The Nervous System
Santiago Ramón y Cajal- The watershed of all studies of the nervous system (1889).
He states that nervous system is composed of individual units (nerve cell) that are structurally independent of one another and whose internal contents do not come into direct contact.
Neuron theory each nerve cell communicates with others through contiguity rather than continuity.
Communication between adjacent but separate cells must take place across the space and barriers separating them.
The Nervous system is the controlling, regulatory, and communicating system in the body.
It transmit electrical impulses from one region of the body to another along specialized nerve cells and neurons.
• An impulse is a sudden change in the electric potential of the cell membrane.
Environment
• External environment – light, temperature, sound, motion, odor, etc.
• Internal environment – organs of the body
STIMULUS AND RESPONSE
STIMULUS
• anything in the environment (light, water, heat, pressure, wind, touch, etc) that triggers a physiological change in an organism
RESPONSE
• the corresponding reaction to an environmental stimulus. In the long run, a series of responses will enable an organism to adapt and survive
Forms and Functions of the Nervous System:
Stimulus-response coordination – Vary between simple (irritability and taxis) and higher (chemical and nervous) forms of animals
Intracellular Systems – between cells
Organelle systems – receptors of stimulus and effectors of response
Transmission of Information in the Nervous System
First thought to be electrical in nature (like metal wire), but:
• the charge carriers in nerves are ions, not electrons, and the density of ions in the axon is much less than that of electrons in a metal wire.
• the membrane of an axon is not a perfect insulator, so that the movement of current along the axon is not complete.
• nerve fibers are smaller than most wires, so that the currents they can carry are limited in amplitude.
Transmission of synapse Electrical transmission
Nervous control under the regulation of the nervous system with its system of neuronal mechanisms; the ionic current flows directly through channels that couple the cells.
• Nervous control-is fast-acting and the effect is short-term, although frequent stimulation can be stored to produce a longer-lasting behavioral response.
Chemical transmission / control-under the regulation of the endocrine system and includes the various hormones; chemical substance called the neurotransmitter passes from one cell to the other, stimulating the second cell to generate its own action potential.
• Chemical control is slow-acting but the effect is long-term.
Endocrine Gland System
Chemical coordination of body functions is mediated by the endocrine system, composed of ductless glands that release hormones.
Hormones are chemical messengers secreted by a gland and affect a specific target tissue or organ.
The endocrine and the nervous system coordinate with each other through a series of feedback mechanisms.
A disorder results when a hormone is under- or over-secreted.
Kinds of Nervous System
• Diffuse – neurons are distributed throughout the organism in a netlike pattern, , usually beneath the outer epidermal layer.
• E. g. Hydra, jellyfish, sea anemones, and flatworms
• Centralized - Presence of ganglia or a brain of the organisms
• E.g. Grasshoppers and cats
The Types of Nerve Cell
• Neurons (85 billion to 200 billion neurons) - the fundamental units of the brain and nervous system that are responsible for receiving sensory input from the external world, for sending motor commands to our muscles, and for transforming and relaying the electrical signals at every step in between.
• The axon may be enveloped by a myelin sheath for faster conduction of impulse.
• Neurons connect with one another through a junction called synapse.
• Neuroglia (10 neurons: 1 neuroglia) - lack axons, but has only one type of process. Also, they do not form synapses, and they retain the ability to divide throughout their life span.
• The four types of neuroglia found in the central nervous system are astrocytes, microglial cells, ependymal cells, and oligodendrocytes.
Nervous systems
The basic pattern of stimulus-response coordination: receptor, adjustor, and effector units.
Neurons (receptor nerve cells)- receive external stimuli that are passed to fibers called axons or extensors
• Sensory (Afferent) – impulse for incoming excitation; Produces response Interneuron (adjustor) - receptor that selects, interprets, or modifies the input from the receptor and sends impulse to an efferent neuron, such as a motor neuron.
• Neurotransmitter – chemical released in junctions (synapses)- can be excited, inhibited, or modulated
Motor (Efferent) - outgoing; makes contact with an effector such as muscle or gland, which produces a response. This create reflex arc.
Neurotransmitter release
• LOCALIZED OR RECEPTOR POTENTIAL - physical stimulus acts on a sensory receptor cell specifically designed to respond to that stimulus, then the energy of the stimulus is transduced, or transformed, into an electrical response.
• ACTION POTENTIAL – A brief reversal of membrane potential
• DEPOLARIZATION (rising phase) – when positively charge sodium ions rush into neurons
• REPOLARIZATION (falling phase) – closing of sodium ion channels and opening of potassium channels
• HYPERPOLARIZATION - excess of open potassium channels and potassium effux from the cell.
