Nervous System

• Cell types in neural tissue:

  • Neurons
  • Neuroglial cells (also known as neuroglia, glia, and glial)
• Divisions of the Nervous System:

  • Central Nervous System (CNS): Brain, Spinal cord
  • Peripheral Nervous System (PNS): Cranial nerves, Spinal nerves
• Divisions of Peripheral Nervous System:

  • Sensory Division: Picks up sensory information and delivers it to the CNS
  • Motor Division: Carries information to muscles and glands
  • Divisions of the Motor Division:
  • Somatic – carries information to skeletal muscle
  • Autonomic – carries information to smooth muscle, cardiac muscle, and glands
• Functions of Nervous System:

  • Sensory Function (receiving information):
  • Sensory receptors gather information
  • Information is carried to the CNS
  • Integrative Function (deciding what to do about information):
  • Sensory information used to create sensations, memory, thoughts, decisions
  • Motor Function (acting on information):
  • Decisions are acted upon
  • Impulses are carried to effectors
• Neuron Structure:

  • Neurons vary in size and shape
  • Neurons share certain features: Dendrites, a cell body, an axon
• Myelination of Axons:

  • White Matter: Contains myelinated axons, considered fiber tracts
  • Gray Matter: Contains unmyelinated structures like cell bodies, dendrites
• Classification of Neurons and Neuroglia:

  • Neurons can be sensory, motor, or integrative
  • Neurons can be classified into three major groups: Bipolar neurons, Unipolar neurons, Multipolar neurons
• Types of Neuroglial Cells:

  • In the PNS:
  • Schwann Cells: Produce myelin found on peripheral myelinated neurons, speed up neurotransmission
  • Satellite Cells: Support clusters of neuron cell bodies (ganglia)
  • In the CNS:
  • Astrocytes: Scar tissue, mop up excess ions, induce synapse formation, connect neurons to blood vessels
  • Oligodendrocytes: Myelinating cell
  • Ependyma or ependymal: Ciliated, line central canal of spinal cord, line ventricles of brain
  • Microglia: Phagocytic cell
• Resting Potential:

  • Resting Membrane Potential (RMP): 70 mV difference from inside to outside of cell, polarized membrane
  • RMP = -70 mV, due to distribution of ions inside vs. outside
  • Na+/K+ pump restores
• Local Potential Changes:

  • Caused by various stimuli like temperature changes, light, pressure
  • Environmental changes affect the membrane potential by opening chemically gated, voltage gated, or mechanically gated ion channels
• The Synapse:

  • Nerve impulses pass from neuron to neuron at synapses, moving from a pre-synaptic neuron to a post-synaptic neuron
• Synaptic Transmission:

  • Neurotransmitters are released when impulse reaches synaptic knob
• Membrane potential changes:

  • If membrane potential becomes more negative, it has hyperpolarized
  • If membrane potential becomes less negative, it has depolarized
  • Graded to intensity of stimulation reaching threshold potential
  • Reaching threshold potential results in a nerve impulse, starting an action potential
• Action potentials:

  • At rest, the membrane is polarized (RMP = -70)
  • Sodium channels open and membrane depolarizes (toward 0)
  • Potassium leaves cytoplasm and membrane repolarizes (+30)
  • Threshold stimulus reached (-55)
  • Brief period of hyperpolarization (-90)
• All-or-None Response:

  • If a neuron responds at all, it responds completely
  • A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon
  • All impulses carried on an axon are the same strength
• Refractory Period:

  • Absolute Refractory Period: Time when threshold stimulus does not start another action potential
  • Relative Refractory Period: Time when stronger threshold stimulus can start another action potential
• Synaptic Potentials:

  • EPSP (Excitatory postsynaptic potential): Depolarizes membrane of postsynaptic neuron, making action potential more likely
  • IPSP (Inhibitory postsynaptic potential): Hyperpolarizes membrane of postsynaptic neuron, making action potential less likely
• Summation of EPSPs and IPSPs:

  • EPSPs and IPSPs are added together in a process called summation
  • More EPSPs lead to greater probability of an action potential
  • More IPSPs lead to lower probability of an action potential
• Neuronal Pools:

  • Groups of interneurons that make synaptic connections with each other
  • Interneurons work together to perform a common function, may be excitatory or inhibitory
  • Each pool receives input from other neurons
  • Each pool generates output to other neurons
• Convergence:

  • Neuron receives input from several neurons
  • Incoming impulses represent information from different types of sensory receptors
  • Allows nervous system to collect, process, and respond to information
  • Makes it possible for a neuron to sum impulses from different sources
• Divergence:

