Spinal Cord Anatomy and Function

Chapter 13: The Spinal Cord and Spinal Nerves

• This chapter identifies and describes the anatomical features of the spinal cord and spinal nerves.
• It discusses the functions of the spinal cord and spinal nerves, and how they maintain homeostasis.
• It explains spinal reflex arcs.

Introduction

• The chapter's aim is to detail the spinal cord's anatomical features and spinal nerves.
• It explains the functions of the spinal cord and spinal nerves, detailing their role in homeostasis.
• It aims to clarify spinal reflex arcs.

Functions of the Spinal Cord

• The spinal cord processes reflexes.
• It integrates EPSPs (excitatory postsynaptic potentials) and IPSPs (inhibitory postsynaptic potentials).
• It transmits sensory impulses to the brain and motor impulses to effectors.

Spinal Cord Anatomy

• The spinal cord is protected by bone (vertebrae), connective tissue (meninges), and cerebrospinal fluid.

Protection of the Spinal Cord

• The spinal cord's protection involves layers of:
  • Bone (vertebrae).
  • Connective tissue (meninges).
  • Fluid (cerebrospinal fluid).

Meninges

• The meninges consist of three layers: dura mater, arachnoid mater, and pia mater.

External Anatomy of the Spinal Cord

• The spinal cord begins at the foramen magnum and ends at the level of L2.
• The spinal cord has two enlargements: cervical and lumbar.
• The cervical enlargement controls sensory and motor input for the upper extremities.
• The lumbar enlargement manages motor and sensory input for the legs.
• The cauda equina comprises the roots of the lower spinal nerves. The nerves angle down alongside the filum terminale, an extension of the pia mater.
• The spinal cord is connected to each spinal nerve via roots and even smaller rootlets that carry axons. The posterior (dorsal) rootlets contain only sensory axons, which transmit nerve impulses from skin, muscle, and internal organs into the central nervous system.

Internal Anatomy of the Spinal Cord

• Spinal nerves connect the central nervous system to sensory receptors, muscles, and glands of the peripheral nervous system.
• 31 pairs of spinal nerves.
• Anterior and posterior roots join a spinal nerve to a section of the spinal cord.
• The white matter of the spinal cord is on the outside, whereas the gray matter is on the inside.

Sensory and Motor Processing

• The spinal cord's internal anatomy allows for processing sensory and motor information in an organized manner.

Comparison of Various Spinal Cord Segments

• Different sections of the spinal cord have distinguishing characteristics concerning diameter, white matter, and gray horns.
• The cervical segment features a relatively larger diameter and greater white matter.
• The thoracic segment is characterized by a smaller diameter and less prominent gray horns.
• The lumbar segment shows a near-circular shape, with larger anterior and posterior gray horns.
• The sacral segment has a smaller shape with larger amounts of gray matter and relatively less white matter.
• The coccygeal segment resembles the sacral segment but is smaller.

Spinal Nerves

• Spinal nerves connect the central nervous system to sensory receptors, muscles, and glands in the peripheral nervous system.
• 31 pairs of spinal nerves exist, each attaching to a spinal cord segment through anterior and posterior roots.

Connective Tissue Covering of Spinal Nerves

• The coverings of spinal nerves include epineurium, perineurium, and endoneurium.

Peripheral Nerves

• Spinal nerves are communication paths between the spinal cord and specific body parts.
• Nerves are comprised of fascicles grouped by perineurium, a connective sheath surrounding the entire nerve.

Peripheral Nerves: Emergence and Branches

• Spinal nerves emerge at regular intervals, except for the first cervical pair that emerges between bordering vertebrae.
• Spinal nerves go through the dura and split into anterior rami, posterior rami, and rami communicantes. Rami communicantes connect to sympathetic ganglia.

Peripheral Nerves: Plexuses

• Anterior rami frequently form plexuses which are large interconnected networks of interwoven nerves.
• Major plexuses include cervical, brachial, celiac (solar), lumbar, sacral, and coccygeal.

