Comparative Anatomy: Nervous System Flashcards

The Nervous System

8.0 What is the Nervous System?

• The nervous system has two divisions:

  • Central nervous system: brain and spinal cord

  • Peripheral nervous system: network of nerves and neural tissues branching throughout the body.

8.1 Types of Neurons

• Neurons: nerve cells, structural and functional units of the nervous system.

• They conduct impulses, enabling the body to interact with internal and external environments.

• Neuroglia: tissues that support nerve cells.

• Motor Neurons:

  • Typically have one axon and several dendrites.

  • Axon: transmits signals long distances, covered by a myelin sheath (insulating fatty layer).

  • Dendrites: short and unsheathed.

  • Function: cause muscle contractions and control secretions from glands and organs.

• Sensory Neurons:

  • Do not have true dendrites.

  • Attach to sensory receptors and transmit impulses to the central nervous system.

  • The central nervous system stimulates interneurons, which then stimulate motor neurons.

• Interneurons:

  • Located entirely within the central nervous system.

  • Intercept impulses from sensory neurons and transmit signals to motor neurons.

8.2 Nerve Fibers

• Nerve fibers in the central nervous system lack Schwann cells, so damage is not reversible.

• A bundle of nerve fibers is called a nerve.

  • Afferent nerves: conduct impulses to the central nervous system.

  • Efferent nerves: conduct impulses to muscles, organs, and glands.

8.3 Synapses

• Nerve impulses are transmitted via synapses (branches).

• Synapses connect dendrites and axons from one neuron to another.

• The number of synapses influences transmission, decreasing with disease, lack of stimulation, drug use, etc.

8.4 Gray and White Matter of the Central Nervous System

• The brain and spinal cord receive impulses, process information, and respond.

• Gray matter: unsheathed nerve fibers in the cortex (surface layer); cannot be regenerated if damaged.

• White matter: myelinated nerve fibers in the internal structure.

8.5 Brain and Spinal Cord Meninges

• The brain and spinal cord are enclosed by three membranes (meninges).

  • Dura mater: outside membrane.

  • Arachnoid: middle membrane.

  • Pia mater: inside membrane.

8.6 Parts of the Brain: The Cerebrum

• The cerebrum takes up 7/8ths of the brain's weight.

• Governs all sensory and motor activity, including sensory perception, emotions, consciousness, memory, and voluntary movements.

• Divided into left and right hemispheres.

• The surface (cortex) is arranged in bulging folds (gyri) and furrows (sulci).

• Cortex Lobes:

  • Frontal lobe: motor activity, personality, and speech.

  • Parietal lobe: language, temperature, pressure, and touch interpretation.

  • Temporal lobe: centers for hearing, smell, and language input.

  • Occipital lobe: specialized in vision.

8.7 Parts of the Brain: The Cerebellum

• The cerebellum is the second largest part of the brain.

• It contains nerve fibers that connect it to every part of the central nervous system.

• Coordinates voluntary and involuntary movements and maintains posture.

8.8 Parts of the Brain: The Diencephalon

• The diencephalon includes the thalamus and hypothalamus.

• Thalamus: relay center for all sensory impulses (except olfactory) and motor areas of the cortex.

• Hypothalamus:

  • Regulates behavior and emotional expression.

  • Regulates body temperature and metabolic activities.

  • Attached to the pituitary gland; controls hormonal secretions.

8.9 Parts of the Brain: The Brainstem

• The brainstem consists of the midbrain, pons, and medulla oblongata.

• Midbrain: gray matter controls visual reflexes and sense of hearing.

• Pons: white matter regulates visceral (internal organ) control.

• Medulla oblongata:

  • Connects the rest of the brain to the spinal cord.

  • Regulates breathing, swallowing, coughing, sneezing, vomiting, heartbeat, and blood pressure.

8.10 The Spinal Cord

• The spinal cord has 31 segments:

  • 8 cervical segments (C1-C8 vertebrae).

  • 12 thoracic segments (T1-T12 vertebrae).

  • 5 lumbar segments (L1-L5 vertebrae).

  • 5 sacral segments (S1-S5 vertebrae).

  • 1 coccygeal segment.

• The 44 cm long spinal cord is shorter than the spinal column, so segments do not perfectly correspond to the vertebrae.

• At each segment, left and right pairs of sensory and motor nerves branch out to the peripheral nervous system.

• Impulses travel back and forth to the brain and muscles.

8.11 Cerebrospinal Fluid

• Colorless fluid produced in the ventricles of the brain.

• Surrounds the brain and spinal cord.

• Cushions the brain and cord from shocks.

• Maintained at a level around 1/2 - 2/3 cup.

8.12 Peripheral Nervous System

• The network of nerves branching out throughout the body from the brain and spinal cord.

