jimtrue.com : school : BSC2011 : CH 48, 49: Animal Systems - Nervous (pp 1022-1062)

Posted by Jim True on December 7, 2004 1:08 PM. Last Updated October 22, 2006 9:23 PM

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CH 48, 49: Animal Systems - Nervous (pp 1022-1062)

Functions

Overall function is to act as the command and control center of the vertebrate body.
Functions via nerve impulses that progressively arise from shifts in ion concentration on the immediate inside and outside of the axon membrane of a neuron.
Specific functions are associated with specific regions:

Sensory Input - Gather information from external and internal sensors via sensory nerves (afferent pathway).
Integration and Processing - Location depends on level of organization.

In invertebrates, occurs in ganglia (clusters of nerve cell bodies), in vertebrates, it occurs in the spinal cord and brain (Central Nervous System - CNS).

Sensory Input - Response commands sent to glands and muscles (effector cells) via motor nerves (efferent pathway).

Sensory Input

Stimulus - Anything that elicits a response.
Three categories of stimuli are perceived by animals:

Mechanical - Includes:

Physical pressure - air, water
Gravity - influences balance, orientation
Sound - used in location, communication
Touch - response to vibrations, pain, used in exploration

Chemical - Includes:

Taste and smell - wide variety of systemic responses elicited by these stimuli. In humans (at least), smell is STRONGLY associated with memory.

Electromagnetic - Stimuli are propagated as waves. Includes:

Light - vision and biorhythms
Heat - detection of radiated energy
Electricity - reception AND production
Magnetism - detection of, and orientation to, Earth's magnetic fields, allows for navigation and migration

Systems

P. Porifera - No nervous tissues or systems. Response to stimuli chemically triggered from cell to cell - very slow.
P. Cnidaria - Posess a "nerve net", in which there are multipolar neurons arranged throughout the body at the base of the epithelial tissue.

Impules can move bidirectionally.
Possess simple receptors for gravity. System mainly for movement and feeding/protection responses. Cubozoa has visual receptors - most advanced of the Cnidarians.

P.Platyhelminthes - Possess one or more brains formed by ganglia, with variable number of nerve cords (number of nerve cords is of taxonomic significance).

The primitive nervous sytems resembles the nerve net of cnidarians, in more advanced forms, there is a "ladder type" system of crosslinked nerve cords.
Each longitudinal cord has anterior ganglion.
Impulses are directional
In the phylum is the first evidence of cephalization, the development of a head, which is a primary sensory platform which typically meets the environment first in animals with directional movement (ie Bilateria).
Sensors include light detectors, chemosensory (taste), tactile (touch) and gravity.

P. Nematoda - Dorsal, ventral and lateral nerve cords (4 total) with circumpharyngeal ganglia.

Sensors are simple, mainly tactile.

P. Mollusca - A "ladder type" system with 2 dorsal and 2 ventral nerve cords with a ring-like brain (circling the digestive system) formed by ganglia.

Sensors include light detectors plus vision, tactile, chemosensors, balance and gravity (only lacking hearing).
Cephalopods possess the most highly integrated nervous systems among invertebrates. Have a much more discrete brain plus enlarged specialized nerves. Actually demonstrate ability to learn by observation, due it is believed to social interaction.

P. Annelida - Segmented nervous system (metameric) with ventral and dorsal nerve cords.

The brain is ring shaped around pharynx.
Sensors include light detection, chemo and tactile sensors.

P. Arthropoda - Well developed system with compartively large gangliar brain.

Usually an enlarged ventral nerve cord with branches extending into segments.
Sensors include full array including excellent vision including visual reception into the ultraviolet spectrum.

P.Echinodermata - Very simple radial arrangement of nerves, which are primarily used for control of ampullae and podia.

Sensors include light detection, gravity and chemosensors.

S.p. Vertebrata - Two main parts to system:

Central Nervous System (CNS) - Brain and Spinal cord.
Peripheral Nervous (PNS) - All nerve paths outside CNS.

Afferent nerves - Sensory pathway from PNS to CNS
Efferent Nerves - Motor (response) pathway from CNS to PNS. Efferent paths can be:

Voluntary - the motor response is to skeletal muscles.
Involuntary (AKA autonomic or automatic) - Involves visceral stimulation (glands, heart, etc.). Within the involuntary portion of the nervous system, there are two further subdivisions:

Sympathetic - Automatic responses to stress, excidted state of involuntary system. Produce the emergency "fight or flight" reactions. [Evolutionary holdover from 'prey/predator response']
Parasympathtetic - Normal body activities (body at rest).

These two subdivisions are antagonistic, ie, the more one of these influences systems, the less the other does and vice versa.

CNS Components

Skip section for Final exam (yellow background)
Spinal Cord - Only found in vertebrates, part of CNS. Consists of two internal regions surrounded by 3 connective tissue layers:

Gray Matter (GM) - Butterly shaped inner region of cord. Form the circuits for sensory nerves. Dendrites are at the sensors, cell bodies, unmyelinated axons and interneurons are in GM.

Lateral input/output at GM. If no higher processing involved, results in a reflex.
Reflex - Reaction to stimulus is immediate, involves no processing by brain. Input and output through spinal cord only.

White Matter (WM) - Surrounds GM in cord. Formed of dendrites plus myelinated axons. Links different levels of the spinal cord with each other and the brain.

Allows for vertical transmission of impulses.

Connective tissues - Three layers called the meninges. (Must be where spinal meningitis comes from).

End of Skip Section

Brain - Five regions in advanced vertebrates:

Telencephalon (Forebrain) - Olfactory lobes (smell) and cerebrum.

In mammals, the cerebrum is very large and is the site of "rational though" in humans (at least).

Diencephalon (Forebrain) - Includes the thalamus (sensory processing), the pineal gland (sets the internal body clock or biorhythm) and hypothalamus (integrations of functions such as heartbeat and respiration, plus "seat of emotions" such as joy, anger, lust, hunger, thirst, etc.).
Mesencephalon (Midbrain) - Optic lobes for input of visual stimuli.
Metencephalon (Hindbrain) - Includes the cerebellum (controls body coordination) and pons (links cerebellum to various other parts of the brain).
Myelencephalon (Hindbrain) - Formed by the medulla oblongata, which appears as an expanded portion of the spinal cord and acts as a reflex center and control center for basal metabolism (heartbeat and respiration).

Except for medulla, all other parts of brain exhibit gray matter on outside, white matter inside (opposite of spinal cord). [ Gray matter is unmyelinated axons & interneurons; White matter is dendrites and myelinated axons, providing faster response as the axons are 'insulated' by the myelin much like electrical insulation on copper wire. ]

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