Branches of a Spinal Nerve

• Shortly after exiting the intervertebral foramen, a spinal nerve divides into several rami (branches).

Dermatomes

• Dermatomes represent specific skin regions associated with spinal nerves.

Cervical Plexus

• The cervical plexus supplies the skin and muscles of the head, neck, upper shoulders, chest, and diaphragm.
• The main nerve to know within this plexus is the phrenic nerve (C3-C5), which controls the diaphragm.

Brachial Plexus

• The brachial plexus supplies nerves to the shoulder and upper limbs. A lesion of the median nerve results in carpal tunnel syndrome.

Nerve Plexuses

• The long thoracic nerve emerges from the brachial plexus and supplies the serratus anterior muscle.
• This nerve is susceptible to injury.
• Various injuries to the brachial plexus are possible.

Nerve Plexuses: Lumbar Plexus

• The lumbar plexus supplies nerves for the anterior wall of the abdomen, the buttocks, the upper part of the thighs, and the medial aspects of legs and feet.
• Key nerves to know in this plexus include ilioinguinal, iliohypogastric, genitofemoral, lateral femoral cutaneous, obturator, and femoral nerves.

Nerve Plexuses: Sacral Plexus

• The sacral plexus supplies nerves for parts of the posterior thigh, buttocks, and lower leg. The main one to know, the sciatic nerve, supplies the back of the leg and foot.

Spinal Cord Physiology

• The spinal cord plays roles in homeostasis by propagating and integrating nerve impulses. White matter tracts transmit impulses to and from the brain. Gray matter receives and integrates information to perform spinal reflexes.

Sensory and Motor Tracts

• White matter tracts conduct sensory and motor impulses to and from the brain.

Reflexes and Reflex Arcs

• Reflexes are rapid involuntary responses to stimuli, crucial for maintaining homeostasis.
• Gray matter within the spinal cord serves as the integrating center for spinal reflexes.

Reflexes: Interactions Animation

• The general components of a reflex arc include: sensory receptor, sensory neuron, integrating center, motor neuron, and effector.

Reflex Arcs: Vocabulary Terms

• Reflex terms like ipsilateral, contralateral, monosynaptic, and polysynaptic describe the pathway of a reflex. Reciprocal innervation explains the simultaneous contraction of one muscle and relaxation of its antagonist.

Stretch Reflex

• The stretch reflex causes a muscle to contract when stretched.

Tendon Reflex

• The tendon reflex causes a muscle to relax when tension is detected in its tendon.

Flexor (Withdrawal) Reflex

• The flexor (withdrawal) reflex moves a limb away from a painful stimulus.

Crossed-Extensor Reflex

• The crossed-extensor reflex maintains balance during a withdrawal reflex by extending the opposite limb.

Reflexes

• Reflexes include the patellar, Achilles, Babinski, and others, and their impact on the spinal cord and body.

Disorders

• Spinal disorders can be classified into traumatic injuries, spinal cord compression, degenerative diseases, syndromes like shingles and Poliomyelitis, and others. The extent and location of injuries determine the degree and nature of resultant paralysis.

Damage to the Spinal Cord

• "Transection" of the spinal cord severing ascending/descending tracts, can lead to paralysis depending on the injury's level. Severing at the base of the skull can lead to asphyxiation.

Other Spinal Cord Disorders

• Spinal cord compression (from various causes) and degenerative diseases (like Multiple Sclerosis and Amyotrophic Lateral Sclerosis) are considered disorders affecting the spinal cord.
• Shingles results from a viral infection.
• Poliomyelitis, caused by a virus, can lead to paralysis.

This quiz covers essential aspects of spinal cord anatomy, including the roles of various nerves and the structural features of the spinal cord. It features questions on the lumbar enlargement, white matter arrangement, and nerve innervation. Test your knowledge about the spinal cord's protective structures and key functions.