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Neurogliaalso known as glial cells•• non-neuronal cells thatmaintain homeostasis,form myelin, and providesupport and protection forthe brains neuronsi.e. astocyte,•oligodendrocyte andmicroglia
Astrocyte biochemical support of endothelial cells that form the • blood-brain barrier • provision of nutrients to the nervous tissue • maintenance of extracellular ion balance with principal role in the repair and scarring process of the • brain and spinal cord following traumatic injuries.
Oligodendrocyte insulation of axons in the central nervous system (the brain • and spinal cord) of higher vertebrates • provision of nutrients to the nervous tissueMicroglia the resident macrophages of the brain and spinal cord, and • thus act as the first and main form of active immune defense in the central nervous system
Patterns of Organization of Nervous System• Nerve nets
Patterns of Organization of Nervous System• with cephalization come more complex nervous systems
Nature of Nerve Signals• every cell has a voltage or membrane potential across its plasma membranes• a membrane potential is a localized electrical gradient across membrane – anions are more concentrated within a cell – cations are more concentrated in the extracellular fluid
• Measuring Membrane Potentials • an unstimulated cell usually has a resting potential of -70mV
• How a Cell Maintains a Membrane Potential – Cations • K+ the principal intracellular cation • Na+ is the principal extracellular cation – Anions • proteins, amino acids, sulfate, and phosphate are the principal intracellular anions • Cl– is principal extracellular anion
• Ungated ion channels allow ions to diffuse across the plasma membrane – these channels are always open• this diffusion does not achieve an equilibrium since Na-K pump transports these ions against their concentration gradients
• changes in membrane potential of a neuron give rise to nerve impulses• excitable cells have the ability to generate large changes in their membrane potentials – gated ion channels open or close in response to stimuli • the subsequent diffusion of ions leads to a change in the membrane potential
• Types of gated ions: – chemically-gated ion channels open or close in response to a chemical stimulus – voltage-gated ion channels open or close in response to a change in membrane potential
• Graded Potentials: Hyperpolarization and Depolarization – graded potentials are changes in membrane potential
• The Action Potential: All or Nothing Depolarization – if graded potentials sum to -55mV a threshold potential is achieved • triggers an action potential – Axons only
• In the resting state closed voltage-gated K+ channels open slowly in response to depolarization• Voltage-gated Na+ channels have two gates – closed activation gates open rapidly in response to depolarization – open inactivation gates close slowly in response to depolarization
• nerve impulses propagate themselves along an axon• the action potential is repeatedly regenerated along the length of the axon
• Saltatory conduction – in myelinated neurons only unmyelinated regions of the axon depolarize • thus, the impulse moves faster than in unmyelinated neurons
• Electrical Synapses – action potential travels directly from the presynaptic to the postsynaptic cells via gap junctions• Chemical Synapses – more common than electrical synapses – postsynaptic chemically-gated channels exist for ions such as Na+, K+, and Cl- • depending on which gates open the postsynaptic neuron can depolarize or hyperpolarize
Neurotransmitters• Acetylcholine – excitatory to skeletal muscle – inhibitory to cardiac muscle – secreted by the CNS, PNS, and at vertebrate neuromuscular junctions
• Biogenic Amines – Epinephrine and norepinephrine • can have excitatory or inhibitory effects • secreted by the CNS and PNS • secreted by the adrenal glands epinephrine norepinephrine
• Dopamine – generally excitatory; may be inhibitory at some sites • widespread in the brain • affects sleep, mood, attention, and learning – secreted by the CNS and PNS – a lack of dopamine in the brain is associated with Parkinson’s disease – excessive dopamine is linked to schizophrenia
Parkinson’s disease • degenerative disorder of the central nervous system that often impairs the sufferers motor skills, speech, and other functions • characterized by muscle rigidity, tremor, postural abnormalities, gait abnormalities, a slowing of physical movement (bradykinesia) and a loss of physical movement (akinesia) in extreme cases
Schizoprenia mental disorder characterized by a disintegration of the • process of thinking and of emotional responsiveness auditory hallucinations, paranoid or bizarre delusions, or • disorganized speech and thinking, and it is accompanied by significant social or occupational dysfunction
• Serotonin – generally inhibitory • widespread in the brain • affects sleep, mood, attention, and learning – secreted by the CNS
• Amino Acids – Gamma aminobutyric acid (GABA) • inhibitory • secreted by the CNS and at invertebrate neuromuscular junctions – Glycine • inhibitory • secreted by the CNS
• Amino Acids – Glutamate • excitatory • secreted by the CNS and at invertebrate neuromuscular junctions – Aspartate • excitatory • secreted by the CNS
• Neuropeptides – Substance P • excitatory • secreted by the CNS and PNS – Met-enkephalin (an endorphin) • generally inhibitory • secreted by the CNS
• Gases that act as local regulators – Nitric oxide – Carbon monoxide
Vertebrate Nervous System
• A ganglion is a cluster of nerve cell bodies within the peripheral nervous system.• A nucleus is a cluster of nerve cell bodies within the central nervous system.
