Neural Control and the Senses Chapter 25

Neurons Communication units of nervous systems Detect information about internal and external conditions Issue commands for responsive actions

interneurons of brain, spinal cord Types of Neurons stimulus (output) receptors Sensory neurons Detect and relay information Interneurons Receive and process information Motor neurons Transmit signals from interneurons to effectors sensory neurons integrators interneurons of brain, spinal cord motor neurons effectors muscles, glands response (output)

Structure of a Neuron dendrites INPUT ZONE cell body axon OUPUT ZONE TRIGGER ZONE CONDUCTING ZONE axon endings

Fig. 25-1b, p.423

Neuroglia Cells that metabolically assist, structurally support, and protect neurons Make up more than half the volume of the vertebrate nervous system

Resting Membrane Potential Electrical gradient across membrane About -70 mV Maintained by sodium-potassium pump Potassium (K+) higher inside Sodium (Na+) higher outside more Na+ flows into the neuron more gated channels for Na+ open neuron becomes more positive inside

Na+ K+ outside plasma membrane inside K+ Na+ p.424a

How Ions Move across Membrane interstitial fluid cytoplasm Na+/K+ pump passive transporters with open channels passive transporters with voltage-sensitive gated channels active transporters lipid bilayer of neuron membrane

Action Potential Brief reversal in membrane potential Voltage change causes voltage-gated channels in membrane to open Inside of neuron briefly becomes more positive than outside

Action Potential 1 2 3 4 Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+

interstitial fluid cytoplasm Fig. 25-4a, p.425

Na+ Na+ Na+ Fig. 25-4b, p.425

K+ K+ K+ Na+ Na+ Na+ Fig. 25-4c, p.425

Na+/K+ pump K+ K+ K+ Na+ Na+ Na+ K+ Fig. 25-4d, p.425

Positive Feedback more Na+ ions flow into the neuron more gated channels for Na+ open neuron becomes more positive inside

All or Nothing All action potentials are the same size If stimulation is below threshold level, no action potential occurs If stimulation is above threshold level, cell always depolarizes to same level

Repolarization Once action potential peak is reached, Na+ gates close and K+ gates open Movement of K+ out of cell The inside of the cell once again becomes more negative than the outside

Recording of Action Potential +20 -20 Membrane potential (millivolts) threshold -40 resting membrane potential -70 1 2 3 4 5 Time (milliseconds)

Propagation of Action Potentials Action potential in one part of an axon brings neighboring region to threshold Action potential moves from one patch of membrane to another Can only move one direction

Chemical Synapses Action potentials cannot jump from cell to cell Signal is transmitted from axon end, across a synaptic cleft, by chemical signals called neurotransmitters

Chemical Synapse Gap between the terminal ending of an axon and the input zone of another cell plasma membrane of axon ending of presynaptic cell plasma membrane of postsynaptic cell synaptic vesicle synaptic cleft membrane receptor

Synaptic Transmission Action potential in axon ending triggers release of neurotransmitter from presynaptic cell into synaptic cleft vesicle inside presynaptic cell synaptic cleft postsynaptic cell

Synaptic Transmission Neurotransmitter diffuses across cleft and binds to receptors on membrane of postsynaptic cell Binding of neurotransmitter to receptors opens ion gates in membrane of postsynaptic cell

Ion Gates Open neurotransmitter ions receptor for neurotransmitter gated channel protein

Synaptic Integration Many signals reach a neuron at the same time Signals may suppress or reinforce one another Whether or not an action potential occurs depends on the sum of the signals the neuron receives

Neuromuscular Junction Synapse between motor neuron and skeletal muscle fiber Neuron releases chemical neurotransmitter acetylcholine (ACh)

A Neuromuscular Junction motor neuron axons from spinal cord to skeletal muscle fibers transverse slice of spinal cord part of a skeletal muscle Fig. 25-6a, p.427

A Neuromuscular Junction muscle fiber axon ending Fig. 25-6b, p.427

Neurotransmitters Acetylcholine (ACh) Norepinephrine Epinephrine Dopamine Serotonin GABA

