The posttrematic nerve goes to the muscles of the gill chamber and monitors the gill filaments along the posterior wall of the gill chamber. This nerve goes only to the 1st gill chamber.
X - Vagus (wanderer) - sensory to skin in gill and ear area, taste buds and pharynx. Autonomic motor fibers go to the heart, lungs and digestive organs. It is important in swallowing and in fish in respiration. In fishes it has an arrangement like C.N. IX. The difference is that the vagus goes to gill chambers 2 - 5.
XI - Accessory - in amniotes - mixed to muscles of the neck and anterior back.
XII - Hypoglossal - motor - tongue, floor of mouth, throat and neck. Only a cranial nerve in amniotes.
The Branchiomeric nerves are cranial nerves V, VII, IX, and X. They are lumped together because in the earliest vertebrates they supplied the pharyngeal arches which were involved in breathing.
Autonomic Nervous System
This portion of the motor system is involuntary. It goes to the heart, blood vessels, lungs gills, endocrine and exocrine glands, digestive organs, excretory and reproductive organs, chromatophores, fat and involuntary eye muscles.
The autonomic nervous system has 2 neurons that lead from the CNS -> effector. They synapse in a ganglion. Therefore, we call the 1st one the preganglionic neuron and the 2nd one the postganglionic neuron.
There are 2 subdivisions of the ANS. The sympathetic nervous system or "fight or flight" system puts the animal into alarm mode. The parasympathetic nervous system is for resting activities. In reality both are functional but to greater or lesser degrees.
Function SNS PNS
Exits CNS thoracolumbar area *craniosacral area
neurotransmitter noradrenalin acetylcholine
neuron short long
neuron long short
Ganglia observable in effector organ
* involves CN III, VII, IX and X
The ANS neurons that are preganglionic always release acetylcholine. Cholinergic fibers release acetylcholine Adrenergic fibers release noradrenaline.
The ganglia of the ANS can be subdivided into 3 groups: 1) paravertebral, 2) collateral, and 3) terminal.
Paravertebral ganglia: these are found lying on either side of the spinal cord. They form the synapse for sympathetic neurons. They are arranged in a strand and there is 1 ganglion per segment. Some parasympathetic fibers run through the ganglia, but do not synapse there.
Collateral ganglia: these are found in the head and abdomen. In the head region they are formed by the synapses of per- and postganglionic fibers of CN III, VII, IX, and X. In the abdomen they are formed by sympathetic fibers innervating the viscera. These sympathetic ganglia do not form a string-like structure and therefore are not paravertebral ganglia.
Terminal ganglia: these are parasympathetic ganglia found in the viscera. The postganglionic fibers are very short and the ganglion is usually part of the outer wall of the organ that it innervates.
The adrenal medulla is really part of the ANS of mammals. The medulla cells produce adrenaline and noradrenaline. They are innervated by preganglionic SNS fibers. Instead of releasing the adrenaline/noradrenaline via the postganglionic fibers, it is released directly into the blood. In the neural connection involving the adrenal medulla we see preganglionic fibers, but no postganglionic fibers. In many ways the medulla is comparable to a big ganglion.
Most of our discussion has centered on the neurons, but there are other important cells in the nervous system. There are several classes of glial cells that are vital to nervous function. Oligodendrocytes form myelin around the axons within the CNS. Astroglia aid in nerve impulse transmission. Microglia are phagocytes that remove debris. Schwann cells make the myelin that surrounds axons that reside outside the CNS.
For a stimulus to be picked up it must cause part of the nervous system to respond. The part of the nervous system that originally responds to the stimulus is the receptor. Receptors are highly selective for a specific modality. They transduce energy from a stimulus into an electrical signal. Some receptors amplify the signal.
Receptors are variable and can be classified by their function. One way to subdivide them is general vs special. Another is mechanical (hearing), vs electromagnetic (vision) vs chemical (taste).
We are most familiar with the senses we are conscious of like sight, pain, temperature etc., but we also have a number of receptors that relay information we can't consciously access. We have receptors for blood oxygen and carbon dioxide levels, blood pressure, muscle tension and other things we aren't aware of.