44 Cranial and Spinal Nerves
• Brain and spinal cord – central canal is continuous with ventricles; contain cerebrospinal fluid (CSF) – white matter is composed of bundles of myelinated axons – gray matter consists of unmyelinated axons, nuclei, and dendrites
• A Simple Nerve Circuit – the Reflex Arc in Vertebrates – A reflex is an autonomic response
(Foramenof Monro) (Opticoel) (Aqueduct of Sylvius or Iter)
– functions in homeostasis, coordination of movement, conduction of impulses to higher brain centers– relays information to and from higher brain centers
• Midbrain – contains nuclei involved in the integration of sensory information • superior colliculi are involved in the regulation of visual reflexes • inferior colliculi are involved in the regulation of auditory reflexes
• Medulla oblongata – contains nuclei that control visceral (autonomic homeostatic) functions – breathing – heartbeat and blood pressure – swallowing – vomiting digestion• Pons – – contains nuclei involved in the regulation of visceral activities such as breathing
Cerebellum• functions for coordination of motor activities, and perceptual and cognitive factors• relays sensory information about joints, muscles, sight, and sound to the cerebrum.• coordinates motor commands issued by the cerebrum
pineal gland– Epithalamus • includes a choroid plexus and the pineal gland
thalamus• relays all sensory information to the cerebrum• relays motor information from the cerebrum• receives input from the cerebrum• receives input from brain centers involved in the regulation of emotion and arousal
hypothalamus• regulates autonomic activity – contains nuclei involved in thermoregulation, hunger, thirst, and sexual and mating behavior – regulates the pituitary gland • in mammals, the hypothalamic suprachiasmatic nuclei (SCN) function as a biological clock
Cerebrum (outer covering of gray matter)
• association areas (where sensory information is integrated and assessed and motor responses are planned)
• Lateralization of Brain Function – The left hemisphere • specializes in language, math, logic operations, and the processing of serial sequences of information, and visual and auditory details • specializes in detailed activities required for motor control – The right hemisphere • specializes in pattern recognition, spatial relationships, nonverbal ideation, emotional processing, and the parallel processing of information
• Language and Speech – Broca’s area • usually located in the left hemisphere’s frontal lobe • responsible for speech production – Wernicke’s area • usually located in the right hemisphere’s temporal lobe • responsible for the comprehension of speech – Other speech areas are involved in generating verbs to match nouns, grouping together related words, etc
The Limbic System - hippocampus - olfactory cortex - inner portions of the cortex’s lobes - parts of the thalamus and hypothalamus
The Limbic System • mediates basic emotions (fear, anger), involved in emotional bonding, establishes emotional memory – e.g., the amygdala is involved in recognizing the emotional content of facial expression
• Memory and Learning – short-term memory stored in the frontal lobes – establishment of long-term memory involves the hippocampus
• The transfer of information from short-term to long-term memory – enhanced by repetition – influenced by emotional states mediated by the amygdala – Influenced by association with previously stored information.
Cranial Nerves• nerves that emerge directly from the brain• In humans, there are 12 pairs of cranial nerves • 1st and 2nd pair – cerebrum • 3rd – 12th pair – brainstem
Sensory Systems• sensations begin as different forms of energy that are detected by sensory receptors – energy is converted to action potentials that travel to appropriate regions of the brain
• Sensations are action potentials that reach the brain via sensory neurons.• Perception is the awareness and interpretation of the sensation.
• Sensory reception begins with the detection of stimulus energy by sensory receptors. – Exteroreceptors detect stimuli originating outside the body. – Interoreceptors detect stimuli originating inside the body. – Sensory receptors convey the energy of stimuli into membrane potentials and transmit signals to the nervous system. • involves sensory transduction, amplification, transmission, and integration.
