Nervous System Overview

Nervous System Overview

• The nervous system is anatomically divided into central nervous system (brain and spinal cord) and peripheral nervous system (cranial and spinal nerves).
• The major functional divisions of the peripheral nervous system (PNS) are sensory (afferent) and motor (efferent).
• Sensory division conveys impulses to the central nervous system (CNS).
• Motor division carries impulses from the CNS.

Nervous System Components

• The motor division includes somatic (voluntary) and autonomic (involuntary) systems.
• Somatic system serves skeletal muscles.
• Autonomic system innervates smooth and cardiac muscle, and glands.
• The nervous system has two cell types: neuroglia and neurons.

Neuroglia

• Neuroglia (supporting cells) support and insulate neurons.
• CNS neuroglia includes astrocytes, microglial cells, ependymal cells, and oligodendrocytes.
• Astrocytes are the most abundant CNS neuroglia.
• Microglial cells are defensive cells in the CNS.
• Ependymal cells line cerebrospinal fluid-filled cavities.
• Oligodendrocytes have processes forming myelin sheaths around CNS nerve fibers.
• PNS neuroglia includes Schwann cells and satellite cells.

Neurons

• Neurons have a cell body and cytoplasmic processes called axons and dendrites.
• A bundle of nerve fibers is called a tract in the CNS and a nerve in the PNS.
• A collection of cell bodies is called a nucleus in the CNS and a ganglion in the PNS.
• The cell body is the biosynthetic (and receptive) center of the neuron.
• Most neurons have many dendrites, receiving signals from other neurons toward the nerve cell body.
• Neurons typically have one axon that transmits nerve impulses away from the nerve cell body.
• Axon terminals release neurotransmitters.
• Bidirectional transport along axons uses ATP-dependent motor proteins "walking" along microtubule tracks.

Myelinated Fibers

• Large nerve fibers (axons) are myelinated.
• Myelin sheath is formed in the PNS by Schwann cells and in the CNS by oligodendrocytes.
• The myelin sheath gaps are called nodes of Ranvier.

Neuron Classification

• Neurons are classified based on the number of processes issuing from the cell body into multipolar, bipolar, or unipolar types.
• Functionally, neurons are classified as sensory, motor, or interneurons.
  • Sensory neurons conduct impulses toward the CNS.
  • Motor neurons conduct impulses away from the CNS.
  • Interneurons connect sensory and motor neurons.

Basic Principles of Electricity

• Voltage (potential) is the measure of potential energy of separated electrical charges.
• Current is the flow of electrical charge.
• Resistance hinders current flow.

Resting Membrane Potential

• A resting neuron has resting membrane potential at -70 mV (inside negative).
• This potential is due to differences in sodium and potassium ion concentrations, and differences in membrane permeability to these ions.
• The sodium-potassium pump maintains these ion concentration differences.

The Synapse

• A synapse is a functional junction between neurons.
• The information-transmitting neuron is the presynaptic neuron.
• The information-receiving neuron is the postsynaptic neuron.
• Electrical synapses allow ions to flow directly from one neuron to another; the cells are electrically coupled.

Chemical Synapses

• These are sites of neurotransmitter release and binding.
• When an impulse reaches the presynaptic axon terminals, voltage-gated Ca2+ channels open, and Ca2+ enters the cell, mediating neurotransmitter release.
• Neurotransmitters diffuse across the synaptic cleft and attach to postsynaptic membrane receptors, opening ion channels.
• Neurotransmitters are removed from the synapse via diffusion, enzymatic breakdown, or reuptake into the presynaptic terminal or astrocytes.

Postsynaptic Potentials and Synaptic Integration

• Excitatory postsynaptic potentials (EPSPs) are local graded potentials caused by simultaneous passage of Na+ and K+.
• Inhibitory postsynaptic potentials (IPSPs) lead to hyperpolarizations caused by opening K+ or Cl- channels.
• The membrane of the axon hillock acts as a neuronal integrator, summing EPSPs and IPSPs.
• Presynaptic inhibition is mediated by axoaxonal synapses, which reduce the amount of neurotransmitter released by the inhibited neuron.

Classification of Neurotransmitters by Function

• Neurotransmitters can be classified by their function (inhibitory or excitatory, or both) and action (direct or indirect).
  • Direct-acting neurotransmitters bind to and open ion channels.
  • Indirect-acting neurotransmitters act through second messengers.

Neurotransmitter Receptors

• These can be channel-linked (fast changes in membrane potential) or G protein-coupled (slow synaptic responses).
  • Second messengers often activate kinases, which act on ion channels or activate other proteins.

Classification of Neurotransmitters by Chemical Structure

• Main classes include acetylcholine, biogenic amines, amino acids, peptides, purines, dissolved gases, and lipids.

Patterns of Neural Processing

• In serial processing, one neuron stimulates the next sequentially.
• A reflex is a rapid, involuntary motor response to a stimulus mediated by reflex arcs.
• The minimum number of elements in a reflex arc is five: receptor, sensory neuron, information processing center, motor neuron, effector.

The Brain Regions and Organization

• The adult brain is divided into cerebral hemispheres, diencephalon, brain stem, and cerebellum.
• The cerebral hemispheres and cerebellum have gray matter nuclei surrounded by white matter and an outer cortex of gray matter.
• The diencephalon and brain stem lack a cortex.

The Brain Ventricles

• The brain contains four ventricles filled with cerebrospinal fluid (CSF).
• Two are lateral ventricles within the cerebral hemispheres.
• The third ventricle is within the diencephalon.
• The fourth ventricle lies between the brain stem and cerebellum and connects with the central canal of the spinal cord.

Cerebral Hemispheres

• The two cerebral hemispheres are connected by the corpus callosum.
• Each hemisphere receives sensory impulses and dispatches motor impulses from the opposite side of the body.

Blood Brain Barrier

• The blood-brain barrier consists of the least permeable capillaries in the body.
• It excludes many potentially harmful substances.
• Some substances that pass through the blood-brain barrier include fats and fat-soluble molecules, respiratory gases, alcohol, nicotine, and anesthetics.

Spinal Cord

• Extends from the medulla oblongata to the region of T12.
• Below T12 is the cauda equina (collection of spinal nerves).

Spinal Cord Anatomy

•  Spinal cord has white matter and central canal, along with dorsal and ventral horns of gray matter, dorsal root ganglion, and spinal nerve.

Peripheral Nervous System: Structure of a Nerve

• Endoneurium surrounds each nerve fiber.
• Groups of fibers are bound into fascicles by the perineurium.
• Fascicles are bound together by the epineurium.

Peripheral Nervous System: Branches of the Efferent System

•  The efferent, or motor system, includes somatic (non-autonomic/voluntary control over skeletal muscles) and autonomic (involuntary control over visceral organs). - The autonomic further branches into sympathetic and parasympathetic systems.

Comparison of Efferent Nervous System Branches

• Acetylcholine (ACh) is the neurotransmitter at the preganglionic synapses of both the parasympathetic and sympathetic.
• The postganglionic neuron of the parasympathetic system releases ACh while the postganglionic neuron of the sympathetic system releases norepinephrine (NE).

Function of Autonomic Divisions

• Parasympathetic generally opposes sympathetic efforts.
• Sympathetic division has a widespread effect throughout the body; generally prepares for action and use of energy resources; increases heart rate, blood pressure and glucose levels.