Chapter 40 NERVOUS SYSTEMS

INVERTEBRATE NERVOUS SYSTEMS

Nerve nets and radial nervous systems are characteristic of radially symmetrical animals.

·  Nerve nets consist of scattered neurons, impulses may flow in both directions of the synapse, and the impulse weakens as it spreads from the point of stimulation.

·  There is no CNS.

·  Found in cnidarians. Some cnidarians have two nerve nets, one for slow for tentacle movement, and another for faster to coordinate swimming.

·  Echinoderms have a nerve ring and nerves that extend into various parts of the body.

Bilateral nervous systems are found in bilateral animals.

·  Neurons aggregate to form ganglia, nerves, nerve cords and a brain.

Trends in the evolution of the nervous system:

  1. An increase number of neurons.
  1. Concentration of neurons to form ganglia and brain, and thick cords of tissue that become nerves and nerve cords.
  1. A specialization of function, e. g. afferent and efferent neurons transmit different type of impulse; parts of the brain perform different functions.
  1. Increase number of association neurons and complex synaptic contacts that allow better integration of incoming messages, provide a greater range and precision of responses.
  1. Cephalization with a concentration of sense organs toward the anterior end.

Planaria has a ladder-type of nervous system. The two anterior ganglia control to some extent the rest of the system.

Annelids and arthropods have one or two ventral nerve cords that extend the length of the body. An anterior pair of ganglia dorsally located is needed to respond adequately to stimuli and to coordinate input.

Mollusks have three pairs of ganglia. Each ganglion controls a different region of the body:

·  Pedal ganglia control the movement of the foot.

·  Visceral ganglia control the opening and closing of the shell.

·  Cerebral ganglion that controls complex reflexes and motor functions.

·  Pedal and visceral ganglia are connected to the cerebral ganglia by nerve cords.

·  In cephalopods, the three ganglia are clustered in a ring surrounding the esophagus in a sort of circular "brain".

DIVISIONS OF THE VERTEBRATE NERVOUS SYSTEM

brain

Central

spinal cord

Vertebrate

Nervous receptors

System Somatic afferent nerves (receptors to CNS)

efferent nerves (CNS to skeletal muscles)

Peripheral

Receptors

Autonomic afferent n. (receptors to CNS)

efferent n. (CNS to organs) sympathetic

parasympathetic

THE VERTEBRATE BRAIN

The embryonic neural tube differentiates into three regions:

  1. Presencephalon or forebrain.
  2. Mesencephalon or midbrain.
  3. Rhombencephalon or hindbrain.

These primary divisions in turn differentiate to give rise to specific structures of the adult brain.

The hindbrain develops into the medulla, pons, and cerebellum.

The midbrain is prominent in amphibians and fishes.

·  It receives sensory information, integrates it and distributes to appropriate motor nerves.

·  It is the center of vision.

In reptiles, birds and mammals it consists of

·  The superior colliculi, centers for visual reflexes, e.g. pupil constriction.

·  The inferior colliculi, center of certain auditory reflexes.

The forebrain gives rise to the thalamus, hypothalamus and cerebrum.

·  Telencephalum develops into the cerebrum.

·  Diencephalum gives rise to the thalamus and hypothalamus.

In vertebrates the thalamus receives sensory messages except olfactory messages, and distributes them to the sensory areas of the brain.

The hypothalamus forms the floor of the third ventricle and receives olfactory messages and regulates the function of internal organs, maintains homeostasis (temperature, respiration, regulation of pituitary gland, appetite, etc.)

The cerebrum of fish and amphibians is almost entirely concerned with the integration of olfactory information.

In other vertebrates it integrates olfactory and other information.

In most vertebrates, the cerebrum is divided into right and left hemispheres.

·  White matter is made of myelinated axons

·  Gray matter (cerebral cortex) is made of cell bodies and dendrites.

·  Folds of the cerebrum are called convolutions or gyri (gyrus).

·  Furrows are called sulci (sulcus) is shallow or fissures if deep.

In birds, the corpus striatum controls behavior and another part controls learning.

In reptiles and mammals, the neopallium, in the cortex, integrates sensory and motor functions and is responsible for higher functions like learning.

·  Neopallium makes the bulk of the mammalian cerebrum.

·  90% of the cortex in humans is the neopallium.

·  Made of six distinct layers.

HUMAN CENTRAL NERVOUS SYSTEM

The CNS is protected by bone: skull and vertebrae, and three membranes, the meninges: dura mater, arachnoid and pia mater.