• Sensory Transduction – conversion of stimulus energy into a change in membrane potential – Receptor potential: a sensory receptor’s version of a graded potential
• Amplification – the strengthening of stimulus energy that can be detected by the nervous system
• Transmission – the conduction of sensory impulses to the CNS – some sensory receptors must transmit chemical signals to sensory neurons • the strength of the stimulus and receptor potential affects the amount of neurotransmitter released by the sensory receptor – some sensory receptors are sensory neurons • the intensity of the receptor potential affects the frequency of action potentials
• Integration – the processing of sensory information. • begins at the sensory receptor – sensory adaptation is a decrease in responsiveness to continued stimulation – the sensitivity of a receptor to a stimulus will vary with environmental conditions
Categories of Sensory Receptors
• Mechanoreceptors respond to mechanical energy. – muscle spindle is an interoreceptor that responds to the stretching of skeletal muscle. – hair cells detect motion Pacinian corpuscle – mechanoreceptor in the skin that detects pressure and vibration
• Pain receptors = nocioceptors – different types of pain receptors respond to different types of pain – Prostaglandins increase pain by decreasing a pain receptor’s threshold
• Thermoreceptors respond to heat or cold – respond to both surface and body core temperature
• Chemoreceptors respond to chemical stimuli. – general chemoreceptors transmit information about total solute concentration – specific chemoreceptors respond to specific types of molecules – internal chemoreceptors respond to glucose, O2, CO2, amino acids, etc. – external chemoreceptors are gustatory receptors and olfactory receptors
• Electromagnetic receptors respond to electromagnetic energy – Photoreceptors respond to the radiation we know as visible light – Electroreceptors: some fish use electric currents to locate objects
Photoreceptors and Vision• Eye cups are among the simplest photoreceptors – detect light intensity and direction — no image formation – the movement of a planarian is integrated with photoreception
• Image-forming eyes – compound eyes of insects and crustaceans. • Each eye consists of ommatidia, each with its own light-focusing lens.
• Single-lens eyes of invertebrates such as jellies, polychaetes, spiders, and mollusks – the eye of an octopus works much like a camera and is similar to the vertebrate eye
• Accommodation is the focusing of light in the retina. – In squid, octopuses, and many fish this is accomplished by moving the lens forward and backward.
– In mammals, accommodation is accomplished by changing the shape of the lens
• Photoreceptors of the human retina – About 125 million rod cells – About 6 million cone cells
• Rhodopsin (retinal + opsin) is the visual pigment of rods.• The absorption of light by rhodopsin initiates a signal- transduction pathway.
• Visual processing begins with rods and cones synapsing with bipolar cells – Bipolar cells synapse with ganglion cells• Visual processing in the retina also involves horizontal cells and amacrine cells
• Lateral pathways: – Photoreceptors horizontal cells other photoreceptors. • Results in lateral inhibition. – More distance photoreceptors and bipolar cells are inhibited sharpens edges and enhances contrast in the image. – Photoreceptors bipolar cells amacrine cells ganglion cells. • Also results in lateral inhibition, this time of the ganglion cells.
• The optic nerves of the two eyes meet at the optic chiasm. – Where the nasal half of each tract crosses to the opposite side.• Ganglion cell axons make up the optic tract. – Most synapse in the lateral geniculate nuclei of the thalamus. • Neurons then convey information to the primary visual cortex of the optic lobe.
Hearing and Equilibrium
• Vibrations in the cochlear fluid basilar membrane vibrates hair cells brush against the tectorial membrane generation of an action potential in a sensory neuron.
• Pitch is based on the location of the hair cells that depolarize.• Volume is determined by the amplitude of the sound wave.
• the inner ear also contains the organs of equilibrium
• Statocysts are mechanoreceptors that function in an invertebrate’s sense of equilibrium.
• sound sensitivity in insects depends on body hairs that vibrate in response to sound waves – different hairs respond to different frequencies• many insects have a tympanic membrane stretched over a hollow chamber
Chemoreception: Taste and Smell• taste receptors in insects are located on their feet
• In mammals, taste receptors are located in taste buds, most of which are on the surface of the tongue.• Each taste receptor responds to a wide array of chemicals.
• Sensory receptors transduce stimulus energy and transmit signals to the nervous system
• In mammals, olfactory receptors line the upper portion of the nasal cavity – the binding of odor molecules to olfactory receptors initiate signal transduction pathways