Untitled Quiz

Nervous System Overview

• The nervous system is composed of the brain and spinal cord (CNS) and all other neural structures outside the brain and spinal cord, (the peripheral nervous system - PNS)
• PNS also includes sensory receptors, peripheral nerves and associated ganglia, and efferent motor endings.
• The CNS and PNS are responsible for sensory input, integration, and motor output.
• The PNS is divided into functional subdivisions:
  • Sensory (afferent) division: conveys impulses to the CNS from sensory receptors.
  • Motor (efferent) division: transmits impulses from the CNS to effector organs (muscles and glands).
  • The motor division is also divided into two parts:
    • Somatic nervous system: conducts impulses from the CNS to skeletal muscles (voluntary).
    • Autonomic nervous system: regulates the activity of smooth muscles, cardiac muscles, and glands (involuntary).

Nervous Tissue

• Nervous tissue is made up of two principal types of cells: neuroglia and neurons.
• Neuroglia are supporting cells, wrapping the neurons, providing supportive scaffolding, segregating and insulating neurons, guiding young neurons to proper connections, and promoting health and growth. There are six types, four in the CNS and two in the PNS.
  • CNS: Astrocytes, Oligodendrocytes, Microglia, Ependymal cells.
  • PNS: Schwann cells, Satellite cells.
• Astrocytes are the most abundant, versatile, and highly branched glial cells that cling to neurons and their synaptic endings, and cover capillaries. Astrocytes are instrumental in supporting and bracing neurons, guiding migration of young neurons, maintaining the appropriate chemical environment for generation of nerve impulses, taking up excess neurotransmitters, and helping to form the blood-brain barrier.
• Neurons are excitable nerve cells that transmit electrical signals.
  • A neuron consists of soma, axons, and dendrites.
  • Cytosol is enclosed by the neuronal membrane.
  • Soma contains the nucleus and organelles, while the axon starts from the axon hillock, with no rough endoplasmic reticulum extending into the axon.
  • Dendrites and the axon terminals have differences in their cytoplasmic content. The axon terminal contains synapses with other cells.
  • The microtubules are nucleated at the centrosome and then released and delivered to either the dendrites or the axon.
  • Neurofilaments are abundant in axons, and their spacing is sensitive to the level of phosphorylation.
  • Microfilaments are dispersed within the cell and are most abundant near the plasma membrane.
• The neuron's structure includes dendrites, cell body, axon, and axon terminal. The axon conveys impulses towards the axon terminal.

Membrane Potentials

• Membrane potential changes are produced by changes in membrane permeability to ions and alterations of ion concentrations across the membrane.
• Two types of signals can be produced: graded potentials and action potentials.
• Graded potentials are small deviations from the membrane potential that decrease in intensity with distance. Current generated is quickly dissipated due to the leaky plasma membrane. Their magnitude varies directly with the strength of the stimulus.
• Action potentials are brief reversals of membrane potential with a total amplitude of about 100 mV.
  • Changes in the membrane potential during an action potential can be depolarization, repolarization, and hyperpolarization. Depolarization occurs when the inside of the membrane becomes less negative; repolarization is when the membrane returns to resting potential; and hyperpolarization occurs when membrane becomes more negative than resting potential.
  • The all-or-none law states that if the stimulus strength is above threshold, the nerve or muscle fiber will give a complete response or no response.
  • The intensity of a stimulus is reflected by the frequency of action potentials, not the amplitude.

Synapses

• Synapses are junctions between neurons and other cells, either axodendritic or axosomatic.
• Synapses are the sites where neurons communicate with other neurons or effector cells.
  • Electrical synapse—ions flow directly from one neuron to the next (fast).
  • Chemical synapse—neurotransmitters are released into a fluid space, then bind to receptors on the next neuron (slow).
• First a nerve impulse arrives at a synaptic end bulb of presynaptic axon, then the increase of Ca2+ triggers exocytosis of synaptic vesicles and release of neurotransmitters into the synaptic cleft. The neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron. The postsynaptic membrane permeability changes, followed by an excitatory or inhibitory effect. Postsynaptic potentials (EPSPs and IPSPs) summation is required for the triggering of an action potential at the axon hillock. Synaptic delay is the rate-limiting step in neural transmission (0.3 to 5 ms).
• Neurotransmitter effects must be terminated, either by degradation or reuptake.

Neurotransmitters and Receptors

• Neurotransmitters can be classified chemically or functionally. Chemical classification types include amino acids, biogenic amines, purinergics, and neuropeptides. Unconventional ones are endocannabinoids and gasotransmitters. Functional classification includes direct or indirect actions.
• Receptors—ionotropic and metabotropic. Ionotropic receptors are membrane-spanning ion channel proteins that open directly in response to ligand binding.
• Metabotropic receptors activate signaling pathways, which indirectly open or close channels or have other effects, often slower than ionotropic responses. Some metabotropic receptors have excitatory and others inhibitory effects, dependent on how they affect ion channels.

The Central Nervous System

• The CNS consists of the brain and the spinal cord. The brain is divided into three main parts: cerebrum, cerebellum, and brainstem.
• The cerebral hemispheres account for 83% of the total brain mass and are the most conspicuous parts of the intact brain.

Peripheral Nervous System

• The PNS includes all neural structures outside the brain and spinal cord. The PNS is comprised of nerves, ganglia, and sensory receptors.
• Sensory receptors are used to classify the PNS into five types: mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors, and nociceptors (pain). There are also exteroceptors, interoceptors, and proprioceptors used to classify sensory receptors by location.
• Receptors can be classified by overall structure as either simple or complex receptors. Simple receptors are modified dendritic endings of sensory neurons and are either encapsulated or free nerve endings. Complex receptors are localized collections of cells associated with the special senses.

Cranial Nerves

• There are twelve pairs of cranial nerves associated with the brain and brain stem. Most are mixed nerves; some are solely sensory or solely motor.

Spinal Nerves

• There are thirty-one pairs of spinal nerves that arise from the spinal cord. They are mixed nerves.
• The spinal nerves are classified according to their point of issue.
• Spinal nerves contain rami which supply and connect areas to the body parts.
• Nerve plexuses are formed by the interlacing neurons.

Reflexes

• Reflexes are rapid, involuntary responses to stimuli.
• Somatic reflexes activate skeletal muscles, whereas autonomic reflexes activate visceral effectors.
• Spinal reflexes are mediated by the spinal cord, with higher brain centers generally providing advice.

Autonomic Nervous System

• The autonomic nervous system is a subdivision of the motor peripheral system and is involuntary.
• The autonomic nervous system is divided into sympathetic and parasympathetic divisions.
  • Sympathetic division - "fight-or-flight"
  • Parasympathetic division - "rest-and-digest"
• The structure of sympathetic and parasympathetic nerves are different, which is relevant for the release of neurotransmitters.
• There are some exceptions to the general rules of how the neurotransmitters are released and act in the sympathetic and parasympathetic system.