Nervous System
n Master controlling and communicating system of the body
n Interacts with the endocrine system to control and coordinate the body’s responses to changes in its environment, as well as growth, development and reproduction
Comparison Between the Nervous system and the Endocrine system
n Nervous system
n Secrete chemicals called neurotransmitters
n Help maintain homeostasis
n Nervous responses are rapid and of short duration
n Nervous impulses are transmitted via neurons
n Endocrine System
n Secrete chemicals called hormones
n Help maintain homeostasis
n Endocrine responses are slow but of long duration
n Hormones are carried by blood plasma
Function of the Nervous System
n Sensation
n Monitoring of changes both inside and outside the body
n Integration
n Interprets sensory input and makes decisions on what has to be done
n Response
n Activation of an effector organs (Fig 11.1)
Divisions of the Nervous System
n Central Nervous System
n Consists of the Brain and Spinal Cord
n Integrating center of the NS
n Peripheral Nervous system
n Part of the NS outside the CNS
n It is the communication link between the CNS and the body parts
n Consists of nerves that extend from the brain and spinal cord
n Sensory (Afferent) Division
n Transmit impulses from the periphery to the CNS
n Motor (Efferent) Division
n Transmit impulses from the CNS to effector organs
n The motor division of the PNS has two main parts
n Somatic Nervous System
n Consists of motor nerves fibers that conduct impulses from the CNS to the skeletal muscles
n Also referred to as voluntary nervous system
n Autonomic Nervous System (ANS)
n Consists of visceral motor nerve fibers that regulate the activities of visceral smooth muscles, cardiac muscles and glands
n Also referred to as the involuntary nervous system
n The ANS consists of two divisions
n The Sympathetic nervous system
n Mobilizes body systems during emergencies (fright or flight response)
n Parasympathetic nervous system
n Conserves energy
n Controls non emergency functions
Nervous System: Cell Types
n Glial cells
n They have branching processes like neurons but are much smaller in size
n Their nuclei stain darker
n In general, glial cells are supportive cells
n Some insulate
n Produce chemicals that guide young neurons to proper connections
Glial cells in the CNS
n Make up about half the brain’s mass and outnumber neuron in a ratio of 10:1
n Astrocytes
n Star-like in appearance and are connected by gap junctions
n Processes anchor neurons to their nutrient supply lines
n Play a role in the exchange between capillaries and neurons, synapse formation and in guiding the migration of young neurons
n Control the chemical environment around neurons
n Microglia
n Ovoid-shaped cells with long thorny processes
n Monitor the health of surrounding neurons
n Transform to macrophages and become phagocytic
n Ependymal cells
n Shape varies from squamous to columnar and may possess cilia
n Line the central cavities of the brain and spinal cord
n Form a permeable barrier between CSF in the cavities and the tissue fluid bathing the CNS cells
n Beating of the cilia helps to circulate CSF
n Oligodendrocytes
n Branching cells but with fewer processes compared to astrocytes
n Wrap their processes around the thicker neuron fibers in the CNS and provide insulation with myelin sheaths (Fig 11.3)
Glial cells in the PNS
n Satellite cells
n Surround neuron cell bodies within ganglia
n Function still unknown
n Schwann cells
n Surround and form myelin sheath around larger nerve fibers in the PNS
n Vital for the regeneration of peripheral nerve fibers
Neurons
n Neurons are specialized cells that conduct messages in the form of electrical impulses throughout the body
n Neurons function optimally for a lifetime, are mostly amitotic (Exceptions occur in the olfactory epithelium and hippocampus) , and have an exceptionally high metabolic rate requiring oxygen and glucose. (Fig 11.4)
n The neuron cell body, also called the perikaryon or soma, is the major biosynthetic center containing the usual organelles except for centrioles.
n Clusters of neuron cell bodies in the CNS are called nuclei. In the PNS, they are called ganglia
n Dendrites are cell processes that are the receptive regions of the cell
n Each neuron has a single axon that generates and conducts nerve impulses away from the cell body to the axon terminals.
n Bundles of neuron processes in the CNS are called tracts. In the PNS, they are called nerves.
n The myelin sheath is a whitish, fatty, segmented covering that protects, insulates, and increases conduction velocity of axons.
Structural Classes of Neurons
n Multipolar neurons have three or more processes. They are found mostly in the CNS
n Bipolar neurons have a single axon and dendrite. They are found only in some special sense organs
n Unipolar neurons have a single process extending from the cell body that is associated with receptors at the distal end. They are found mainly in ganglia in the PNS. (Tab 11.1)
Functional Classes of neurons
n Sensory, or afferent, neurons conduct impulses toward the CNS from receptors.
n Motor, or efferent, neurons conduct impulses from the CNS to effectors.
n Interneurons, or association neurons, conduct impulses between sensory and motor neurons, or in CNS integration pathways.
