PREPARED BY MOSES KAZEVU INTRODUCTION TO NEUROPHYSIOLOGY The nervous system is a complex network that allows an organism to communicate with its environment. The environment includes both external (world outside the body) and internal (components and cavities inside the body) environment. The nervous system includes: Sensory components: detect changes in environmental stimuli. Motor components: generates movement, contractions which cause cardiac and smooth muscle to carry out their respective functions as well as glandular secretions. Integrative components: receive, store and process sensory information and then orchestrate the appropriate motor response. The nervous system is composed of 2 divisions: 1. Central nervous system This includes the brain and spinal cord. It is formed by neurons and supporting cells called neuroglia. Structures of brain and spinal cord are arranged into gray matter (representing nerve cell bodies and the proximal parts of the nerve fibers, arising from nerve cell body) and white matter (representing the remaining parts of nerve fibers). In the brain white matter is on the inner part and gray matter is on the outer part, in the spinal cord white matter is on the outer part and gray matter is in the inner part. 2. Peripheral nervous system Has both afferent inputs (sensory) and efferent outputs (motor). This includes sensory receptors, sensory neurons, ganglia outside the CNS, somatic motor neurons and autonomic motor neurons. PREPARED BY MOSES KAZEVU It consists of cranial nerves arising from the brain and spinal nerve The peripheral nervous system can be divided into sensory (afferent) and motor (efferent) divisions. The motor (efferent) division consists of a somatic component and an autonomic component. o The autonomic nervous system consists of the sympathetic division (thoracolumbar) and the parasympathetic division (craniosacral). The peripheral nervous system (PNS) provides an interface between the environment and the CNS. The sensory/afferent division brings information into the nervous system usually beginning with events in sensory receptors in the periphery. PREPARED BY MOSES KAZEVU These receptors include, but are not limited to visual receptors, auditory receptors, chemoreceptors and somatosensory (touch) receptors. This afferent information is then transmitted to progressively higher levels of the nervous system and finally to the cerebral cortex. The motor or efferent division relays information out of the nervous system to the periphery. This results in skeletal/cardiac/smooth muscle contraction or glandular (endocrine and exocrine) secretion. The CNS includes the brain and spinal cord. The major divisions of the CNS are the spinal cord, brain stem (medulla, pons and midbrain), cerebellum, diencephalon (thalamus and hypothalamus) and cerebral hemispheres (cerebral cortex, white matter, basal ganglia, hippocampal formation and amygadala). The brain consists of 3 major divisions: Prosencephalon (forebrain) o Divided into the telencephalon (cerebral hemispheres, basal ganglia, hippocampus, amydaloid nuclus) and diencephalon (thalamus, hypothalamus, metathalamus, subthalamus) Mesencephalon (midbrain): is a part of the brain stem. Rhombencephalon (hind brain) o Divided into the metencephalon (pons and cerebellum) and myelencephalon (medulla oblongata) PREPARED BY MOSES KAZEVU The spinal cord is the most caudal portion of the CNS, extending from the base of the skull to the first lumbar vertebra. It consists of 31 pairs of spinal nerves which have both sensory and motor nerves. PREPARED BY MOSES KAZEVU Information travels up and down within the spinal cord through pathways. The ascending pathways in the spinal cord carry sensory information from the periphery to higher levels of the CNS. Descending pathways in the spinal cord carry motor information from higher levels of the CNS to the motor nerves that innervate the periphery. The medulla, pons and midbrain are collectively called the brain stem. The 12 cranial nerves arise from the brain stem. The medulla is the rostral extension of the spinal cord. It contains autonomic centers that regulate breathing, blood pressure and reflexes such as swallowing, coughing and vomiting. The pons is rostral to the medulla and together with centers in the medulla participate in maintenance of posture and in regulation of breathing. Information from cerebral hemispheres is also relayed to the cerebellum by the pons. The midbrain is rostral to the pons and participates in control of eye movements. It also contains relay nuclei of the auditory and visual systems. The cerebellum is attached to the brain stem and lies dorsal to the pons and medulla. It functions in coordination of movement, planning and execution of movements, maintenance of posture and coordination of head and eye movements. The diencephalon is made up of the thalamus and the hypothalamus. The thalamus processes almost all sensory information going to the cerebral cortex and almost all motor information coming from the cerebral cortex to the brain stem and spinal cord. The hypothalamus lies ventral to the thalamus and contains centers that regulate body temperature, food intake and water balance. The hypothalamus also has an endocrine function. The cerebral hemispheres consist of the cerebral cortex, an underlying white matter and 3 deep nuclei (basal ganglia, hippocampus and PREPARED BY MOSES KAZEVU amygdala). The