The simplest reception device is a naked nerve ending (some temperature reception). Slightly more complex is a sense capsule. The nerve endings (dendrites) are covered by a capsule. Some senses have both types of reception - touch and pressure.
The receptor forms a generator potential which is of graded strength depending upon the strength of the impulse. In the axon of the 1st sensory neuron the impulse is transmitted as an action potential. If the generator potential is strong enough it elicits an action potential. If not the impulse stops. Action potentials in a given axon are the same intensity : all-or-none.
1) Olfaction - This is widespread and often acute in vertebrates. We think that in early vertebrates much of the cerebrum was olfactory in function. The receptors sit in olfactory epithelium. Filaments from the olfactory cells sit in mucus. The mucus keeps the filaments moist so the molecules we smell can dissolve.
Vertebrates have 7 olfactory receptors according to one theory. This is still not totally worked out. A vertebrate's ability to sense hundreds of odors comes from how combinations of the 7 types of receptors are stimulated.
All fish have olfaction, but don't need the mucus glands. Air breathers have the mucous glands. Smell is not well developed in most birds and aquatic mammals.
Just in front of the embryonic stomodeum a pair of olfactory placodes forms. These sink into the head and become the sensory, supportive, and mucus cells. The olfactory cells extend from the nasal area to the brain as C.N. I.
2) Vomeronasal or Jacobson's Organ - This is a smell-taste type sense. We don't know if it exists in fishes. Salamanders have this sense and so do snakes and lizards. It is present in some mammals, but absent in other mammals and all birds. We have a vomeronasal organ as an embryo and lose it before birth. It sits in the ventral part of the nasal cavity.
The organ has been best studied in reptiles. It is used in exploration, recognition of others, food seeking, courtship and maternal care. Tongue flicking by snakes and lizards involves bringing odor molecules to the vomeronasal organ.
3) Taste - the sense of taste in vertebrates is less acute than the sense of smell. The receptor is a taste bud. Dissolved food particles are detected by the receptor and an impulse is sent to the brain. Vertebrates have 4 taste modalities: sour, sweet, bitter and salt. The 7th, 9th and 10th cranial nerves innervate the buds in different areas.
Taste buds are found in the mouth and pharynx of fishes. Some fish also have them outside the mouth - particularly those that burrow into the sand and muck to find food. They are found on the whiskers of a catfish, and in some species on the gills, skin and fins. The hypoglossal nerve supplies skin to the external taste buds of fishes.
Amphibians and mammals have taste buds on the tongue and pharynx and reptiles and birds have them in the pharynx, but have a poorly developed sense of taste. Nectar-feeding birds can discriminate sweet tastes.
4) Vision - vertebrates have 2 types of eyes. All have a pair of lateral eyes. Some have a dorsal eye.
The parietal eye has a lens, cornea and retina with rod-like structures. It can distinguish amount and wavelength of light hitting it - but it can't focus. It is found in lampreys, amphibian larvae, lizards and tuataras.
The parietal eye is useful for monitoring things associated with light levels - daylength and season. This is important for timing daily retreat, reproduction, hibernation etc.
The parietal eye is an outgrowth of the roof of the diencephalon. In lizards it lies in the parietal foramen with a translucent scale above it.
Many nocturnal vertebrates have eyeshine. This is caused by the tapetum lucidum which acts as a mirror. It sits behind the rods and sends light through a 2nd time. This means they are more able to see in dim light.
Accommodation is the process of focusing. The lens of an aquatic vertebrate is more spherical than that of a terrestrial vertebrate. Amphibians have the problem of needing to see in water as well as in air. They solve this by having a flat cornea and a very spherical lens.
Fish, amphibians, and most reptiles accommodate by moving the lens forward or backwards. Lizards, birds, and mammals accommodate by changing the shape of the lens.
The retina starts as an outgrowth of the diencephalon, the optic vesicle. As development continues the vesicle moves out and becomes cup-shaped, the optic cup. As it migrates it pulls an optic stalk behind it. The optic stalk will become the optic nerve.
