Lecture 5. Neurophysiology PDF

Neurophysiology Introduction Our Topics • Organization of the Nervous System • Spinal Cord • Brain stem • Cerebellum • Different parts of Brain • Cells of the Nervous System • General Features of Sensory Systems The Nervous System A complex network that allows to communicate with its environme...

Neurophysiology Introduction Our Topics • Organization of the Nervous System • Spinal Cord • Brain stem • Cerebellum • Different parts of Brain • Cells of the Nervous System • General Features of Sensory Systems The Nervous System A complex network that allows to communicate with its environment. • Sensory components (afferent), which detect changes in environmental stimuli, • Integrative components, receive, store, and process sensory information and then orchestrate the appropriate motor responses. • Motor components (efferent), which generate movement, contraction of cardiac and smooth muscle, and glandular secretions. ORGANIZATION OF THE NERVOUS SYSTEM Central Nervous System (CNS) • The CNS includes the brain and spinal cord. Fig. 3.1 Midsagittal section of the brain. Relationships are shown between the lobes of the cerebral cortex, the cerebellum, the thalamus and hypothalamus, the brain stem, and the spinal cord. Spinal Cord • The spinal cord is segmented, with 31 pairs of spinal nerves that contain both sensory (afferent) nerves and motor (efferent) nerves. • Sensory nerves carry information to the spinal cord from the skin, joints, muscles, and visceral organs in the periphery via dorsal root. • Efferent nerves carry information from the spinal cord to the periphery include both somatic motor nerves and autonomic nerves. Descending and ascending pathways in the spinal cord It is related to • Somatic motor functions, • Visceral functions • Reflexes • Somatosensory functions (touch, proprioception), temperature,noxious stimuli senses) Brain Stem • The medulla, pons, and midbrain are collectively called the brain stem. • Medulla: It contains autonomic centers that regulate • Breathing, • Blood pressure, • Cardioregulatory and • Swallowing, Coughing, Vomiting reflexes Pons • Together with centers in the medulla, participates in balance and maintenance of posture • Some of breathing center located in here. • In addition, the pons relays information from the cerebral hemispheres to the cerebellum • The pontine micturition center is the exit site for impulses going to the bladder. • Superior olive complex, part of the auditory pathway, located in here Midbrain • It participates in control of eye movements and eye reflexes. • It also contains nuclei of the auditory and visual systems. Clinical box • Auditory midbrain implant (AMI) is using to stimulate the inferior colliculus in deaf patients who cannot benefit sufficiently from cochlear implants. The brain auditory pathway Cerebellum Integrates; • Sensory information about position from the spinal cord, • Motor information from the cerebral cortex, • Balance information from the vestibular organs of the inner ear. The functions; • Coordination of movement, planning and execution of movement, • Maintenance of posture, • Coordination of head and eye movements. Thalamus • The thalamus processes almost all sensory information going to the cerebral cortex (except smell) . • Almost all motor information coming from the cerebral cortex to the brain stem and spinal cord via thalamus. Hypothalamus The hypothalamus lies ventral to the thalamus and contains centers that regulate ; • Body temperature, • Food intake, • Water balance • The hypothalamus is also an endocrine gland that controls the hormone secretions of the pituitary gland. The hypothalamus secretes releasing and release-inhibiting hormones into hypophysial portal blood that cause release (or inhibition of release) of the anterior pituitary hormones. The hypothalamus also contains neurons that secrete antidiuretic hormone (ADH) and oxytocin. Cerebral Hemispheres The cerebral hemispheres consist of ; • The cerebral cortex, • Underlying white matter, • Deep nuclei such as Basal ganglia and Limbic system). The main functions of the cerebral hemispheres are; • Perception, • Higher motor functions, • Cognition, • Memory, and • Emotion Cerebral cortex • It consist of 4 lobe; Frontal, Parieal, Temporal, Occipital Basal ganglia • It receive input from all lobes of the cerebral cortex and send projections, via the thalamus, to the motor cortex to assist in regulating movement. • It have an inhibitory effect on many motor systems. Hippocampus, and amygdala • They are part of the limbic system. • The hippocampus is involved in memory; • The amygdala is involved with the emotions and communicates with the autonomic nervous system via the hypothalamus • The amygdala determines emotional responses to odors. • It is the main center of creation of emotions such as excitement, anxiety and pleasure due to scents. CELLS OF THE NERVOUS SYSTEM Nature volume 457, pages675–677 (2009) Dendrites; are tapering processes that arise from the cell body. They receive information and contain receptors for neurotransmitters. Axons; each neuron has a single axon, which can be quite long. Axon contains of microtubules that rapidly move materials between the cell body and the axon terminus. SENSORY SYSTEMS SENSORY SYSTEMS Sensory Pathways • Sensory systems receive information from the environment via specialized receptors in the periphery and transmit this information through a series of neurons and synaptic relays to the CNS. Synapses are made in relay nuclei between first- and second, second- and third, and third- and fourth-order neurons. Second-order neurons cross the midline either in the spinal cord (shown) or in the brain stem (not shown) so that information from one side of the body is transmitted to the contralateral thalamus and cerebral cortex. 