General organization of Nervous system

Organization of the Nervous System Central Nervous System (CNS) Peripheral Nervous System (PNS) Brain Spinal Cord Cranial Nerves (and their branches) Spinal Nerves (and their branches) Somatic Nervous System (SNS) Autonomic Nervous System (ANS) Enteric Nervous System (ENS) Sympathetic Division Parasympathetic Division Sensory: External stimuli Somatic receptors (touch, pain) Special senses (vision, hearing) Sensory: Internal stimuli (i.e. visceral organs) Sensory: Internal stimuli (i.e. GI tract) Motor: Skeletal muscles Motor: Visceral organs Motor: GI tract

Cells of the Nervous System Nervous tissue consists of 2 types of cells Neurons: Functional unit cells “Unique functions” of the nervous system (e.g. sensation, information processing, control functions) Communicate via electrical signals and chemical messengers Neuroglia: Support cells Nourish and protect neurons Preserve the physical and biochemical structure of nervous tissue

Neurons Dendrites Axon Cell body Axon terminals “INPUT” “OUTPUT” Slender processes that branch out from the cell body “OUTPUT” Dendrites Axon Single long process extending out from the cell body Cell body Expanded portion that contains most of the typical cellular “bits” Axon terminals Transmit signals to a nerve/effector “Typical neuron”

Ganglion (pl = ganglia) Neuronal cell bodies and axons tend to group together throughout the nervous system Cluster of Cell Bodies Bundle of Axons CNS Nucleus (pl = nuclei) Tract PNS Ganglion (pl = ganglia) Nerve

Structural Classification of Neurons Multipolar Bipolar Unipolar Many dendrites and one axon One dendrite and one axon Dendrites and axon are fused Most common Special sense organs (eye, ear) Sensory neurons of the PNS

Intricate web of dendrites Cell body shaped like a pyramid Purkinje cells Pyramidal cells Intricate web of dendrites Cell body shaped like a pyramid Cerebellum Cerebral cortex

Functional Classification of Neurons Sensory (Afferent) Neurons: Deliver signals from peripheral receptors to the CNS Primarily unipolar neurons whose cell bodies are located in peripheral sensory ganglia Motor (Efferent) Neurons: Deliver signals from the CNS to effectors (e.g. muscles, glands) in the periphery Primarily multipolar neurons whose cell bodies are located in spinal cord nuclei Interneurons: Transmit signals between sensory and motor neurons (i.e. distribute sensory information and coordinate motor activity) Primarily multipolar neurons which are contained entirely within the CNS

Neuroglia Much more abundant than neurons The CNS and PNS have different types of neuroglia CNS: Astrocytes Oligodendrocytes Ependymal cells Microglia PNS: Satellite cells Schwann cells

Neuroglia of the CNS Astrocytes Oligodendrocytes Ependymal cells Guide neuron development Create a supportive framework Maintain the blood-brain barrier Regulate the composition of the interstitial fluid Oligodendrocytes Produce the myelin sheaths of (some) CNS neurons Ependymal cells Line the canals and ventricles of the CNS Produce cerebrospinal fluid (CSF) and assist in its circulation Microglia Act as macrophages (remove cellular debris and pathogens)

“Oligodendrocyte of the PNS” Neuroglia of the PNS Satellite cells Create a supportive framework Regulate the composition of the interstitial fluid “Astrocyte of the PNS” Schwann cells Produce the myelin sheaths of (some) PNS neurons “Oligodendrocyte of the PNS”

Myelin “White matter” “Grey matter” Fatty substance that gets wrapped around the axons of some neurons Functions: Protects/insulates the axon Increases the conduction speed of the axon “White matter” Region of myelinated axons “Grey matter” Region of unmyelinated axons and/or neuronal cell bodies

Electrical Signals in Neurons Like most cells, neuronal cell membranes have a resting membrane potential (a difference in electrical charge between the inside/outside) This is due to a difference in the number of positive and negative ions on either side of the cell membrane, with more negative ions being present within the cell (giving a negative value to the membrane potential) Resting membrane potential -70mV

Neurons and muscle cells are “electrically excitable”, meaning that their resting membrane potential changes in response to certain stimuli In neurons, such changes take one of two forms: GRADED POTENTIALS (Dendrites & cell bodies) ACTION POTENTIALS (Axons)

