Physiology

Nervous System

• Nervous system is responsible for all our behaviors, memories and movements.

• Graded potential and nerve muscle action potentials are involved in the relay of senosry stimuli, integrative functions such as perception and motor activities.

• Neuron = bundle of neuron fibers.

Function of Nervous System

• Sensory Function: sensory receptors detect internal stimuli. This info is carried to the brain and spinal cord through cranial and spinal nerves.

• Integrative Function: nervous system processes sensory info by analysing it and making decisions for appropriate responses.

• Motor Function: once sensory info is integrated, the nervous system may elicit an appropriate motor response by activating effectors.

Nervous Tissue

It consists of :

• Neurons (nerve cells): cells that send and receive signals

  • Consist of cell body and processes extending from the cell body (dendrites and axons)

• Neuralgia (glial cells): cekks that support and protect neurons

  • Do not generate or conduct nerve impulses

  • Make up half the volume of the CNS

Types of Neuroglial Cells

• Oligodendrocytes (CNS)

  • Responsible for forming and maintaining the myelin sheath around CNS axons.

  • Each wraps around many nerve fibers.

• Ependymal Cells (CNS)

  • Line central ventricle of the brain and produce and assist in circulation of CSF

  • CSF = Cerebrospinal fluid that protects and nourishes the brain and spinal cord.

• Astrocytes (CNS)

  • Most abundant glial cells

  • Contain microfilaments- give strength and biochemical support to neurons

  • Maintain appropriate chemical environment for the generation of nerve impulses

  • Form blood barrier

  • Provision of nutrients to nervous tissue

  • Role in Repair and scarring process of brain and spinal cord following injuries

• Microglia (CNS)

  • Formed from monocytes

  • Remove cellular debris or invading micro-organisms

  • In areas of infection, trauma or stroke

• Schwann Cells (PNS)

  • Myelinate fibers of PNS

• Satellite Cells (PNS)

  • Supply nutrients to surroundings neurons

  • Provide Structure

  • Protective ad cushioning cells that surround and support cell bodies

Myelination

• Myelin Sheath: multilayerd lipid (phospholipids) and proteins covering that surround the axons of most neurons.

  • Produced by Schwann cells (PNS) and Oligodendrocytes (CNS)

  • Surrounds the axons of most neurons

  • Insulates axons and increases the speed of nerve impulse conduction

• Astrocytes monitor neuron axons for activity

• Myelinated sheaths have gaps between cells which allow the signals to jump the gaps faster

Ions Channels

• Gated Channels: open and close in response to stimulus which results in neuron excitability

• Transition of a channel between closed and open state is called gating

• Leakage (non-gated) Channels: are always open

  • Nerve cells have more K+ than Na+ leakage channels resulting in higher membrane permeability for K+ than for Na+ at rest.

  • Resting membrane potential of -70 mV in nerve tissue

Generation of an Action Potential (AP)

• Occrus in dendrites and cell body

• Affects small portion of cell membrane

  • Whereas AP affects entire surface of membrane

• Allows for communication over short distances

  • AP allows for communication over long distances

• Loaded graded potential: small deviation from resting MP that makes membrane more polarised (inside more negative) or less polarised (inside more positive)

  • Occur in response to opening or closing of mechanically gated or ligand gated ion channels

  • Degree of deviation depends on the strength of Stimulus.

• Hyperpolarizing graded potential: when response makes membranes more positive (excitatory postsynaptic potential)

• Depolarising graded potential: when membrane becomes less positive (inhibitory post synaptic potential)

• Summation: Method of signal transduction between neurons

  • Determines if an action potential will be triggered by effects of postsynaptic potentials

  • Can be Spatial or Temporal

    • Spatial: summation of postsynaptic potentials in response to stimulus that occur at different locations in the membrane of postsynaptic cell at the same time.

    • Temporal: summation of postsynaptic potentials in response to stimulus that occur at the same location but at different times.

Conduction Vs Saltatory Conduction

• Continuous Conduction (unmyelinated fibers): step by step depolarization of each portion of the length of the axolemma

  • No myelin sheath = Slower

• Saltatory Conduction: depolarization only at nodes of Ranvier where there is a high density of voltage gated ion channels.

  • Current carried by ion flows through ECF from node to node

  • Voltage gated channels needed for APs

Propogation of an Impulse from one neuron to another

• Synapse: functinal gap between one neuron to another where the message is passed chemically

  • At synapse chemicals known as neurotransmitters diffuse across gaps

  • Action Potentials are transmitted from neuron to neuron across synapses

• Impulses as a whole is equivalent to electrochemical changes

1. Arrival of depolarisatio wave at presynaptic terminal (Knob)

2. Wave causing opening of Ca2+ (activated channels and an inflow of Ca2+ ions into the cells)

3. Exocytosis: Ca2+ promote fusion of synaptic vesicles with the presynaptic membrane and secretion of neurotransmitter into the synaptic cleft

4. Neurotransmitter diffuses across synaptic cleft which binds to receptors on the post synaptic membrane

5. Binding opens ion channels (Na+ channels)

6. Ions flow into cell resulting in depolarisation and AP

7. When depolarising post synaptic potential reaches threshold it triggers an AP

8. Neurotransmitter is formed in the cell body and transported down the axon to the knob where it is packaged into vesicles

Spinal Nerves

• 31 pairs of spinal nerves: numbered according to region

  • 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral and 1 Coccygeal

• Connect to CNS to sensory receptors, muscles and glands in all parts of the body.

• Dorsal Root: contains axons of neurons whose cell bodies are in Dorsal Root Ganglion (DRG)

• DRG: contains sensory neuron cell bodies. Each spinal segment contains one on each side.

• Central Root: conains axons of motor neurons extending into PNS to control muscles, glands and visceral organs.

• Spinal Nerve: contains axons of both motor and sensory neurons

  • Sensory enter CNS via doral root and Motor exit via Ventral Root

Spinal Reflex

• Integration takes place in Spinal Cord

• Rapid, automated response to specific stimuli

• Preserve homeostasis by making rapid adjustments in functions of organas

• 5 components of Reflex Arc

  • Receptors

  • Sensory Neurons

  • Integrating Center

  • Motor Neuron

  • Effector

Stretch vs Tendon Reflex

• Stretch Reflex: feedback mechanism which control muscle length by causing a muscle contraction via muscle spindles which respond to change in length.

  • Prevents injury from overstretching

  • Stretch Reflex (Hitting Patellar With Hammer)

    • Slight stretch of quadriceps (stimulates muscle spindles)

    • Sensory Neuron excited and carries impulses to SC

    • Sensory neurons activate motor neurons to quadriceps which causes contraction to relieve initial stretch and extend the knee in a brief kick

    • Branch of sensory neuron connects to an inhibitory neuron that connects to a motor neurons which inhibitis hamstring so it relaxes

    • Contraction and relaxation of muscle is called reciprocal inhibition

• Tendon Reflex: Feedback mechanism to control muscle tension

  • Causes relaxation of the muscle attached to the stimulated tendon organ

  • Increase tension applied to tendon which stimulates sensory receptors tendon organ

  • Nerve impulses propogate to SC along a sensory neuron

  • Within SC sensory neuron activates an inhiitory neuron which synapses with motor neuron

  • Inhibitory neurotransmitter inhibites (hyperpolarizes) motor neurons (generates fewer impulses)

  • Muscles relaxes and relieves tension

  • Sensory neuron synapses with an excitatory interneuron in SC

  • Interneuron Synapses with motor neurons controlling antagonistic muscle (reciprocal activation)

Cranial Nerves

Autonomic Nervous System