functions of the cerebral hemispheres are perception, higher motor functions, cognition, memory and emotion. The cerebral cortex consists of 4 lobes (frontal, parietal, temporal and occipital) separated by sulci or grooves. The cerebral cortex receives and processes sensory information and integrates motor functions. o Sensory and motor areas of the cortex are further designated as “Primary”, “Secondary” and “tertiary” depending on how directly they deal with sensory or motor processing. o The primary areas are the most direct and involve the fewest number of synapses, the tertiary areas require the most complex processing and involve the greatest number of synapses. o Association areas integrate diverse information for purposeful actions The basal ganglia consist of the caudate nucleus, the putamen and globus pallidus. The basal ganglia receive input from all lobes of the cerebral cortex and have projections, via the thalamus to the frontal cortex to assist in regulating movement. The hippocampus and amygdala 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. NEURONS NEURONS AND THEIR CLASSIFICATION Neurons/ nerve cells are the structural and functional unit of the nervous system. A nerve consists of a collection of neurons. These terms should thus not be used synonymously. PREPARED BY MOSES KAZEVU PARTS OF NEURON They have cell bodies with a nucleus and all cytoplasmic organelles as well as processes (axons and dendrites). Neurons do not have centrosomes and so do not undergo division (terminally differentiated). TABLE 2.1: Parts of a neuron Part of neuron Nerve cell body Dendrite Axon Function Also known as the soma/ perikaryon Irregular shape. Consists of cytoplasm (neuroplasm) covered by a cell membrane. Neuroplasm has a large nucleus (with 1 or more prominent nucleoli), neurofibrils (cytoskeleton), mitochondria, Nissl bodies (basophilic granules, contain ribosomes) and Golgi apparatus. These are short processes. Transmits impulses towards the nerve cell body, usually shorter than axon. These are long processes. Transmits impulses away from the nerve cell body. Longest axon is about 1 meter. TYPES OF NEURONS Neurons are classified according to: Number of poles Function Length of axon PREPARED BY MOSES KAZEVU TABLE 2.2: CLASSIFICATION OF NEURONS Classification Number of poles (Fig 2.1) Neuron type Unipolar Bipolar Multipolar Function Sensory or Afferent Motor or Efferent Comment Only has one pole. Axon and dendrite arise from one pole. Only present in embryonic stage in human beings. (In adults there is a pseudounipolar neuron) Neuron has two poles. Axon arises from one pole and dendrite arises from the other pole. Neurons have many pole. One of the pole gives rise to axon and all other poles give rise to dendrites Carry sensory impulses from periphery to central nervous system. They are pseudounipolar in nature Carry motor impulses from the CNS to the peripheral effector organs Most are multipolar in nature PREPARED BY MOSES KAZEVU Length Golgi type I neuron Golgi type II neuron Have long axons. Cell body is in different parts of the CNS and their axons in peripheral organs Have short axons. These neurons are present in the cerebral cortex and spinal cord Note: each nerve is formed by many bundles or groups of nerve fibers. Each bundle of nerve fibers is called a fasciculus. PREPARED BY MOSES KAZEVU CLASSIFICATION OF NERVE FIBERS Nerve fibers are classified by 6 different methods including: Structure Distribution Origin Function Secretion or neurotransmitter Diameter and conduction of impulse (Erlanger-Gasser classification) CLASSIFICATION BY STRUCTURE These are either Myelinated: covered by myelin sheath Non-Myelinated: not covered by myelin sheath CLASSIFICATION BY DISTRIBUTION These include: Somatic nerve fibers: supply skeletal muscles Visceral or autonomic nerve fibers: control various internal organs of the body CLASSIFICATION UPON ORIGIN Consist of: Cranial nerve fibers: arise from the brain Spinal nerve fibers: arise from the spinal cord CLASSIFICATION BY FUNCTION Are of two types (discussed above) Sensory Motor CLASSIFICATION UPON SECRETION OF NERUOTRANSMITTER They include Adrenergic nerve fibers: secrete noradrenalin PREPARED BY MOSES KAZEVU Cholinergic nerve fibers: secrete acetylcholine CLASSIFICATION BY DIAMETER AND CONDUCTION OF IMPULSE Erlanger and Gasser classified the nerve fibers into 3 major types, on the basis of diameter (thickness) of the nerve fibers and velocity of conduction of impulses. They are divided into: Type A nerve fibers: these are the thickest. These are myelinated. They are further divided into o Type A alpha or type I nerve fibers o Type A beta or type II nerve fibers o Type A gamma nerve fibers o Type A delta or type III nerve fibers Type B nerve fibers: these are myelinated. Type C nerve fibers (also called type IV fibers): these are the smallest and are unmyelinated The velocity of impulse is directly proportional to the thickness of the fiber. TABLE 2.3: Type of nerve fibers Type A alpha (Type I) A beta (Type II) A gamma A delta (Type III) B C (Type IV) Diameter () 12 to 24 6 to 12 5 to 6 2 to 5 1 to 2 <1.5 Velocity of conduction (m/s) 70 to 120 30 to 70 15 to 30 12 to 15 3 to 10 0.5 to 2 Just like any other excitable tissue neurons have the property of excitability, conductivity, refractory period, summation, adaptation (Accomodation), infatigability and abide to the all or non-law. PREPARED BY MOSES KAZEVU