• Includes 31 pairs of spinal nerves and 12 pairs of cranial nerves:

  1. Olfactory nerve: smell.

  2. Optic nerve: vision.

  3. Oculomotor nerve: eye and eyelid muscles.

  4. Trochlear nerve: eyeball.

  5. Trigeminal nerve: face, nose, mouth, forehead, top of head, and jaw.

  6. Abducens nerve: eyeball.

  7. Facial nerve: face and scalp muscles, tongue (taste).

  8. Auditory or cochlear nerve: hearing and equilibrium.

  9. Glossopharyngeal nerve: saliva, swallowing, and taste.

  10. Vagus nerve: heart, lungs, stomach, and intestines.

  11. Accessory nerve: head and shoulder movement.

  12. Hypoglossal nerve: tongue muscles.

8.13 Autonomic Nervous System

• Part of the peripheral nervous system.

• Controls involuntary bodily functions (sweating, gland secretions, blood pressure, heart).

• Divisions:

  • Sympathetic: 'flight or fight' responses (increased alertness, metabolic rate, respiration, blood pressure, heart rate, sweating; decreased digestive and urinary function).

  • Parasympathetic: counteracts sympathetic responses, restoring homeostasis.

ANIMAL SENSORY SYSTEM

8.14 Sensations

• Result from stimuli that initiate afferent impulses.

• Eventually reach a conscious level in the cerebral cortex.

• All sensations involve receptor organs.

• Simplest receptor organs are bare nerve endings.

  • Pain

  • Temperature

  • Pressure

  • Touch

• Special Senses

  • Sight

  • Hearing

  • Taste

  • Smell

  • Orientation in space

8.15 Receptors

• Exteroceptors

  • Detect stimuli near outer body surface

    • Cold

    • Warmth

    • Touch

    • Pressure

  • Special senses

    • Hearing

    • Vision

• Interoceptors

  • Detect stimuli from inside the body

    • Taste

    • Smell

    • pH

    • Distension

    • Spasm

    • Flow

• Proprioceptors

  • Detect stimuli deep within the body

    • Located in skeletal muscles, tendons, ligaments, and joint capsules.

    • Provide information to CNS on posture, orientation in space, pressure, etc.

    • Fibers are heavily myelinated for rapid transmission.

8.16 Sensory Receptor

• Peripheral component of an afferent axon and the centrally located nerve cell body of that axon.

• Convert different types of energy into nerve signals (sound, light, thermal, chemical, and mechanical).

• Generally receptors are specific and only respond to one form of energy.

8.17 Pain Receptor

• Receptors are termed nociceptors

  • Bare nerve endings of pain neurons

  • Pain stimulus causes cell damage resulting in firing of the neuron

  • Essentially a chemoreceptor

  • Either myelinated or unmyelinated

  • Myelinated fibers have a short lag time between stimulus and reaction "bright quality"

8.18 Visceral Pain

• Pain can arise from organs in abdominal cavity (viscera)

  • Peritonitis and pleuritis are two examples

  • Intestinal pain is another

  • Heart can also be a source of pain

8.19 Referred Pain

• Usually felt on surface of the body, but source is deep within viscera

• Caused by convergence of cutaneous and visceral pain afferent fibers on the same neuron in the sensory pathway

• Example (traumatic pericarditis) hardware

• Pressure applied to withers causes pain response

8.20 Taste

• Sense of taste is called gustation

• The receptor organ is the taste bud

• Taste buds are found on the tongue, palate, pharynx, and larynx.

• Taste buds have gustatory cells and supporting cells.

• Gustatory cells are receptors for taste.

8.21 Taste Reception

• Taste bud pit communicates with the oral cavity by way of the pore.

• Any substance tasted must get into solution and enter the pore of the taste bud.

• Hair of the gustatory cell is affected causing stimulation of the gustatory cell.

• The impulse is transmitted by cranial nerves VII and IX to the brain.

8.22 Taste Sensations

• Classified as salty, sweet, bitter, or sour.

• Each taste sensation is some combination of the above.

• Taste perception by animals is based on preference.

• Considerable variation within a species

8.23 Temperature and Taste

• In humans, the temperature of a beverage or food markedly affects its taste.

• In humans, the temperature of a beverage or food markedly affects its taste.

8.24 Smell

• As evolution progressed, nerve cell bodies migrated centrally so that only the nerve fibers remained in a peripheral position.

• This provided protection for nerve cells, which do not regenerate.

• Central migration did not occur for the nerve cell bodies of Cranial Nerve I (olfactory).

• Cell bodies of Cranial Nerve I are found in the mucous membrane of the nasal cavity.

• This location is known as the olfactory region.