Basic Principles of Electricity
n Voltage is a measure of the amount of difference in electrical charge between two points, called the potential difference.
n The flow of electrical charge from point to point is called current, and is dependent on voltage and resistance (hindrance to current flow).
n In the body, electrical currents are due to the movement of ions across cellular membranes.
The Role of Membrane Ion Channels
n The plasma membrane has many ion channels, some of which are always open, called leakage channels, and some that have a protein “gate” that changes shape or opens in response to the proper signal.
The Resting Membrane Potential
n The neuron cell membrane is polarized, being more negatively charged inside than outside. The degree of this difference in electrical charge is the resting membrane potential.
n The resting membrane potential is generated by differences in ionic makeup of intracellular and extracellular fluids, and differential membrane permeability to solutes.
Membrane Potentials That Act as Signals
n Neurons use changes in membrane potential as communication signals. These can be brought on by changes in membrane permeability to any ion, or alteration of ion concentrations on the two sides of the membrane.
n Changes in membrane potential relative to resting membrane potential can either be depolarizations, in which the interior of the cell becomes less negative, or hyperpolarizations, in which the interior of the cell becomes more negatively charged. (Fig 11.9)
n Graded potentials are short-lived, local changes in membrane potentials. They can either be depolarizations or hyperpolarizations, and are critical to the generation of action potentials.
Action Potentials
n Action potentials, or nerve impulses, occur on axons and are the principle way neurons communicate.
n Generation of an action potential involves a transient increase in Na+ permeability, followed by restoration of Na+ impermeability, and then a short-lived increase in K+ permeability.(Fig 11.12)
n Propagation, or transmission, of an action potential occurs as the local currents of an area undergoing depolarization cause depolarization of the forward adjacent area.
n Repolarization, which restores resting membrane potential, follows depolarization along the membrane.
Threshold
n A critical minimum, or threshold, depolarization is defined by the amount of influx of Na+ that at least equals the amount of efflux of K+.
n Action potentials are an all-or-none phenomena: they either happen completely, in the case of a threshold stimulus, or not at all, in the event of a subthreshold stimulus. (Fig 11.13)
Coding for stimulus intensity
n Stimulus intensity is coded in the frequency of action potentials.
n The refractory period of an axon is related to the period of time required so that a neuron can generate another action potential. (Fig 11.14 & 11.15)
Factors Affecting conduction Velocity
n Axons with larger diameters conduct impulses faster than axons with smaller diameters.
n Unmyelinated axons conduct impulses relatively slowly, while myelinated axons have a high conduction velocity.
The Synapse
n A synapse is a junction that mediates information transfer between neurons or between a neuron and an effector cell.
n Neurons conducting impulses toward the synapse are presynaptic cells, and neurons carrying impulses away from the synapse are postsynaptic cells.
n Electrical synapses have neurons that are electrically coupled via protein channels and allow direct exchange of ions from cell to cell.
n Chemical synapses are specialized for release and reception of chemical neurotransmitters. (Fig 11.18)
Termination of neurotransmitter effect
n Neurotransmitter effects are terminated in three ways: degradation by enzymes from the postsynaptic cell or within the synaptic cleft; reuptake by astrocytes or the presynaptic cell; or diffusion away from the synapse.
n Synaptic Delay
n Synaptic delay is related to the period of time required for release and binding of neurotransmitters.
Postsynaptic Potentials and Synaptic Integration
n Neurotransmitters mediate graded potentials on the postsynaptic cell that may be excitatory or inhibitory.
n Summation by the postsynaptic neuron is accomplished in two ways: temporal summation, which occurs in response to several successive releases of neurotransmitter, and spatial summation, which occurs when the postsynaptic cell is stimulated at the same time by multiple terminals.
n Synaptic potentiation results when a presynaptic cell is stimulated repeatedly or continuously, resulting in an enhanced release of neurotransmitter.
n Presynaptic inhibition results when another neuron inhibits the release of excitatory neurotransmitter from a presynaptic cell.
Neurotransmitters and Their Receptors
n Neurotransmitters are one of the ways neurons communicate, and they have several chemical classes.
n Functional classifications of neurotransmitters consider whether the effects are excitatory or inhibitory, and whether the effects are direct or indirect.
n There are two main types of neurotransmitter receptors: channel-linked receptors mediate direct transmitter action and result in brief, localized changes; and G protein-linked receptors mediate indirect transmitter action resulting in slow, persistent, and often diffuse changes.
Acetylcholine
n First neurotransmitter identified, and best understood
n Released at the neuromuscular junction
n Synthesized and enclosed in synaptic vesicles
n Degraded by the enzyme acetylcholinesterase (AChE)
n Released by:
n All neurons that stimulate skeletal muscle
n Some neurons in the autonomic nervous system