Cleaning Up After neurotransmitter has acted, it is quickly removed from synaptic cleft Molecules diffuse away, are pumped out, or broken down

Information Flow interneuron motor neuron sensory neuron

Organization Neurons are bundled in nerves Nerves are organized in circuits and reflex pathways Information from sensory neurons is relayed to interneurons in spinal cord and brain Motor neurons carry signals to body

Nerve A bundle of axons enclosed within a connective tissue sheath myelin sheath A bundle of axons enclosed within a connective tissue sheath many neurons inside a connective tissue sheath

Myelin Sheath Sheath blocks ion movements Action potential must “jump” from node to node Greatly enhances speed of transmission

Multiple Sclerosis A condition in which nerve fibers lose their myelin Slows conduction Symptoms include visual problems, numbness, muscle weakness, and fatigue

Reflexes Automatic movements in response to stimuli In simplest reflex arcs, sensory neurons synapse directly on motor neurons Most reflexes involve an interneuron

Stretch Reflex STIMULUS Biceps stretches. sensory neuron motor neuron RESPONSE Biceps contracts.

Invertebrate Nervous Systems All animals except sponges have some sort of nervous system Nerve cells interact with one another in signal- conducting and information-processing highways

Bilateral Nervous System rudimentary brain branching nerve nerve cord ganglion (one in most body segments)

Vertebrate Development Earliest fishlike vertebrates had a hollow, tubular nerve cord Modification and expansion of nerve cord produced spinal cord and brain Nerve cord persists in vertebrate embryos as a neural tube

Central and Peripheral Nervous Systems Central nervous system (CNS) Brain Spinal cord Peripheral nervous system Nerves that thread through the body

Vertebrate Nervous Systems

Major Nerves Brain cervical nerves cranial nerves (eight pairs) (twelve pairs) Spinal Cord thoracic nerves (twelve pairs) ulnar nerve (one in each arm) lumbar nerves (five pairs) sacral nerves (five pairs) sciatic nerve (one in each leg) coccygeal nerves (one pair) Fig. 25-12, p.431

Peripheral Nervous System Somatic nerves Motor functions (Shown in green) Autonomic nerves Visceral functions (Shown in red)

Two Types of Autonomic Nerves Sympathetic Parasympathetic Most organs receive input from both Usually have opposite effects on organ

eggs optic nerve midbrain medulla oblongata salivary glands heart vagus nerve cervical nerves (8pairs) larynx bronchi lungs stomach liver spleen pancreas thoracic nerves (12 pairs) kidneys adrenal glands small intestine upper colon lower colon rectum lumbar nerves (five pairs) (all ganglia in walls of organs) (most ganglia near spinal cord) bladder sacral nerves (five pairs) uterus pelvic nerve genitals Autonomic Nervous System Fig. 25-13, p.432

Sympathetic Nerves Originate in thoracic and lumbar regions of spinal cord Ganglia are near the spinal cord Respond to stress or physical activity (fight-or-flight response)

Parasympathetic Nerves Originate in brain and sacral region of spinal cord Ganglia are in walls of organs Promote housekeeping responses such as digestion

Opposing Systems Most organs receive both sympathetic and parasympathetic signals Example: Sympathetic nerves signal heart to speed up; parasympathetic stimulate it to slow down Synaptic integration determines response

Structure of CNS White matter Gray matter Meninges Tracts with myelin sheaths Sensory and motor neurons Gray matter Unmyelinated Cell bodies, dendrites, neuroglia Meninges Protective coverings

Table 25-1, p.434

Function of Spinal Cord Expressway for signals between brain and peripheral nerves Sensory and motor neurons make direct reflex connections in spinal cord Spinal reflexes do not involve brain

Spinal Cord ventral dorsal spinal cord meninges (protective coverings) spinal nerve vertebra location of intervertebral disk Spinal Cord Fig. 25-14, p.433