• The size of this region is directly related to the development of the sense of smell.

• Dogs can detect substances 1:1000 of that detectable by humans.

• Sensation of smell is known as olfaction.

• Animals with greatly developed sense of smell are macrosmatic.

• Animals with less developed sense are microsmatic. (e.g. humans and monkeys)

• Animals with no sense of smell are anosmatic. (many aquatic animals)

• Each olfactory receptor has a cell body and a nerve fiber extending from each end. One is an axon and the other a dendrite.

• The dendritic process of the olfactory cell extends to the outside of the olfactory region membrane in crevices between sustentacular cells.

• Sustentacular cells provide major support to the dendritic processes and shield the nerve cell body from the olfactory cavity.

• Dendritic processes form hair-like structures (olfactory cilia) that extend into the nasal cavity.

• Cilia are covered with secretions from the glands of Bowman.

• Ducts from the glands of Bowman lead through the epithelium of the nasal cavity to its surface.

• Secretions constantly refresh the thin layer of fluid bathing the olfactory cilia.

• Sniffing allows for back-and-forth movement of air, providing a greater chance for substances to go into solution.

• Once the compound is in solution it binds to olfactory cilia and provides a stimulus for the impulse to be transmitted.

• Axons of the olfactory cells join and proceed as fibers and branches of the olfactory nerves.

• Basal cells divide and become sustentacular cells or olfactory cells replacing those lost.

• It is unlikely that a specific olfactory cell exists for each smell.

• It is probable that the basic smells combine to provide the sensation of a particular odor.

• Only one odor can be perceived at any one time.

• Olfactory cells adapt to odors.

8.25 Pheromones

• Animals use odors to communicate with each other.

• A chemical secreted by one animal which influences the behavior of another is called a pheromone.

• Pheromones are used to identify species, mark territories, emit alarms, mark food location, and identify animals in estrus.

8.26 Ear/Hearing

• Outer visible part

• Tube (external acoustic meatus) which extends from the pinna into the substance of the skull to the middle ear (tympanic cavity)

• Varying degrees of muscle attachment lend movement to the ear

8.27 Middle Ear

• Middle ear separated by inner ear by membranes that close the vestibular (oval) window and cochlear (round) window

• Middle ear communicates with the pharynx by way of the auditory tube (Eustachian tube)

• Auditory tube allows for pressure equalization

• Within the middle ear, a mechanical linkage is provided between the tympanic membrane and the membrane closing the vestibular window by three auditory ossicles (bones).

  • Incus - hammer

  • Malleus - anvil

  • Stapes stirrup

• Amplification of sound waves is provided by leverage of the ossicles and by the greater surface area of the tympanic membrane which transmits sound to the smaller surface area of the vestibular window

• Excessively loud sounds are dampened by two skeletal muscles (tensor tympani and the stapedius)

8.28 Inner Ear

• Can be divided into two parts according to function

  • Vestibular portion which is sensory for position and balance and receives branch of cranial nerve VIII (vestibulocochlear)

  • Cochlear portion which is sensory for sound and receives the cochlear nerve, a branch of cranial nerve VIII

• Contained with a bony excavation known as the osseous labyrinth ('labyrinth' referring to an intricate combination of passages)

• Because the cochlea is coiled, it can occupy limited space. An uncoiled cochlea would project into the brain.

• Vestibular portion is housed in the parts of the osseous labyrinth called the vestibule and three semicircular canals

  • Anterior, lateral, and posterior

• Each canal leaves and returns to the vestibule

• Cochlear portion housed mostly in the cochlear portion of the osseous labyrinth

• Within the osseous labyrinth is a membranous labyrinth, which is a completely enclosed connective tissue structure

  • Contains a fluid known as endolymph (composition is similar to intracellular fluid)

  • Outside membranous labyrinth and within osseous labyrinth is another fluid known as perilymph (composition similar to spinal fluid)

• Within the vestibular portion, the membranous labyrinth also includes three semicircular canals and two sacs within the vestibule known as the utricle and saccule.

• As each membranous labyrinth occupying the semicircular canals leaves the utricle, a dilated portion is noted - the ampula

• Each of the three ampullae contains sensory receptors for equilibrium known as the crista ampullaris. The utricle and saccule each contains a sensory receptor area known as the macula.

  • Macula receptors are more or less stimulated depending on the position of the head in space.

  • The cristae are stimulated during head movement

• Extension of the membranous labyrinth into the cochlea is known as the cochlear duct or scala media. This divides the cochlea into a part above the scala media (scala vestibuli) and a part below (scala tympani).

• Along the length of the scala media are a large number of structures each individually called an organ of corti.