CONDUCTION
• velocity increases at high temperatures and decreases at low temperatures
• Since ions cannot cross the lipid content of the myelin sheath, they spread passively down the nerve fiber until reaching the unmyelinated nodes of Ranvier.
• The nodes of Ranvier are packed with a high concentration of ion channels, which, upon stimulation, propagate the nerve impulse to the next node.
• Therefore, action potential jumps quickly from node to node along the fiber in a process called saltatory conduction (from Latin saltare meaning “to jump”).
NERVOUS SYSTEM OF DIFFERENT ANIMALS (Asymmetrical)
• Porifera – no nervous system; touch or pressure outside the cell cause local contractions of the body.
NERVOUS SYSTEM OF DIFFERENT ANIMALS (Radiata)
• Ctenophore and cnidarians – has diffuse nerve net (a mesh-like system of individual and separate nerve cells and Fibers dispersed over the organism).
• The development in the net of rapidly conducting bundles of Fibers and of pacemaker systems allowed rapid withdrawal and rhythmic swimming activities, respectively, in some cnidarians.
NERVOUS SYSTEM OF DIFFERENT ANIMALS (Bilatera)
• Flatworms (Platyhelminthes) - appears a longitudinal nerve cord and an anterior collection of nerve cells that can be called ganglia or brain concentrated at the cephalic (head) end. Longitudinal nerve cords, usually three to five pairs, extend posteriorly from the brain; it has seven types of nerve cell bodies and two types of neuroglia
• Nematodes (phylum Aschelminthes) - have a high degree of centralization, with three-quarters of all nerve cells concentrated in a group of anteriorly placed ganglia and no peripheral plexuses or nets. They usually have eight longitudinal cords, commissures between dorsal and ventral cords, six cephalic nerves, a few special ganglia, and nerves in the tail, and two sympathetic systems (one anterior and one posterior).
• Annelid worms – consist of small brain and a ventral nerve cord, with each segmental ganglion largely responsible for sensory and motor functions within the segment.
• Mollusks – vary among groups, but have well-centralized nervous systems dominated by large brain.
• Arthropods - brain consists of three main regions/compartmentalization: the protocerebrum, deutocerebrum, and tritocerebrum.
• Anterior protocerebrum - receives the nerves of the eyes and other organs, contains centers, or neuropils, such as the optic centers and bodies known as corpora pedunculata.
• Deutocerebrum - contains the association centers for the first antennae.
• Posterior tritocerebrum - contains association neuropils for the second antennae (of crustaceans) and gives rise to nerves that innervate the mouthparts and the anterior digestive canal.
MAIN PARTS OF NERVOUS SYSTEM OF VERTEBRATES
• Central Nervous System (CNS) – the integrative and control center; contains brain and spinal cord
• Peripheral Nervous System (PNS)- mainly of nerves, which are long fibers that connect to CNS and other part of the body.
• Motor and Sensory
• Somatic versus autonomic
• Sympathetic ganglia – mobilizes body system (fight or flight)
• Parasympathetic ganglia – conserves energy; promotes housekeeping function during rest; restores the body to a state of calm.
• Enteric nervous system - large division of the peripheral nervous system (PNS) that can control gastrointestinal behaviour
Homeostasis
The homeostatic control has three components:
• A receptor (sense organ) to detect a change
• A center of control (the brain or the spinal cord) that will process and integrate what is happening
• An effector (muscle cells or organs/ glands) to produce a response appropriate to the change.
• Negative feedback: a reaction in which the system responds in such a way as to revers the rection of change (Thermoregulation, Carbon dioxide concentration, and Blood sugar level)
• Positive feedback: a response is occurring to amplify the change in the variable. (This has a destabilizing effect, so does not result in homeostasis. Positive feedback is less common in naturally occurring systems than negative feedback, but it has its applications.) (E.g., in nerves, a threshold electric potential triggers the generation of a much larger action potential, Blood clotting, and Events in childbirth)
PLANT RESPONSE
Tropism is a biological mechanism that enables plant to move toward (positive tropism) or against (negative tropism) the source of a stimulus.
Thigmotropism / Nastic movement - Leaflets of makahiya plant closing; Venus flytrap trapping an insect.
Phototropism - Sunflower growing in the sun’s direction
Geotropism AND Hydrotropism - Roots of a plant growing underground
Hormones are chemical messengers in plants.
They regulate various biochemical and physiological responses that include seed germination, flowering, photosynthesis, fruit ripening and shoot and root development.
Sensory Receptors
Mechanoreceptors - respond to physical stimuli such as sound or touch
Thermoreceptors – response to heat
Chemoreceptors - detect chemicals
Photoreceptors - respond to light
Pain receptors - detect possible tissue damage