Introduction to the Nervous System

Introduction to the Nervous System

• The nervous system is morphologically divided into two parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain, brainstem, cerebellum, and spinal cord.
• The PNS includes cranial and spinal nerves, sensory ganglia, and autonomic ganglia.

Central Nervous System

• The brain can be referred to as the cerebrum, prosencephalon, or forebrain.
• The telencephalon comprises the cerebral hemispheres.
• The diencephalon includes the epithalamus, thalamus, hypothalamus, and subthalamus.
• The brainstem contains the mesencephalon (midbrain), pons, and bulbus (medulla oblongata).
• The cerebellum, along with the pons, is sometimes referred to as the metencephalon.
• The CNS also consists of the spinal cord.
• The brain, brainstem, and cerebellum lie within the cranial cavity.
• The spinal cord extends within the vertebral canal.

Organization of the Nervous System

• Neurons related to a specific function form clusters called "nuclei."
• Axons performing similar functions form clusters called "tracts."
• The CNS is divisible into white and gray areas.
• Gray areas contain neurons, referred to as gray matter.
• White areas contain nerves, referred to as white matter.

Peripheral Nervous System

• Consists of peripheral nerves and ganglia.
• Peripheral nerves include cranial and spinal nerves.
• Ganglia can be autonomic (sympathetic and parasympathetic) or sensory.
• Nerves with sensory fibers have sensory ganglia.
• Spinal nerves have sensory ganglia which are 31 pairs called dorsal root ganglia or spinal ganglia.
• Cranial nerves with sensory fibers also have sensory ganglia, like the trigeminal nerve with its trigeminal ganglion.

Functional Divisions

• The nervous system has two functional parts: somatic and autonomic.
• The somatic nervous system is related to sensations from the outside environment and the locomotor system, controlling skeletal muscles. It's conscious and voluntary.
• The autonomic nervous system is related to sensations from the internal environment, controlling the function of organs and systems. It is unconscious and involuntary.

Microscopic Structure: Cell Types

• The nervous system consists of two types of cells: neurons and neuroglia.
• Neurons are the functional units.
• Neuroglia are the supporting cells.

Neuron Structure and Function

• Neurons are excitable cells responsible for: perception of sensation, production of motor and emotional responses, learning, memory, and control of organs and systems.
• A typical neuron has: a cell body, axon, dendrites and synaptic terminals to make connections with other neurons, referred to as synapses.
• Some axons leave the CNS to form nerves.
• Axons with a large diameter are typically enclosed by a myelin sheath and called myelinated axons.
• The myelin sheath is formed by supporting cells.
• Oligodendrocytes form the myelin sheath in the CNS.
• Schwann cells form the myelin sheath in the PNS.
• Small diameter axons in the CNS, postganglionic axons in the PNS, and some fine axons associated with pain reception are non-myelinated.
• Axons produce collaterals that have synaptic terminals at their ends and make synapses with other neurons.
• The cell membrane of neurons can be excited by chemical, electrical, or mechanical stimuli.
• Excitation results in a change in transmembrane potential called an action potential that is approximately around 40 mV.
• The action potential spreads over the cell membrane away from the initiation site, called a nerve impulse.
• The nerve impulse is transmitted to other neurons by synapses.

Neuron Variety

• Unipolar neurons: The cell body has a single neurite that gives off two axons, i.e. in the posterior root ganglion.
• Bipolar neurons have two neurites, i.e. retinal bipolar cells, cells of the vestibular and cochlear ganglia.
• Multipolar neurons: The cell body has multiple neurites; A longer neurite is called the axon, and shorter neurites are the dendrites.
• Golgi type I neurons have a long axon that may be up to 1 meter long or more. The axons form fiber tracts of the CNS and PNS, i.e. pyramidal cells of the cerebral cortex, purkinje cells of the cerebellar cortex, motor cells in the anterior horn of the spinal cord.
• Golgi type II neurons have a short axon or may be absent. They are mostly found in cerebral and cerebellar cortex and outnumber the Golgi type I neurons.