The subarachnoid space is found between the arachnoid and the pia mater, and it contains the cerebrospinal fluid.

The choroid plexus is a network of capillaries that secretes the cerebrospinal fluid (CSF).

The plexus and the arachnoid act as a barrier between the blood and the CSF.

·  Exchanges nutrients and waste between the blood and the CNS.

·  Prevent harmful substances in the blood to enter the CNS.

·  Shock absorbing fluid.

The CSF travels through the sinuses and the cerebral ventricles.

The spinal cord.

Function so the spinal cord:

  1. Transmits impulses to the from the brain
  2. Controls many reflex activities.

The spinal cord extends from the base of the brain to the second lumbar vertebrae.

The spinal cord consists of gray matter and white matter.

·  It has a small central canal.

·  The white matter surrounds the gray matter.

·  The gray matter has the shape of an H.

·  The gray matter consists of cell bodies, dendrites and unmyelinated axons.

·  The white matter is made of myelinated axons arranged into tracts or pathways.

A reflex action or withdrawal reflex is a fixed response to a simple stimulus.

A message is also send to the cerebrum and pain, touch, etc. is felt.

Many activities such as breathing are controlled by reflex action.

The cerebrum.

The cerebrum is the largest and most prominent part of the human brain.

Cerebral cortex is made of gray matter arranged into sulci.

·  Sensory areas receive information from senses and receptors.

·  Motor areas control the movement of voluntary muscles.

·  Association areas are the site of intellect, learning, memory, language, and emotion; interprets sensory information.

The cortex has been mapped into areas responsible for certain functions:

·  Occipital lobe: visual centers.

·  Temporal lobes: auditory centers.

·  Parietal lobes receive information about heat, touch and pressure.

·  Other areas are involved in complex integrative activities.

The size of the motor area in the brain for any given part of the body is proportional to the complexity of movement involved and not to the amount of muscle.

White matter lies beneath the cerebral cortex.

·  Corpus callosum connects right and left hemispheres.

·  Axons are arranged into bundles (tracts).

The basal ganglia, a cluster of nuclei within the white matter, are important centers of motor function.

The basal ganglia send information to the substantia nigra and receive input from it.

Other neurons found in the substantia nigra relay information to the motor cortex.

BRAIN ACTIVITY

EEG or electroencephalogram is a recording of the brain's electrical activity.

Activity follows a wake-sleep pattern.

·  Alpha waves are produced during relaxed periods when the eyes are closed.

·  Beta waves are produced during heightened activity, e.g. reading a book.

·  Delta waves are produced when the person is asleep during non-REM sleep.

·  Theta waves are produced in children and in frustrated adults.

Reticular activating system (RAS) is a complex pathway within the brain stem and thalamus.

·  Maintains consciousness and determines the degree of alertness.

·  Receives messages from the spinal cord and communicates with the cerebral cortex.

Sleep is a state of unconsciousness with decrease brain activity.

·  During REM, rapid eye movement state, the eyes move rapidly with the eyelids closed, delta waves become erratic and dreams occur.

·  REM caused by a release of norepinephrine in the brain stem.

·  During REM blood flow in the frontal lobes was reduced and blood flow increased in areas that produce visual scenes and emotions.

·  During Non-REM sleep, metabolic rate and breathing slows down, blood pressure decreases and delta waves are produced.

Neurons in the RAS fatigue after many hours of activity and the sleep centers secrete serotonin, a neurotransmitter involved in sleep.

Neurophysiologists do not know why sleep is necessary.

Both non-REM and REM are necessary.

The limbic system controls emotions, sexual behavior, motivation, pain, pleasure, rhythms and autonomic responses.

·  It consists of the hypothalamus, hippocampus and amygdala.

·  The substantia nigra is also involved in motivation and release dopamine when stimulated.

·  Dopamine may be involved in concentration and attention. Its role is controversial at present.

Other areas of the brain are involved in emotions like the frontal area of the cerebral cortex.

Learning is a change in behavior that results from experience.

Memory is the storage of knowledge and the ability to retrieve it.

·  Information is transferred from sensory memory to short-term memory to long-term memory.

·  Short term memory allows recalling information for a few minutes and it may be based on reverberating circuits.

·  Information may be transferred to long term memory.

·  Long-term memory may require gene activation and formation of new synaptic connections.

·  Memories are stored throughout the sensory and association areas of the cerebrum.

·  Wernicke's area in the temporal lobe is an association area involved in language function.