On the surface of the head the lens placode develops from ectoderm. It is induced by the optic cup to develop into the lens. Mesenchyme around the optic cup is induced to form the horoid, sclera, cornea, and ciliary body. Ectoderm external to the lens becomes the conjunctiva. The edge of the optic cup becomes the iris.
5) Infrared reception:
A number of snake species can detect IR radiation. Pit vipers and boids have pits that are used for sensing IR. Boids have a row of labial pits used to detect IR. Pit vipers have a pair of facial organs. Each of the facial organs has an outer chamber and an inner chamber separated by a thermosensory membrane. IR radiation hits this membrane and warms it. Nerve endings in the membrane are excited and send a message via CN V to the brain. Light hitting the membrane won't elicit this response.
This receptor is very sensitive. Temperature changes of
0.003 C can trigger a response. The arrangement of 2 facial pits allows a binocular heat vision. Like vision, information from the pits goes to the optic tectum where it along with visual information is mapped.
The combination of heat and visual input probably makes the snake's aim more precise. Vision isn't required though. Even a blind or blindfolded rattlesnake can follow a rodent.
There are several senses served by CN VIII. We are familiar with hearing and balance. Aquatic vertebrates up through amphibians also have a lateral line system. Along with CN VIII, Cranial nerves VII, IX, and X also serve the lateral line.
6) Lateral line system - this system has 2 parts. Electroreceptors detect electrical fields and mechanoreceptors detect pressure. The receptors are neuromasts. They are filled with hair cells that are stimulated by pressure or electrical potentials.
The hair cells of the inner ear start as auditory (otic) placodes near the myelencephalon. Epidermis near this becomes the lateral line placodes. These migrate out along the forming body to make the lateral line system.
The hair cells have hairlike extensions that stick up into a cupula. Each hair cell has a long kinocilium and short stereocilia. Pressure bends the cupula and when it does the hairs get bent. The lateral line system is set up so that some hair cells respond to being bent in 1 direction and others respond to being bent in the other direction.
The pressure detecting neuromasts allow fish to respond to disturbances in the water. This allows fish to school and detect objects in the water. It allows blind species to navigate and feed. The electroreception allows fish to pick up muscle movements of other fish.
7) Balance - The inner ear sits in a hollow area called the labyrinth. The labyrinth has 2 subparts the utricle and saccule. The semicircular canals come off the utricle. Sensory patches in the utricle and saccule, called maculae, have hair cells that are most sensitive to linear changes. The semicircular canals have hair cells found on cristae, that respond to rotation of the head.
Besides being able to track which end is up, the equilibrium centers must be in contact with other body parts. Information is transmitted to various brain regions and from there it goes to muscles of the body. This helps us keep balance and keep our head pointed forward. Information via CN III, IV, and VI goes to the external eye muscles so we continue to look straight forward.
If you spin around a lot you display nystagmus - side-to-side movement of the eyes. This occurs because we try to focus on an object, but as we continue to spin we can't, so we pick a new spot to focus on. This gives us a slow tracking movement as our eyes stay glued to the object we are focused on and a fast "pick a new spot" movement as the eyes move from the old spot we were focused on to a new spot. CN III, IV, and VI get information from the vestibular input and move the eyes.
Lots of reflexes for body and eye movement are tied to the labyrinth. Our ability to maintain upright posture is based on combining input from both the visual and balance receptors.
8) Hearing: The hearing mechanism starts with the tympanum in terrestrial animals. It contacts a stapes (columella) and in mammals an incus and malleus.
Sound causes the tympanum to vibrate and this vibration is carried through the (malleus and incus if present) and the stapes, of the middle ear. Because the surface area of the tympanum is larger than that of the ossicles, the sound vibrations are amplified as they go through the middle ear. In all terrestrial vertebrates the stapes inserts into the oval window of the lagena.
In amphibians the sacculus has 2 receptor sites for hearing: the amphibian papilla (found only in amphibians) and the basilar papilla.
In birds, reptiles, and mammals the lagena is much larger than in amphibians and the basilar papilla is part of the Organ of Corti. In placental mammals the lagena become very large and coiled like a snail shell, so we call it the cochlear duct.