  • One neuron sends impulses to several neurons via its branched axon
  • Can amplify an impulse
  • Impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction or glandular secretion
• Meninges are membranes surrounding the central nervous system (CNS) that protect it, consisting of three layers:

  • Dura mater: the outermost layer, known as the "hard mother," is a double layer with fluid and blood vessels, containing large veins called dural sinuses
  • Arachnoid mater: a weblike structure, with the subarachnoid space containing cerebrospinal fluid (CSF)
  • Pia mater: the innermost delicate layer
• Cerebrospinal fluid (CSF) is secreted by the choroid plexus within the lateral ventricles, flowing through interconnected cavities within the cerebral hemispheres and brain stem, and is reabsorbed by granulations that penetrate the inner dura mater (Dural Sinus), completely surrounding the brain and spinal cord, providing nutrition and protection while helping maintain stable ion concentrations in the CNS
• The spinal cord extends from the foramen magnum to the 2nd lumbar vertebra, serving as the center for spinal reflexes and a conduit for nerve impulses to and from the brain
• Reflexes are automatic, subconscious responses to stimuli, including the knee-jerk reflex that helps maintain posture, the withdrawal reflex for protection, and the crossed-extensor reflex where flexor muscles contract on one side while extensor muscles contract on the opposite side for balance
• The brain interprets sensations, determines perception, stores memory, reasoning, makes decisions, coordinates muscular movements, regulates visceral activities, and determines personality, with major parts including the cerebrum, two cerebellar hemispheres, diencephalon, brain stem, and cerebellum
• The cerebral cortex, a thin layer of gray matter constituting the outermost portion of the cerebrum, contains 75% of all neurons and is responsible for various functions such as motor control, interpretation of sensory experiences, memory, reasoning, verbalization, judgment, and emotions
• Memory involves short-term working memory and long-term memory that changes the structure and function of neurons, enhancing synaptic transmission
• The limbic system, consisting of portions of the frontal lobe, temporal lobe, hypothalamus, thalamus, basal nuclei, and other deep nuclei, controls emotions, produces feelings, and interprets sensory impulses
• The brain stem, comprising the midbrain, pons, and medulla oblongata, plays vital roles in conducting nerve impulses, regulating vital functions like breathing and heart rate, and maintaining wakefulness through the reticular formation
• The cerebellum, located inferior to the occipital lobes and posterior to the pons and medulla oblongata, integrates sensory information, coordinates skeletal muscle activity, and maintains posture
• Cerebellum:

  • Integrates sensory information concerning the position of body parts
  • Coordinates skeletal muscle activity
  • Maintains posture
• Peripheral Nervous System:

  • Cranial nerves arise from the brain
  • Somatic fibers connect to the skin and skeletal muscles
  • Autonomic fibers connect to viscera
  • Spinal nerves arise from the spinal cord
  • Somatic fibers connect to the skin and skeletal muscles
  • Autonomic fibers connect to viscera
• Structure of a Peripheral Nerve:

  • Sensory Nerves conduct impulses into the CNS
  • Motor Nerves conduct impulses to muscles or glands
  • Mixed Nerves contain both sensory and motor nerve fibers
• Nerve Fiber Classification:

  • General somatic efferent fibers carry motor impulses from the CNS to skeletal muscles
  • General visceral efferent fibers carry motor impulses away from the CNS to smooth muscles and glands
  • General somatic afferent fibers carry sensory impulses to the CNS from skin and skeletal muscles
  • General visceral afferent fibers carry sensory impulses to the CNS from blood vessels and internal organs
• Special somatic efferent fibers carry motor impulses from the brain to muscles used in chewing, swallowing, speaking, and forming facial expressions
• Cranial Nerves I and II:

  • Olfactory (I) sensory fibers transmit impulses associated with smell
  • Optic (II) sensory fibers transmit impulses associated with vision
• Cranial Nerves III and IV:

  • Trochlear (IV) primarily motor, motor impulses to muscles that move the eyes
  • Oculomotor (III) primarily motor, motor impulses to muscles that raise eyelids, move the eyes, focus lens, adjust light entering the eye
• Cranial Nerve V:

  • Trigeminal (V) mixed nerve
  • Opthalmic division: sensory from surface of eyes, tear glands, scalp, forehead, and upper eyelids
  • Maxillary division: sensory from upper teeth, upper gum, upper lip, palate, and skin of face
  • Mandibular division: sensory from scalp, skin of jaw, lower teeth, lower gum, and lower lip; motor to muscles of mastication and muscles in the floor of the mouth
• Cranial Nerves VI and VII:

  • Abducens (VI) primarily motor, motor impulses to muscles that move the eyes
  • Facial (VII) mixed nerve, sensory from taste receptors, motor to muscles of facial expression, tear glands, and salivary glands