  • Convert sound waves to nerve impulses

  • Location of organ of corti within scala media determines frequency of sound perceived

  • Organ of cortis is composed of hair cells that have hairs projecting toward the tectorial membrane. Displacement of the hair cell cilia against the tectorial membrane by oscillations of the basilar membrane causes the hair cells to depolarize and create a nerve impulse.

8.29 Summary of Sound Reception

• Sound waves are directed into the external auditory meatus by the pinna.

• Sound wave strikes the tympanic membrane (eardrum) and sets it in motion.

• The motion of the eardrum is transmitted through the middle ear by the auditory ossicles to the vestibular (oval) window.

• The stapes moves back and forth pushing the membrane of the oval window in and out.

• The movement of the oval window sets up fluid pressure waves in the incompressible perilymph of the cochlea.

• Pressure waves are transmitted through the scala vestibuli.

• The pressure waves deform the walls of the scala vestibuli, scala tympani, and vestibular membrane, resulting in fluctuating increasing and decreasing pressure of the endolymph in the cochlear duct.

• Endolymph pressure fluctuations move the basilar membrane, resulting in vibrations that cause the hair cells of the spiral organ to move against the tectorial membrane. The bending of microvilli results in receptor potentials that ultimately lead to nerve impulses.

• Pressure waves eventually cause the round window to bulge into the middle ear.

8.30 The Eye/Sight

• Protected by orbital socket of skull, eyebrows, eyelashes and eyelids.

• Bathed in fluid from Lacrimal Glands

  • (tears empty into nasal cavity)

  • Conjunctiva - thin membrane that lines the eyelids and covers part of the eye.

• Wall of Eye is made up of 3 layers or coats

  • Sclera, Choroid Coat, and the Retina

• SCLERA

  • Outer layer

  • White of the eye

  • Tough, fibrous capsule helps maintain the shape of the eye and protects the structure within.

  • EXTRINSIC MUSCLES - muscle responsible for moving the eye that are attached to the sclera.

• Cornea

  • Front of sclerotic coat (part of sclera)

  • Clear part (no blood vessels) - called the "window of the eye"

  • Transparent so light rays can pass through

  • Gets O2 and nutrients through lymph

• Choroid Coat

  • Middle layer

  • Contains blood vessels

  • Circular opening in front is the PUPIL

  • Colored, muscular layer surrounding pupil is IRIS

  • INTRINSIC MUSCLES - change size of iris to control amount of light entering through the pupil - exposed to light = constriction

• Lens

  • Crystalline structure located behind iris and pupil - focuses light rays on retina - function is accommodation -= adjusting for near / far vision

  • Elastic, disc-shaped, biconvex

  • Situated between the anterior and posterior chambers

• ANTERIOR CHAMBER filled with AQUEOUS HUMOR, a watery fluid.

• POSTERIOR CHAMBER filled with transparent, jellylike substance -VITREOUS HUMOR

• Both substances maintain eye's spherical shape and refract light rays

• Retina

  • Innermost layer - 3rd coat of the eye - located between the posterior chamber and choroid coat

  • Light rays focus an image on the retina

  • The image then travels to the cerebral cortex via the OPTIC NERVE.

• If light rays do not focus correctly on the retina, the condition may be corrected with properly fitted contact lenses, or eyeglasses, which bend the light rays as required.

  • Retina contains specialized cells, visual receptors - rods and cones

• RODS- sensitive to dim light…problems driving at night….damage to rods

• CONES - sensitive to bright light -responsible for color vision

• OPTIC DISC- on the retina, known as the

• blind spot- nerve fibers gather here to form the optic nerve, no rods or cones.

8.31 Why do dogs and cats have better hearing and smell?

• Predatory Adaptations: Enhanced hearing and smell are crucial for hunting, as they help them detect prey even from a distance.

• Sensory Cortices: They have larger olfactory bulbs and more developed sensory cortices in their brains which means they have more neurons dedicated to processing smells and sounds, allowing for more acute senses

• Social Communication: Dogs use scent to mark territories, identify other animals, and even detect emotional states. Cats rely on scent for communication but to a lesser extent.

• Survival Instincts: Enhanced hearing and smell help them detect potential dangers, such as predators or approaching threats, allowing them to react quickly and appropriately to ensure survival.

• Evolutionary Selection: Over millennia, animals with better senses of hearing and smell would have been more successful in hunting, avoiding predators, and finding mates, thus passing on these advantageous traits to future generations through natural selection.

8.32 Why do bats have better vision at night?

• Bats don't have better vision at night; they rely primarily on echolocation rather than vision for navigation and hunting in low-light conditions.

• Echolocation is a biological sonar system that bats use to navigate and locate prey in darkness by emitting high-frequency sound waves and interpreting the echoes that bounce back from objects in their environment.