Unit 3: Spinal Cord and Brainstem

Unit 3: Spinal Cord and Brainstem

• The spinal cord is the CNS part within the spinal column.
• The spinal column consists of vertebrae.
• The spinal cord is the main communication pathway between the brain and body, but cranial nerves are the main pathway for communication between the brain and head.
• The spinal cord has a degree of autonomy from the brain, managing reflexes.
• The spinal cord is made up of grey and white matter.
• Grey matter is densely packed with cell bodies and dendrites with axons of white matter communicating to other segments.
• Sensory and motor nerves are on both sides of the spinal cord,.
• Sensory nerves carry information from the body to the spinal cord.
• Motor nerves carry information from the spinal cord to the body.
• The spinal cord has a segmented structure, with 31 pairs of spinal nerves.
• If the spinal cord is cut at a particular segment, sensation and motor control below that segment are lost.
• The spinal cord is surrounded by meninges (protective membranes) and cerebrospinal fluid (CSF).

Protection of the Spinal Cord

• The spinal cord is protected by the spinal/vertebral column, meninges and the cerebrospinal fluid (CSF).
• The spinal column consists of many vertebrae: cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and coccygeal (pelvic).
• The spinal cord runs through the spinal foramen in the centre of each vertebra.
• Meninges are the three-layered protective membranes around the brain and spinal cord.
• The pia mater is the innermost layer adhering to the spinal cord.
• The arachnoid is the middle layer, with cerebrospinal fluid (CSF) between it and the pia mater.
• The dura mater is the outermost, strong and inflexible layer.

External Organisation

• The spinal cord is roughly the size of a little finger, but not uniform in diameter.
• The spinal cord has two enlargements where spinal nerves leave to innervate the upper and lower limbs.
• Each pair of spinal nerves connects to the left and right side of the spinal cord, respectively.
• The dorsal roots and ventral roots join to form the spinal nerve.
• Dorsal roots contain primary afferent sensory neurons from the body periphery to the CNS.
• Sensory neurons' cell bodies are in the dorsal root ganglia.
• Ventral roots contain efferent motor neurons innervating skeletal muscles.

Internal Organisation: Grey Matter

• Spinal cord grey matter has four protrusions: dorsal and ventral horns, extending to the dorsal and ventral roots of spinal nerves.
• The dorsal horn is where the sensory neurons arrive.
• The ventral horn contains motor neurons' cell bodies, exiting via ventral roots to effector organs.
• The lateral horn, found in some spinal cord segments, accommodates sympathetic preganglionic neurons.
• Grey matter forms an 'H' (or butterfly) shape around the central canal, which contains cerebrospinal fluid (CSF).
• The dorsal and ventral horns can be further divided into laminae, based on cell properties and function.

Internal Organisation: White Matter

• Spinal cord white matter consists of axons, communicating between the spinal cord and other CNS areas.
• Axons organized into columns (dorsal, lateral and ventral).
• Tracts (axon pathways), grouped by origin, termination, and function, are within the white matter columns.
• Ascending tracts carry information from the periphery to the brain.
• Descending tracts carry information from the brain to the spinal cord, influencing motor activity, reflexes, and autonomic functions.

Reflexes

• Reflexes are involuntary responses to stimuli.
• Monosynaptic reflex arcs involve a sensory neuron, an interneuron, and a motor neuron, with a single synapse.
• Polysynaptic reflex arcs involve more than one synapse, often involving interneurons.
• The reflex action involves sensory neurons sending impulses to the spinal cord, which then communicates a signal to a motor neuron causing a response.

Introduction: Brainstem

• The brainstem consists of the midbrain, pons, and medulla.
• The brainstem plays a crucial role in sensory and motor information pathways as they pass through the brainstem.
• The brainstem attaches to the spinal cord caudally, while it is connected to the diencephalon rostrally.

Brainstem: Medulla and Pons

• The medulla is a vital extension of the spinal cord.
• Vital reflexes like breathing, vomiting, coughing, and sneezing are controlled by the medulla.
• The pons has a transverse fibre system (pontocerebellar fibres) connecting cerebral cortex and cerebellum, coordinating body movements.

Brainstem: Midbrain

• The midbrain has the tectum (upper and lower colliculi) and the tegmentum.
• The superior colliculus is important for the visual system.
• The inferior colliculus plays a vital role in auditory processing.
• The tegmentum has nuclei for various functions (including reticular formation, periaqueductal grey matter, the red nucleus and substantia nigra).

Brainstem: Reticular Formation

• The reticular formation is a complex network of nuclei running through the brainstem.
• It plays a crucial role in many functions such as motor activity, breathing, cardiovascular function, muscle tone, posture and pain perception.
• It's crucial for arousal, sleep and consciousness.

Brainstem: Periaqueductal Grey Matter

• The periaqueductal grey matter encircles the cerebral aqueduct.
• It participates in primitive behaviors such as aggression and mating and impacts how opiates produce pain relief.

Brainstem: Substantia Nigra

• The substantia nigra is part of the tegmentum, linking to basal ganglia (caudate nucleus and putamen)
• It's involved in voluntary movement control.
• Degeneration of substantia nigra (especially its connections with the caudate nucleus) is a symptom of Parkinson's disease.

Cranial Nerves

• The brainstem also contains the nuclei of most cranial nerves.
• Cranial nerves control sensory and motor functions of the head and neck (and some internal organs).
• There are twelve pairs of cranial nerves.