The cochlear duct its between 2 other chambers: the scala vestibuli and the scala tympani. These 2 are filled with perilymph. The cochlear duct is filled with endolymph. The scala vestibuli and the cochlear duct are separated by the vestibular membrane. The basilar membrane separates the cochlear duct from the scala tympani. Receptor cells sit on the basilar membrane. Just above them is the tectorial membrane.
Sound enters the inner ear via the oval window. The sound waves displace the perilymph and enter the cochlear duct via the vestibular and basilar membranes. This allows them to stimulate the receptor cells.
Echolocation is orientation by sound. It is found in marine mammale, bats, and oil birds. We know the most about bat echolocation. Bats send out a very loud ultrasonic pulse of 5 - 10 msec. If they are cruising for bugs they emit about 10/sec. Once they find something it goes up to 200/sec. The echo from this cry hitting the object comes back to the bat.
Just a brief time after emitting this really loud cry, they have to be able to hear a very faint echo. This is comparable to standing next to a jet engine and immediately hearing a whisper. Our ears can't do that, but a bat's can. Their ear anatomy is the key to how.
The pinna is large and has ridges. This funnel picks up returning echoes. The pinna can bend forward and a flap in the auditory canal can close to reduce the amount of sound getting to the inner ear curing the cry. The eardrum is thin. This is an adaptation to hearing high frequencies. The middle ear bones are tiny relative to the bat's size, but the 2 middle ear muscles are huge (stapedius and tensor tympani). When the bat cries these contract and decrease the amount of sound getting to the inner ear. Blood sinuses, fat, and connective tissue insulate the inner ear from skull bones which would conduct sound from the mouth to the inner ear.
The endocrine system is somewhat different from our other systems in that a) it is hard to make the cut off between nervous and endocrine systems; b) it's not really a system. It is lumped together because all of the glands secrete hormones. These are chemical messengers that are secreted into the blood. Therefore, endocrine glands are ductless.
Hormones enter the circulatory system and circulate throughout the body. A few hormones act on all body cells, but most affect only specific cells. Only cells with receptors specific for a given hormone are acted on by that hormone.
Hormones are of 2 classes. Peptide hormones bind to the surface of the target cell. Steroid hormones enter the target cell and bind to the chromosomes. They cause their effect by altering protein synthesis.
The shape of endocrine glands is highly variable, but their shape has little bearing on function so this isn't surprising. We know very little about endocrine physiology in non-mammals.
Glands Derived from Ectoderm
Hypothalamus and Pituitary: Among the numerous functions of the hypothalamus is its endocrine function. All of its endocrine function is directed towards the pituitary. The pituitary releases 9 hormones that effect every cell in the body. It is distinctly subdivided and actually starts out as 2 distinct structures.
The hypothalamus and the neurohypophysis originate in the diencephalon. The neurohypophysis consists of the posterior pituitary, the infundibulum, and the median eminance which sits just behind the optic chiasma.
The adenohypophysis comes from the back of the mouth cavity. In many vertebrates it starts as a hollow bud called the Rathke's pouch. This breaks away from the stomodeum and moves up below the hypothalamus. The adenohypophysis has two main parts, the intermediate lobe and the anterior lobe of the pituitary. Over time the intermediate lobe comes to lie closer to the neurohypophysis than to the anterior lobe.
Axons from the hypothalamus extend directly into the posterior lobe of the pituitary and release their hormones. These then are dumped into the blood. Other axons release hormones within the hypothalamus that travel through the portal system to the anterior lobe of the pituitary.. The posterior pituitary releases 2 hormones. Oxytocin is only produced in mammals. It causes smooth muscle contraction in the uterus and milk ejection. The other hormone acts on the kidney to maintain water balance. It is vasotocin or vasopressin in mammals. Vasopressin is also called antidiuretic hormone (ADH).
The pars intermedia only releases melanocyte-stimulating hormone (MSH). It increases melanin production and/or dispersal. It is involved in both morphological and physiological color change.
The anterior pituitary releases 6 hormones.
1) Adrenocorticotropic hormone (ACTH) which causes the adrenal gland to secrete hormones involved in glucose metabolism.