1. Sensory receptors Different sensory modalities are detected by various receptors • In the visual, taste, and auditory systems; the receptors are specialized epithelial cells • In the somatosensory and olfactory systems; the receptors are also first-order, or primary afferent, neurons Sensory Transduction • The basic function of the receptors is the same: to convert a stimulus (e.g., sound waves, electromagnetic waves, or pressure) into electrochemical energy. • The conversion process, called sensory transduction, is mediated through the receptor potential. Receptor Potential • The receptor potential increases or decreases the probability an action potential will occur, depending on whether it is depolarizing or hyperpolarizing • They are graded electronic potentials, whose amplitude correlates with the size of the stimulus. Types of Receptors • Mechanoreceptors are activated by pressure or changes in pressure. • The pacinian corpuscles in subcutaneous tissue, • Meissner corpuscles in nonhairy skin (touch), • Baroreceptors in the carotid sinus (blood pressure), • Hair cells on the organ of Corti (audition) and in the Semicircular canals (vestibular system). Types of Receptors • Photoreceptors are activated by light and are involved in vision. • Chemoreceptors are activated by chemicals and are involved in olfaction, taste, and detection of oxygen and carbon dioxide in the control of breathing. • Thermoreceptors are activated by temperature or changes in temperature. • Nociceptors are activated by extreme stimulus of pressure, temperature, or noxious chemicals. 2. First-order sensory afferent neurons • The primary afferent neuron usually has its cell body in a dorsal root or spinal cord ganglion. Exceptions are the auditory, olfactory, and visual systems. (In these system the receptors are specialized epithelial cells) 3. Second-order sensory afferent neurons • First-order neurons synapse on second-order neurons in relay nuclei, which are located in the spinal cord or in the brain stem. • Interneurons, also located in the relay nuclei, may be excitatory or inhibitory. • These interneurons process and modify the sensory information received from the first-order neurons. 4. Third-order sensory afferent neurons. • Third-order neurons typically reside in relay nuclei in the thalamus. • Again, many second-order neurons synapse on a single third-order neuron. • The relay nuclei process the information they receive via local interneurons, which may be excitatory or inhibitory. 4. Fourth-order sensory afferent neurons. • Fourth order neurons reside in the appropriate sensory area of the cerebral cortex. • For example, in the auditory pathway, fourth-order neurons are found in the primary auditory cortex; • in the visual pathway, they reside in the primary visual cortex; and so forth. • There are secondary and tertiary areas, as well as association areas in the cortex, all of which integrate complex sensory information. The receptive field • The receptive field is the area of the body that changes the firing rate of a sensory neuron when stimulated. • If the sensory neuron's firing rate increases, the receptive field is excitatory. • If the sensory neuron's firing rate decreases, the receptive field is inhibitory. • The smaller receptive field, more precisely sensation can be localized or identified. • In higher order neurons, the receptive field becomes more complex because more neurons converge in the relay nucleus in the higher orders. Sensory Coding Stimulus intensity is encoded in three ways. 1. The number of receptors that are activated. 2. The differences in firing rates of sensory neurons in the pathway. 3. Activating different types of receptors; a light touch of the skin may activate only mechanoreceptors, whereas an intense damaging stimulus to the skin may activate mechanoreceptors and nociceptors. What happens if a receptor receives stimulation that continues with the same intensity? Adaptation • Depending on the duration of stimulation and the type of receptor, its ability to generate an impulse gradually decreases and eventually it cannot generate an impulse. • This phenomenon is called adaptation of the receptor. For example; When a prosthesis is placed on a person, the presence of foreign matter in the body becomes increasingly unnoticeable. • Receptors are divided into two groups according to their adaptation rate. • Some of the mechanoreceptors are phasic, they are only when the stimulus intensity changes, so they cannot emit a continuous signal.Pacinian corpuscles, which detect rapid changes in the stimulus or vibrations. • Photoreceptors, chemoreceptors, thermoreceptors, pain receptors and some of mechanoreceptors are tonic so continue to send impulses to the brain as long as the stimulus is present. • Thanks for your attention… • Next week we will continue with the topic Somatosensory System and Pain (Page 80).