The Brain corpus callosum hypothalamus thalamus pineal gland location part of optic nerve midbrain cerebellum pons medulla oblongata Fig. 25-15, p.434

Development of the Brain Brain develops from a hollow neural tube Forebrain, midbrain, and hindbrain form from three successive regions of tube Most evolutionarily ancient nervous tissue persists as the brain stem

Divisions of Brain Division Main Parts Forebrain Cerebrum Olfactory lobes Thalamus Hypothalamus Limbic system Pituitary gland Pineal gland Midbrain Tectum Hindbrain Pons Cerebellum Medulla oblongata

Cerebrospinal Fluid Surrounds the spinal cord Fills ventricles within the brain Blood-brain barrier controls which solutes enter the cerebrospinal fluid

Anatomy of the Cerebrum Largest and most complex part of human brain Outer layer (cerebral cortex) is highly folded A longitudinal fissure divides cerebrum into left and right hemispheres

Lobes of the Cerebrum parietal frontal occipital temporal primary somatosensory cortex primary motor cortex parietal frontal occipital temporal

Limbic System Controls emotions and has role in memory (olfactory tract) cingulate gyrus thalamus amygdala hypothalamus hippocampus

Sensory Receptors Mechanoreceptors Thermoreceptors Pain receptors Convert stimulus into action potentials Mechanoreceptors Thermoreceptors Pain receptors Chemoreceptors Osmoreceptors Photoreceptors

Stimulus Strength Which pathway carries the signal Action potentials don’t vary in size Brain integrate information by Which pathway carries the signal Frequency of action potentials along each axon Number of axons recruited

Touch Pressure Temperature Pain Motion Position Somatic Sensations Touch Pressure Temperature Pain Motion Position

The Somatosensory Cortex

Receptors in Skin Free nerve ending Ruffini ending Pacinian corpuscle Bulb of Krause Meissner’s corpuscle

Smell A special sense Olfactory receptors Receptor axons lead to olfactory lobe olfactory bulb receptor cell

Taste A special sense Chemoreceptors Five primary sensations: sweet, sour, salty, bitter, and umami

Vision Sensitivity to light is not vision Vision requires Eyes Capacity for image formation in the brain

The Eye Perceives visual field Lens collects light Image formed on retina Contains visual pigments Stimulate photoreceptors

Human Eye sclera retina choroid iris fovea optic lens disk pupil cornea part of optic nerve aqueous humor ciliary muscle vitreous body

Pattern of Stimulation Image on retina is upside down and reversed right to left compared with the stimulus Brain corrects during processing

Organization of Retina Photoreceptors at back of retina, in front of pigmented epithelium For light to reach photoreceptors, it must pass layers of neurons involved in visual processing

Organization of Retina Signals from photoreceptors are passed to bipolar sensory neurons, then to ganglion cells Axons of ganglion cells form the two optic nerves Cone Rod Ganglion cell Bipolar sensory neuron

The Photoreceptors Rods Cones Contain the pigment rhodopsin Detect very dim light, changes in light intensity Cones Three kinds; detect red, blue, or green Provide color sense and daytime vision

Rods and Cones cone cell stacked, pigmented membrane rod cell Fig. 25-28, p.443

Eye Diseases Macular degeneration Cataract Glaucoma fovea start of an optic nerve in back of the eyeball

Hearing Outer ear Middle ear Inner ear new

Properties of Sound Ear detects pressure waves Amplitude of waves corresponds to perceived loudness Frequency of waves (number per second) corresponds to perceived pitch

Anatomy of Human Ear stirrup anvil auditory nerve hammer auditory canal eardrum cochlea

Sound Reception Sound waves make the eardrum vibrate Vibrations are transmitted to the bones of the middle ear The stirrup transmits force to the oval window of the fluid-filled cochlea

Sound Reception hair cells in organ of Corti lumen of cochlear duct tectorial membrane basilar membrane to auditory nerve lumen of scala tympani

Organ of Corti Hair cells

Balance and Equilibrium Mechanoreceptors located in the inner ear Maintains body position semicircular canals vestibular apparatus