Graded Potentials Relatively small change in the resting potential that is localized to a small area of the cell membrane Variable effect size Can undergo summation Diminish in intensity as they travel Can be either excitatory or inhibitory

Action Potentials Relatively large electrical impulse that is generated when the resting potential reaches a certain threshold (usually ~-55mv) “All or none” Travel along the length of the neuron at a constant intensity Always excitatory

Signal Propagation Between Neurons When an action potential travelling along an axon reaches a synaptic terminal, it is “transmitted” to an effector across a synapse Synapses can be: Electrical – Direct transmission via small connecting channels Chemical – Indirect transmission via the release of neurotransmitters

Neurotransmitters have variable effects on an effector neuron Excitatory – Produces a graded potential that brings the neuron’s cell membrane closer to threshold Inhibitory – Produces a graded potential that brings the neuron’s cell membrane further from threshold Neurons constantly receive input from hundreds/thousands of other neurons These inputs undergo summation, and if the cell membrane reaches threshold as a result, an action potential will be generated in the neuron’s axon

External Anatomy of the Spinal Cord In adults, the spinal cord extends from the medulla oblongata (the inferior portion of the brain stem) to the level of the L1-2 vertebrae Therefore, the spinal cord is shorter than the vertebral column Its caudal end tapers to a point called the conus medullaris Extending inferiorly from this point is a strand of fibrous tissue (filum terminale) that anchors the spinal cord to the coccyx

Medulla oblongata Spinal cord Conus medullaris L1-2 Filum terminale

The spinal cord has two regions of increased diameter: The spinal cord is divided into 31 “segments”, each of which gives rise to paired spinal nerves: 8 pairs of cervical spinal nerves (C1-C8) 12 pairs of thoracic spinal nerves (T1-T12) 5 pairs of lumbar spinal nerves (L1-L5) 5 pairs of sacral spinal nerves (S1-S5) 1 pair of coccygeal spinal nerve (Co1) The spinal cord has two regions of increased diameter: Cervical enlargement (C3-T2 vertebrae) – Gives rise to the spinal nerves that innvervate the upper limbs Lumbar enlargement (T9-T12 vertebrae) – Gives rise to the spinal nerves that innervate the lower limbs

Spinal nerves Cervical enlargement Lumbar enlargement Spinal nerves

Internal Anatomy of the Spinal Cord When looked at in cross-section, the spinal cord has a ring of white matter that surrounds a core of grey matter There are two grooves that extend into the white matter and separate the spinal cord into left and right sides In the centre of the grey matter is a central canal, which runs the length of the spinal cord and is filled with CSF Posterior median sulcus Central canal White matter Grey matter Anterior median fissure

Regions of Grey Matter Posterior (dorsal) horn Lateral horn Cell bodies of interneurons that relay sensory signals up the spinal cord Posterior (dorsal) horn Posterior Lateral horn Cell bodies of autonomic motor neurons Only present in the T1-L2 and S2-S4 spinal cord segments. Anterior Anterior (ventral) horn Cell bodies of somatic motor neurons

Regions of White Matter Posterior columns Lateral column Lateral column Anterior columns Each of these columns contains ascending (sensory) tracts and/or descending (motor) tracts.

Spinal Nerves Posterior (dorsal) root Spinal nerve Made up of axons of sensory neurons. Cell bodies are within a posterior (dorsal) root ganglion. Posterior (dorsal) root Posterior (dorsal) root ganglion Spinal nerve Spinal nerves are “mixed” (sensory and motor axons). Anterior (ventral) root Made up of axons of motor neurons. Cell bodies are within the grey matter of the spinal cord.

Intervertebral foramen The spinal nerves emerge from the vertebral column between adjacent vertebrae through the intervertebral foramina (IVF) C1-C7 spinal nerves exit superior to their corresponding vertebrae C8 spinal nerves exit between C7 & T1 Thoracic and lumbar spinal nerves exit inferior to their corresponding vertebrae Intervertebral foramen

Since the spinal cord is shorter than the vertebral column, most spinal nerve roots angle inferiorly before joining within their respective IVFs The nerve roots of the L2-Co1 spinal segments extend caudally from the conus medularis (along with the filum terminale) until they emerge from the vertebral canal at their respective levels Since this collection of fibres resembles a horse’s tail, it is referred to as the cauda equina Cauda equina