Nervous System

1. Neurons: nerve cells - the fundamental unit of nervous tissue 

   1. Afferent neurons: sensory neurons 

      • Respond to stimuli including pain, temperature, touch, & joint position (proprioception) 

      • Receptors are located in the skin, muscles, viscera, and joints 

      • Cell bodies are within the posterior root ganglion; axons form the posterior root 

   2. Efferent neurons: motor neurons 

      • Located in the frontal lobe and anterior horn of the spinal cord 

      • Large cell body with multi-branched dendrites and long axons 

      • Anterior roots: combined axons that exit the anterior horn together located outside the spinal cord in the intervertebral foramen 

   3. Spinal nerves: nerve fibers that exit the intervertebral foramen to transmit sensory info from the periphery to the CNS and motor info from the CNS to the periphery 

   4. Tract: group of myelinated nerve fibers organized within the spinal cord into columns 

      • Sensory and motor tracts from one side of the body cross to the contralateral side within the CNS 

   5. Upper motor neurons: cell bodies located in the cerebral cortex, brainstem and cerebellum 

   6. Lower motor neurons: cell bodies located in the anterior horn of the spinal cord 

   7. Interneurons: function to transmit or integrate signals from one or more neurons, relaying impulses to other neurons 

2. Lesions: injured tissues that have pathological change 

   1. Upper motor neuron lesion: lesion occurring proximal to the anterior horn of the spinal cord 

   2. Lower motor neuron lesion: lesion occurring to motor cell bodies or axons of the anterior horn of the spinal cord 

3. Cell body: central structure of a neuron containing the nucleus and plasma that supports neuronal function 

   2. • Impulses conducted within the cell body are activated in response to specific sensory stimuli 

4. Axon: a projection from a cell body that conducts electrical impulses away from the cell body, connecting to other cells and cell bodies 

5. Dendrites: projection from cell bodys that conduct electrical impulses to the cell body from other structures 

6. Synapse: small gap that is the junction between an axon, dendrite, interneuron, etc 

7. Neurotransmitter: a chemical that is released from the synapse in response to electrical stimuli conduction 

8. Myelin: white, fatty substance covering axons and dendrites to speed up the conduction of impulses 

   2. • Only surrounds nerve fibers in the PNS/CNS, not cell bodies or other fibers 

• Node of Ranvier: interruption in myelin sheaths used to increase the speed of impulse conduction in myelinated fibers by permitting impulses to leap from node to node  

9. Gray matter: unmyelinated nerve fibers and cell bodies found in the CNS 

• Resembles a butterfly shape when visualized in the spinal cord 

• Posterior horn: top portion of the butterfly-shaped gray matter in the spinal cord that receives sensory input from the periphery 

• Anterior horn: lower portion of the butterfly-shaped gray matter in the spinal cord that transmits motor impulses to the periphery 

10. White matter: myelinated nerve fibers  

• Posterior columns: white matter located in the posterior-medial aspect of spina cord 

10. 2. • Ascending tracts transmit the sensations of proprioception, pressure, and vibration 

11. Lateral corticospinal tract: located in the lateral column containing fibers that descend from the motor area in the cerebral cortex to the spinal cord 

• Fibers synapse with motor nuclei in the anterior horn 

• Fibers cross contralaterally at the level of the lower part of the brainstem 

12. Motor endplate: termination area of an efferent axon in the peripheral nerve 

13. Neuromuscular junction: the synapse between the nerve cell and muscle that when activated, initiates a muscle contraction 

Brachial Plexus: formed from the anterior rami of C5-T1 

1. 5 spinal roots from C5-T1 combine to form 3 trunks 

   1. Superior trunk: C5-C6 

   2. Middle trunk: C7 

   3. Inferior trunk: C8-T1 

2. 3 trunks divide into anterior/posterior = 6 divisions 

3. 3 cords are formed by combining the 6 divisions via their relationship with the axillary artery 

   1. Lateral cord: combines the anterior of the superior and middle trunks 

   2. Posterior cord: combines the posterior of all three trunks 

   3. Medial cord: continues the anterior of the inferior trunk 

4. 3 cords divide into 5 peripheral (terminal) nerves 

   1. Axillary nerve: branch of the posterior cord 

      • C5-C6 

      • Deltoid / teres minor muscles 

      • Sensory: lateral arm over portion of deltoid 

      • Impairment: loss of shoulder abduction & weak shoulder lateral rotation 

   2. Musculocutaneous nerve: branch of the lateral cord 

      • C5-C7 

      • Coracobrachialis. Biceps brachii, brachialis muscles 

      • Sensory: anterior lateral forearm 

      • Impairment: loss of elbow flexion, weak supination 

   3. Radial nerve: continuation of the posterior cord 

      • C5-T1 

      • Triceps brachii, anconeus, supinator, wrist/finger/thumb extensors & abductors 

      • Sensory: posterior arm & forearm; radial side posterior hand 

      • Impairment: loss of elbow, wrist, thumb, finger extension; weak supination 

   4. Median nerve: combines the lateral & medial cords  

      • C6-T1 

      • Pronators, wrist/finger flexors on radial side, thumb, first/second lumbricals 

      • Sensory: palmar aspect thumb, second to fourth fingers 

      • Impairment: loss of forearm pronation, loss of thumb opposition/flexion/abduction; weak wrist flexion on radial side; weak wrist radial deviation; weak second & third finger flexion 

   5. Ulnar nerve: continuation of the medial cord 

      • C8, T1 

      • Flexor carpi ulnaris, flexor digitorum profundis-medial half, Interossei, third/fourth lumbricals, fifth finger muscles 

      • Sensory: medial portion fourth finger, fifth finger 

      • Impairment: loss of ulnar deviation; weak wrist/finger flexion; loss of thumb adduction; loss of most intrinsic muscles 

• 13 total nerves arise from the brachial plexus 

• There are 8 nerves that branch prior to forming the 5 major peripheral nerves that contribute primarily to innervation of a single muscle of the shoulder girdle & upper extremity