2) Thyroid-stimulating hormone (TSH) causes the thyroid to release hormones. 3) Growth hormone (GH) acts on all cells for protein synthesis and growth. 4) Follicle-stimulating hormone (FSH) and 5) Luteinizing hormone (LH) act on the gonads.
6) Prolactin causes milk production, parental behavior, affects skin, and stimulates growth.
The pineal gland is also endocrine. It releases melatonin. We still have a lot to learn about it, but we know that it is involved in daily cycles, reproduction and skin color. It is closely associated with the parietal eye and is connected via the parietal nerve.
The adrenal gland is really 2 glands in 1 in tetrapods, but is 2 separate glands in other vertebrates. The adrenal gland of mammals has 2 distinct parts - cortex and medulla. In reptiles and birds the distinction is not clear anatomically.
The adrenal medulla or chromaffin bodies of lower vertebrates is where some preganglionic neurons of the autonomic nervous system terminate. The adrenal medulla or chromaffin bodies make adrenaline and noradrenaline.
Glands Derived from Mesoderm
In fishes and amphibians the equivalent of the adrenal cortex is called the interrenal organ. It may be diffuse cells sitting along the major vessels or a discrete gland as in anurans. Regardless of its name it is derived from mesoderm.
The adrenal cortex/interrenal organ secretes adrenocorticosteroids. This is really many hormones. As a group they function in metabolism of carbohydrates, proteins, salt and water. ACTH stimulates the adrenal cortex.
The cortex of mammals is subdivided into 3 zones. The outer zona glomerulosa, the middle zona fasciculata and the inner zona reticularis.
The gonads are also endocrine, but their endocrine function is strictly reproductive. The gonads are distinct organs. The ovary produces estrogen and progesterone. Estrogen is involved in growth and development of the female reproductive tract. It also produces secondary sex characteristics.
In non-mammals after the eggs are laid sex hormones aren't needed for reproduction. In mammals the embryo has to be supported. The corpus luteum the secretes progesterone which helps to maintain the uterus during pregnancy.
The placenta takes over production of estrogen and progesterone as the corpus luteum degenerates.
The testis produces androgens. These are used for the development of the male reproductive tract and secondary sex traits and sexual behavior.
Glands Derived from Endoderm
The thyroid gland sits in the neck region near the 2nd pharyngeal pouch in the embryo. It is derived from the digestive tract. At the microscopic level it is filled with follicles surrounded by a single cell layer. Secretions fill the follicles. They contain thyroglobin. This is converted to triiodothyronine (T3) and thyroxine (T4). It functions in growth, amphibian metamorphosis, thermogenesis and metabolic rate.
In agnathans and teleosts it is diffuse. In other vertebrates it is single or paired.
The parathyroid gland sits next to the thyroid in tetrapods. Fishes may have an equivalent structure, but little is known. The hormone produced is parathormone. It deals with calcium and phosphorus balance.
Ultimobranchial bodies of agnathans -> aves and immature mammals or parafollicular cells of adult mammals are located near or in the thyroid. They produce calcitonin which affects calcium metabolism.
The pancreas is both endocrine and exocrine. The islets of Langerhans are the endocrine tissue. ß - cells of the islets secrete insulin. It decreases blood glucose by facilitating uptake by the cells. - cells of the islets secrete glucagon. It increases blood glucose by stimulating glucose release from the liver.
The pancreas is derived from the wall of the embryonic gut. It starts out as a dorsal diverticulum and a ventral diverticulum. In fishes the 2 parts never join and the pancreas is not a discrete structure. Rather it is diffuse and lies along the midline. Tetrapods have a distinct pancreas and the dorsal and ventral parts fuse.
A lot of organs produce hormones, but are not really considered endocrine organs. The kidney produces erythropoietin which causes red blood cell formation and release. It also produces renin which affects blood pressure.
The digestive tract produces a number of hormones that aid in digestion. Although all vertebrates seem to have these they have only been studied in mammals.
Mammals produce prostaglandins. They have all sorts of actions including smooth muscle contraction, blood vessel constriction and dilation and induce inflammation. All cells seem to produce them.
Last updated on 19 Apr 2000
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