Nervous System

• The Brain


  The brain has four regions: the cerebrum, the diencephalon, the brainstem, and the cerebellum. Each cerebral hemisphere is subdivided into frontal, parietal, temporal, and occipital lobes.
• Brain tissue


  May be gray or white
• Gray matter


  Consists of aggregations of neuronal cell bodies
• White matter


  Consists of neuronal axons that are coated with myelin
• Internal capsule


  A white-matter structure where myelinated fibers converge from all parts of the cerebral cortex and descend into the brainstem
• Brainstem


  Has three sections: the midbrain, the pons, and the medulla
• Consciousness


  Relies on the interaction between intact cerebral hemispheres and a structure in the diencephalon and upper brainstem, the reticular activating (arousal) system
• Cerebellum


  Coordinates all movement and helps maintain the body upright in space
• Spinal Cord


  Divided into segments: cervical, thoracic, lumbar, sacral, and coccygeal. The spinal cord is thickest in the cervical segment, which contains nerve tracts to both the upper and lower extremities.
• Cauda equina


  The lumbar and sacral roots travel the longest intraspinal distance and fan out like a horse's tail at L1–2
• Peripheral Nervous System


  Consists of both cranial nerves and peripheral nerves that project to the heart, visceral organs, skin, and limbs. It controls the somatic nervous system, which regulates muscle movements and response to the sensations of touch and pain, and the autonomic nervous system that connects to internal organs and generates autonomic reflex responses.
• Sympathetic Nervous System


  Mobilizes organs and their functions during times of stress and arousal
• Parasympathetic Nervous System


  Conserves energy and resources during times of rest and relaxation
• Cranial Nerves


  Twelve pairs of special nerves that emerge from the cranial vault through skull foramina and canals to structures in the head and neck
• Motor Pathways


  Complex avenues that extend from upper motor neurons through long white matter tracts to synapses with lower motor neurons, and continue to the periphery through peripheral nerve structures
• Upper Motor Neurons


  Nerve cell bodies that lie in the motor strip of the cerebral cortex and in several brainstem nuclei; their axons synapse with motor nuclei in the brainstem (for cranial nerves) and in the spinal cord (for peripheral nerves)
• Lower Motor Neurons


  Have cell bodies in the spinal cord, termed anterior horn cells; their axons transmit impulses through the anterior roots and spinal nerves into peripheral nerves, terminating at the neuromuscular junction
• Corticospinal (Pyramidal) Tract


  Mediates voluntary movement and integrates skilled, complicated, or delicate movements by stimulating selected muscular actions and inhibiting others. It also carries impulses that inhibit muscle tone.
• Corticobulbar


  Tracts synapsing in the brainstem with motor nuclei of the cranial nerves
• Basal Ganglia System


  Helps to maintain muscle tone and to control body movements, especially gross automatic movements such as walking
• Cerebellar System


  Receives both sensory and motor input and coordinates motor activity, maintains equilibrium, and helps to control posture
• Upper Motor Neuron Lesions

  Muscle tone is increased and deep tendon reflexes are exaggerated
• Lower Motor Neuron Lesions

  Cause ipsilateral weakness and paralysis, but muscle tone and reflexes are decreased or absent
• Spinothalamic Tract


  Consists of smaller sensory neurons with unmyelinated or thinly myelinated axons that register pain, temperature, and crude touch
• Posterior Columns


  Have larger neurons with heavily myelinated axons that transmit the sensations of vibration, proprioception, kinesthesia, pressure, and fine touch from skin and joint receptors
• Dermatome


  The band of skin innervated by the sensory root of a single spinal nerve
• Reflex


  An involuntary stereotypical response that may involve as few as two neurons, one afferent (sensory) and one efferent (motor), across a single synapse
• Deep Tendon Reflexes


  Monosynaptic reflexes that can be elicited by briskly tapping the tendon of a partially stretched muscle
• Each deep tendon reflex involves specific spinal segments, together with their sensory and motor fibers, so an abnormal reflex can help locate a pathologic lesion
• Superficial (Cutaneous) Reflexes


  Reflexes initiated by stimulating skin as well as muscle, such as the abdominal reflex
• Near syncope


  Feeling light-headed or weak, but failing to actually lose consciousness
• Presyncope


  Feeling light-headed or weak, but failing to actually lose consciousness
• True syncope


  Sudden but temporary loss of consciousness and postural tone from transient global hypoperfusion to the brain
• Seizures


  Sudden excessive electrical discharge from cortical neurons
• Symptomatic seizures


  Seizures with an identifiable cause
• Idiopathic seizures


  Seizures without an identifiable cause
• Tremor


  Rhythmic oscillatory movement of a body part resulting from the contraction of opposing muscle groups
• Tremor may be an isolated finding or part of a neurologic disorder
• Rest tremor


  Tremor that occurs at rest
• Intention tremor


  Tremor that gets worse with voluntary intentional movement or with sustained postures