Spinal Cord Anatomy

Spinal Cord

• Spinal cord is a major communication link between the brain and the peripheral nervous system which includes spinal nerves
• The spinal cord helps integrate incoming information and produces responses through reflex mechanisms
• The spinal cord generates 31 pairs of spinal nerves
  • Cervical and lumbosacral enlargements give rise to the spinal nerves of the limbs
  • Nerves from the end of the spinal cord form the cauda equina
• It is shorter than the vertebral column

Meninges of the Spinal Cord

• Three meningeal layers surround the spinal cord: dura mater, arachnoid mater, and pia mater, from superficial to deep
• The epidural space exists between the periosteum of the vertebral canal and the dura mater
• The subarachnoid space exists between the arachnoid mater and the pia mater

Spinal Cord Cross Section

• Spinal cord consists of peripheral white matter and central gray matter
• White matter is organized into columns (funiculi) which are subdivided into tracts (fasciculi or pathways); these consist of ascending and descending axons
• Gray matter is divided into horns
  • The dorsal horns contain sensory axons that synapse with interneurons
  • The ventral horns contain the neuron cell bodies of somatic motor neurons
  • The lateral horns contain the neuron cell bodies of autonomic motor neurons
  • The gray and white commissures connect each half of the spinal cord
• The dorsal root contains sensory axons
• The ventral root has motor axons
• Spinal nerves have both sensory and motor axons

Reflexes

• Reflexes are stereotypic, unconscious, involuntary responses to stimuli and maintain homeostasis
• Two general types of reflexes:
  • Somatic reflexes: Mediated through the somatic motor nervous system and include responses that remove the body from painful stimuli, keep the body from falling, and cause movement because of external forces
  • Autonomic reflexes: Mediated through the ANS and responsible for maintaining variables within their normal ranges including blood pressure, blood carbon dioxide levels, and water intake
• A reflex arc is the functional unit of the nervous system
  1. Sensory receptors respond to stimuli and produce action potentials in sensory neurons
  2. Sensory neurons propagate action potentials to the CNS
  3. Interneurons in the CNS synapse with sensory neurons and with motor neurons
  4. Motor neurons carry action potentials from the CNS to effector organs
  5. Effector organs, including muscles or glands, respond to the action potential

Spinal Nerves

• There are 31 pairs of spinal nerves: eight cervical, twelve thoracic, five lumbar, five sacral, one coccygeal
• Spinal nerves have specific cutaneous distributions called dermatomes

Spinal Nerves and Plexuses

• Spinal nerves branch to form rami
  • The dorsal rami supply the muscles and skin near the middle of the back
  • The ventral rami in the thoracic region form intercostal nerves, which supply the thorax and upper abdomen
  • Cervical, lumbar, sacral and coccygeal ventral rami join to form plexuses
  • Communicating rami supply sympathetic nerves

Brain

• The brain is located in the cranial cavity, and is the control center for many of the body's functions
• The brain consists of the brainstem, cerebellum, diencephalon, and cerebrum

Brainstem

• The brainstem consists of the medulla oblongata, pons, and midbrain
• It connects the spinal cord and cerebellum to the remainder of the brain
• Ten of the 12 pairs of cranial nerves arise from the brainstem
• Damage to small areas of the brainstem can cause death, whereas damage to relatively large areas of the cerebrum or cerebellum do not

Medulla Oblongata

• It is continuous with the spinal cord and contains ascending and descending tracts
• Medullary nuclei regulate the heart, blood vessels, breathing, swallowing, vomiting, coughing, sneezing, hiccupping, balance and coordination
• The pyramids are tracts controlling voluntary muscle movement

Pons

• It is superior to the medulla
• Ascending and descending tracts pass through the pons
• It connects the cerebrum and the cerebellum
• Pontine nuclei regulate breathing, swallowing, balance, chewing, and salivation

Midbrain

• It is superior to the pons
• Corpora quadrigemina consist of four colliculi
  • Two inferior colliculi are involved in hearing
  • Two superior colliculi are involved in visual reflexes
• Substantia nigra and the red nucleus help regulate body movements
• Cerebral peduncles are the major descending motor pathway

Reticular Formation

• It consists of nuclei scattered throughout the brainstem
• Regulates cyclic motor functions, like breathing, walking, and chewing
• Reticular activating system maintains consciousness and regulates the sleep-wake cycle, and is part of the reticular formation

Cerebellum

• Gray matter forms the cortex and nuclei of the cerebellum
• White matter:
  • Arbor vitae
  • Connects the cerebellum to the rest of the CNS
  • Connects the cerebellar cortex and cerebellar nuclei
• The cerebellum has three parts:
  • Flocculonodular lobe: controls balance and eye movements
  • Vermis and medial part of the lateral hemispheres: controls posture, locomotion, and fine motor coordination
  • The lateral hemispheres are involved with the planning, practice, and learning of complex movements

Diencephalon

• Found between the brainstem and the cerebrum
• Consists of the thalamus, subthalamus, epithalamus, and hypothalamus

Thalamus

• Two lobes are connected by the interthalamic adhesion
• Functions as an integration center
• All sensory input that reaches the cerebrum, except for the sense of smell, synapses in the thalamus
• Interacts with other parts of the brain to control motor activity
• Involved in emotions and pain perception

Subthalamus

• It is inferior to the thalamus
• Involved in motor function

Epithalamus

• It is superior and posterior to the thalamus
• Consists of the:
  • Habenular nuclei, which influence emotions through the sense of smell
  • Pineal body, which may play a role in the onset of puberty and the sleep-wake cycle

Hypothalamus

• Main visceral control center of the body, vitally important to overall body homeostasis.
  • Autonomic control center (heart rate, blood pressure, etc.)
  • Center for emotional response/behavior
  • Body temperature regulation
  • Regulation of food intake
  • Regulation of water balance
  • Control of endocrine system functioning
  • Mammillary bodies are reflex centers for olfaction

Cerebrum

• Cortex of the cerebrum is folded into ridges called gyri and grooves called sulci or fissures
• Longitudinal fissure divides the cerebrum into left and right hemispheres
• Each hemisphere has five lobes: frontal, parietal, occipital, temporal, and insula
• Frontal lobes: Involved in voluntary motor function, motivation, aggression, the sense of smell, and mood
• Parietal lobes: Contain the major sensory areas receiving sensory input like touch, pain, temperature, balance, and taste
• Occipital lobes: Contain the visual centers
• Temporal lobes: Evaluate smell and hearing input, and are involved in memory, abstract thought, and judgment
• Insula: Located deep within the lateral fissure

Cerebrum - White Matter

• Gray matter forms the cortex and nuclei