Nervous System Lecture 5

Nervous System Lecture 5

• The nervous system controls and adjusts bodily activity and provides swift, brief responses
• The nervous system includes the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)
• The CNS comprises the brain and spinal cord and is responsible for integrating, processing, and coordinating sensory input and motor output. It's the center of intelligence, memory, learning, and emotions
• The PNS consists of all peripheral nerves and nervous tissue outside the CNS. It conveys sensory information to the CNS and carries motor commands away from the CNS
• The PNS comprises two divisions:
  • Afferent: brings sensory information to the CNS from sensory receptors such as muscles and glands
  • Efferent: carries motor commands from the CNS to muscles and glands

Neural Tissue

• Composed of two types of cells:
  • Neurons: responsible for transferring and processing information in the nervous system; consisting of a soma, axon, and dendrites
  • Neuroglia: supporting cells that protect neurons.
• Neuroglia also provides framework and maintains the intercellular environment. This includes acting as phagocytes (immune cells) and having the ability to reproduce

Neuron Structure

• Dendrites: receive stimuli from other neurons or the environment
• Cell body: contains the nucleus and other organelles
• Axon: carries nerve impulses toward other neurons or effector organs
• Axon hillock: junction between cell body and axon
• Terminal boutons: end of axon branches that transmit signals to other neurons or effectors
• Neurotransmitters: chemical messengers that carry signals across synapses

Nissl Bodies

• Clusters of rough endoplasmic reticulum (RER) and free ribosomes

Types of Neurons

• Anaxonic neurons: have more than two processes, but axons cannot be distinguished from dendrites
• Bipolar neurons: have two processes separated by the cell body
• Pseudounipolar neurons: a single elongate process with the cell body situated to one side
• Multipolar neurons: have a single axon and multiple dendrites

Synapses

• Site of intercellular communication
• Three types of synapses:
  • Synapses with another neuron
  • Neuromuscular synapses
  • Neuroglandular synapses

Multiple Synapses

• Terminal boutons of multiple neurons are distributed to numberous cells.

Neuronal Circuit Organization

• Divergence: spreading of stimulation to multiple neurons
• Convergence: input to a single neuron from multiple sources
• Serial processing; sequential processing of information
• Parallel processing: information processed simultaneously
• Reverberation: feedback mechanism; may be excitatory or inhibitory

Neuroglia of the CNS

• Astrocytes: maintain blood-brain barrier, provide structural support, regulate ion, nutrient, and dissolved-gas concentrations, absorb and recycle neurotransmitters, from scar tissue after injury
• Oligodendrocytes: myelinate CNS axons, provide structural framework
• Microglia: remove cell debris, wastes, and pathogens by phagocytosis
• Ependymal: line ventricles (brain) and central canal (spinal cord); assist in producing, circulating, and monitoring cerebrospinal fluid

Neuroglia of the PNS

• Satellite cells: regulate the exchange of material between the cell body and the environment
• Schwann cells (neurolemmocytes): form a myelin sheath that surrounds every peripheral axon.

White Matter and Grey Matter

• White matter: myelinated axons
• Gray matter: cell bodies, dendrites, and unmyelinated axons

Spinal Cord

• Conducts nerve impulses to and from the brain
• Processes some sensory information to allow reflex actions (motor actions)
• Composed of 31 spinal segments (Cervical, Thoracic, Lumbar, Sacral, Coccygeal regions)
• Consists of dorsal root ganglia, dorsal roots, ventral roots, and spinal nerves

Spinal Meninges

• Specialized membranes that protect, provide physical stability, and absorb shock
• Continuous with the cranial meninges
• Anchored by denticulate ligaments

Additional details about the Spinal Cord

• The spinal cord extends from the brain to the first lumbar vertebrae
• The spinal cord contains tracts of white matter which conveys sensory & motor information
• Sensory information will ascend while motor information will descend
• The spinal cord is segmented and the segments are interconnected and form specific nerves
• The spinal cord is protected by meninges, which are layers of protective tissue that surround the spinal cord. These meninges help to cushion and protect the spinal cord from injury, in addition to provide the supporting framework and physical stability of the spinal cord.

Reflexes

• Reflex: immediate, involuntary motor response
• Reflex arc: neural "wiring" of a single reflex; begins at a sensory receptor and ends at peripheral receptor
• Pathway of a reflex arc:
  • Activation of a sensory receptor
  • Relay of information to the CNS
  • Information processing
  • Activation of a motor neuron
  • Response by effectors
• Spinal reflexes: e.g., stretch reflex (stimulus stretches a muscle, activates sensory neuron, information in the spinal cord, motor neurons become active, muscle contracts)