Neuron Classification

• Sensory (afferent) neurons: Receive impulses from peripheral receptors in the skin, eye, inner ear, nose, and tongue and transmit these impulses to the CNS.
• Motor (efferent) neurons: Transmit the impulses generated by the CNS to the muscles or organs
• Interneurons: comprise most of the neurons in the CNS, their axons do not leave the CNS, analyze the sensory information, and produce a response.
• General somatic afferent (GSA): Sensations arising from skin receptors for touch, pain, and temperature, and from muscle spindles, golgi tendon organs, and joint receptors.
• General visceral afferent (GVA): Sensations arising from organs.
• General somatic efferent (GSE): To skeletal muscles.
• General visceral efferent (GVE): To organs.
• Special somatic afferent (SSA): Vision, audition, equilibrium.
• Special visceral afferent (SVA): Smell and taste.
• Special visceral efferent (SVE): To muscles having the developmental origin from the 3rd and 4th branchiomeric arch (e.g. muscles of the pharynx and larynx, facial expression, and mastication, as well as muscles in the middle ear).

Synapses

• The synapse is the region where interneuronal communications occur.
• A neuron synapses with a single neuron or with multiple neurons.
• Most neurons synapse with 1,000-15,000 other neurons.
• The flow of information in a synapse is always in the same direction.
• Types of synapses: axodendritic, axosomatic, and axoaxonic.
• Impulse conduction in synapses may be chemical or electrical.
• Most synapses are chemical, and impulse conduction occurs through neurotransmitters.
• Electrical synapses, formed by gap junctions, allow bidirectional impulse conduction.
• Neurotransmitter release into the synaptic cleft causes an action potential in the second neuron.
• Chemical synapses can contain multiple neurotransmitters.
• While several neurotransmitters may be present, one is the primary activator for the postsynaptic neuron.
• Other neurotransmitters act as neuromodulators, modifying the activity of the principal neurotransmitter.
• Principal neurotransmitters include: acetycoline-nicotinic (rapid excitator), L-glutamate (rapid excitator) and GABA (rapid inhibitor).
• Principal neuromodulators include: acetycoline-muscarinic, seratonin, histamine and adenosine.

Neuroglia

• Supporting cells of the CNS.
• Smaller than neurons and outnumber them 5-10 times.
• Astrocytes: Form a supporting framework, serve as phagocytes.
• Oligodentrocytes: Form the myelin sheath in the CNS.
• Microglia: Phagocytic cells that become active in inflammatory and degenerative lesions of the CNS.
• Ependymal cells: Single layer of cells that line the cavities of the brain and spinal cord and function in circulation, absorption, and secretion of the CSF.

Spinal Nerves

• Spinal nerves arise from the spinal cord.
• 31 pairs of spinal nerves arise from each side of the spinal cord.
  • there are: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
• A spinal nerve is formed by the union of the sensory (dorsal) and motor (ventral) roots arising from a segment of the spinal cord.
• Autonomic fibers contribute to spinal nerve formation.
• A spinal nerve carries types of axons: GSA, GSE, GVA, and GVE.
• A spinal nerve gives off a ventral ramus and a dorsal ramus.
  • Dorsal rami go to innervate the skin and muscles of the back.
• Ventral rami form plexuses except in the thoracic region, where they continue as intercostal nerves.
• The plexuses are: cervical, brachial, lumbar and sacral.

Receptors

• Sensations from the outside and inside environment are received by special sensory nerve endings or receptors.
• Receptors are classified into types:
  • Mechanoreceptors
  • Thermoreceptors
  • Nociceptors
  • Photoreceptors
  • Chemoreceptors and baroreceptors

Skin Receptors

• Noncapsulated receptors:
• Free nerve endings: Widely spread through the body(skin, cornea, digestive tract, etc.) and detect mostly pain.
• Merkel’s discs: Found in hairless skin and in hair follicles and they detect degree of pressure.
• Capsulated receptors:
  • Meissner’s corpuscles: Found in the dermal papillae of the skin, especially at the palm of the hand and sole of the foot, selectively sensitive to touch, enabling the detection of two-point tactile discrimination.
  • Pacinian corpuscles: Widely spread through the body and are sensitive to vibration.
  • Ruffini’s corpuscles: Found in the dermis of the hairy skin and sensitive to stretch of the skin.

Kinesthesia Receptors

• These receptors are within joint capsules, muscles, and tendons.
• They detect position and movement.
• Joint capsule receptors inform the CNS about the position of the joint.
• Muscle spindles found in the skeletal muscles inform the CNS about the degree of contraction, sensitive to the changes of muscle length so CNS can control muscle activity.
• Golgi tendon organs are present in the tendons of the muscles, and are sensitive to the tension of the muscle.