Short term memory involves brief changes in neurotransmitter receptors and second messengers are linked to ion channels in postsynaptic neurons.

·  Specific protein kinases are activated by the secondary messenger cyclic AMP.

·  These kinases phosphorilate and affect specific ion channels.

In long term memory, the changes in the postsynaptic neuron are slower but longer lasting.

·  Cyclic AMP activate protein kinases which enter the nucleus and leading to gene activation.

·  The kinases phosphorylate a regulatory protein known as CREB.

·  CREB turns on the transcription of certain genes.

Some types of learning are related to permanent changes in presynaptic terminals and postsynaptic neurons.

·  Neurotransmitter release may be enhanced or inhibited.

·  Calcium ions accumulate inside the presynaptic terminal.

·  Synaptic facilitation occurs after a single action potential.

·  Potentiation is a longer form of enhancement that lasts for several minutes.

Habituation is a decrease response due to repeated exposure to a harmless stimulus.

·  Calcium channels are inactivated in presynaptic neurons.

·  It results in a decrease in neurotransmitter release.

Sensitization results in an increase response after experiencing an unpleasant stimulus.

·  The presynaptic terminals release more neurotransmitters.

·  Sensitization usually lasts for a few minutes.

Classical conditioning is an association between an unconditioned stimulus and a conditioned stimulus.

·  The conditioned stimulus provokes a conditioned response.

·  This reaction may involved a second messenger, protein kinases, gene activation and protein synthesis.

Neurons have the ability to change in response to environmental stimuli, this is known as neural plasticity.

Early environmental stimulation can enhance the development of motor areas.

PERIPHERAL NERVOUS SYSTEM, PNS.

It consists of sensory receptors and nerves.

The somatic system allows the body to adjust to external environment.

·  12 cranial nerves emerge from the brain.

·  Involved with the head region: sensory and motor.

·  31 pairs of spinal nerves.

·  Originate from the spinal cord.

·  Each has a dorsal and a ventral root.

Dorsal root forms a ganglion of sensory neurons before entering the spinal cord.

Ventral root contains motor neurons whose body is in the gray matter of the spinal cord.

The autonomic nervous system regulates the internal activities of the body.

The efferent portion of the autonomic nervous system is divided into:

1. Sympathetic NS permits the body to respond to stressful situations.

·  Preganglionic neurons, paravertebral sympathetic ganglion chain and postganglionic neurons transmit the message to the effector.

2. Parasympathetic NS restores the body to resting state, and actively maintains normal body functions.

·  Parasympathetic preganglionic neurons synapse with postganglionic neurons in ganglia near or within the walls of the effector organs.

The autonomic nervous system uses a relay of two neurons between the CNS and the effector.

Preganglionic neuron has the cell body and dendrite within the CNS.

·  Its axon is part of a peripheral nerve and synapses with the postganglionic neuron.

Postganglionic neuron is entirely outside the CNS and its axon ends near the effector.

The paravertebral sympathetic ganglion chain is found on each side of the spinal cord from the neck to the abdomen.

Some sympathetic preganglionic neurons end in ganglia of the abdomen close to the aorta and its major branches known collateral ganglia.

·  Celiac ganglion, superior mesenteric ganglion and inferior mesenteric ganglion.

Parasympathetic preganglionic neurons synapse with the postganglionic neurons in terminal ganglia near or within the wall of the organs they innervate.

DRUGS

Many drugs alter mood by increasing or decreasing the concentrations of specific neurotransmitters within the brain.

Habitual use of almost any mood-altering drug can cause psychological dependence.

Physical addiction occurs when the drug has components similar to substances manufactured by cells.

Drug addition involves a mechanism of neurons that release dopamine.

These neurons are located in the midbrain and through dopamine affect behavioral control centers in the limbic system.

Norepinephrine, serotonin and dopamine influence mood.

Alcohol metabolism occurs at a fixed rate in the liver and it is not affected by coffee.

·  Inhibits water reabsorption in the kidneys and more water is excreted as urine than consumed.

·  It results in dehydration and low blood sugar level that may cause stupor.

·  Depresses the CNS, impairs coordination and judgment, lengthens reaction time.

·  Damage to pancreas, liver and brain; physical dependence.

Many drugs induce tolerance, in which the body response to the drug decreases so that greater amounts are needed to obtain the desired effect.

·  Tolerance occurs when response to the drug decreases.

·  The liver increase of enzyme concentration that metabolize the drug.

·  Addiction is the result of psychological changes or a lowered production of a needed substance due to drug use.